From 06bfe6ffacecd09f82ca54e761117aa7d9bca565 Mon Sep 17 00:00:00 2001
From: Sameer Agarwal <sameeragarwal@google.com>
Date: Sat, 26 Nov 2022 16:02:28 -0800
Subject: [PATCH] Remove OpenMP and No threading backends.

Since c++11, we can depend on C++ threads always being available.
With the recent work on the performance of CXX threading, the
additional complexity of maintaining multiple backends for some
minor performance delta is not worth it

https://github.com/ceres-solver/ceres-solver/issues/886

Change-Id: Idee480b22a498daec9c4366da8589aa58eaf36a1
---
 .github/workflows/linux.yml                   | 11 +--
 CMakeLists.txt                                | 21 +----
 cmake/CeresCompileOptionsToComponents.cmake   |  4 -
 cmake/CeresConfig.cmake.in                    |  3 +-
 cmake/CeresThreadingModels.cmake              | 82 -------------------
 cmake/config.h.in                             | 22 -----
 internal/ceres/CMakeLists.txt                 | 26 ++----
 internal/ceres/concurrent_queue_test.cc       |  9 +-
 internal/ceres/context_impl.cc                |  4 +-
 internal/ceres/context_impl.h                 |  4 -
 internal/ceres/covariance_impl.cc             |  9 --
 internal/ceres/covariance_test.cc             |  8 +-
 .../ceres/generate_bundle_adjustment_tests.py |  5 --
 .../ba_denseschur_cuda_auto_threads_test.cc   |  2 -
 .../ba_denseschur_cuda_user_threads_test.cc   |  2 -
 .../ba_denseschur_eigen_auto_threads_test.cc  |  4 -
 .../ba_denseschur_eigen_user_threads_test.cc  |  4 -
 .../ba_denseschur_lapack_auto_threads_test.cc |  2 -
 .../ba_denseschur_lapack_user_threads_test.cc |  2 -
 ...atesparse_clustjacobi_auto_threads_test.cc |  2 -
 ...atesparse_clustjacobi_user_threads_test.cc |  2 -
 ...leratesparse_clusttri_auto_threads_test.cc |  2 -
 ...leratesparse_clusttri_user_threads_test.cc |  2 -
 ...gensparse_clustjacobi_auto_threads_test.cc |  2 -
 ...gensparse_clustjacobi_user_threads_test.cc |  2 -
 ..._eigensparse_clusttri_auto_threads_test.cc |  2 -
 ..._eigensparse_clusttri_user_threads_test.cc |  2 -
 .../ba_iterschur_jacobi_auto_threads_test.cc  |  4 -
 .../ba_iterschur_jacobi_user_threads_test.cc  |  4 -
 ...iterschur_schurjacobi_auto_threads_test.cc |  4 -
 ...iterschur_schurjacobi_user_threads_test.cc |  4 -
 ...itesparse_clustjacobi_auto_threads_test.cc |  2 -
 ...itesparse_clustjacobi_user_threads_test.cc |  2 -
 ..._suitesparse_clusttri_auto_threads_test.cc |  2 -
 ..._suitesparse_clusttri_user_threads_test.cc |  2 -
 ...esky_acceleratesparse_auto_threads_test.cc |  2 -
 ...esky_acceleratesparse_user_threads_test.cc |  2 -
 ...echolesky_eigensparse_auto_threads_test.cc |  2 -
 ...echolesky_eigensparse_user_threads_test.cc |  2 -
 ...echolesky_suitesparse_auto_threads_test.cc |  2 -
 ...echolesky_suitesparse_user_threads_test.cc |  2 -
 ...chur_acceleratesparse_auto_threads_test.cc |  2 -
 ...chur_acceleratesparse_user_threads_test.cc |  2 -
 ...arseschur_eigensparse_auto_threads_test.cc |  2 -
 ...arseschur_eigensparse_user_threads_test.cc |  2 -
 ...arseschur_suitesparse_auto_threads_test.cc |  2 -
 ...arseschur_suitesparse_user_threads_test.cc |  2 -
 internal/ceres/parallel_for.h                 | 16 +---
 internal/ceres/parallel_for_cxx.cc            |  8 +-
 internal/ceres/parallel_for_cxx.h             | 12 +--
 internal/ceres/parallel_for_nothreads.cc      | 47 -----------
 internal/ceres/parallel_for_openmp.cc         | 48 -----------
 internal/ceres/parallel_for_openmp.h          | 70 ----------------
 internal/ceres/parallel_for_test.cc           |  9 +-
 internal/ceres/parallel_utils_test.cc         |  6 +-
 internal/ceres/problem_impl.cc                | 10 ---
 internal/ceres/program_evaluator.h            |  9 --
 internal/ceres/solver_utils.cc                |  6 --
 internal/ceres/thread_pool.cc                 |  9 +-
 internal/ceres/thread_pool_test.cc            |  9 +-
 internal/ceres/thread_token_provider.cc       | 24 +-----
 internal/ceres/thread_token_provider.h        | 14 +---
 internal/ceres/wall_time.cc                   | 12 +--
 63 files changed, 34 insertions(+), 563 deletions(-)
 delete mode 100644 cmake/CeresThreadingModels.cmake
 delete mode 100644 internal/ceres/parallel_for_nothreads.cc
 delete mode 100644 internal/ceres/parallel_for_openmp.cc
 delete mode 100644 internal/ceres/parallel_for_openmp.h

diff --git a/.github/workflows/linux.yml b/.github/workflows/linux.yml
index 4f9d29bdd..630998f95 100644
--- a/.github/workflows/linux.yml
+++ b/.github/workflows/linux.yml
@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
-    name: ${{matrix.os}}-${{matrix.build_type}}-${{matrix.lib}}-${{matrix.threading_model}}
+    name: ${{matrix.os}}-${{matrix.build_type}}-${{matrix.lib}}
     runs-on: ubuntu-latest
     container: ${{matrix.os}}
     defaults:
@@ -25,10 +25,6 @@ jobs:
         lib:
           - shared
           - static
-        threading_model:
-          - CXX_THREADS
-          - NO_THREADS
-          - OPENMP
 
     steps:
       - uses: actions/checkout@v2
@@ -55,8 +51,8 @@ jobs:
         uses: actions/cache@v2
         with:
           path: ${{env.CCACHE_DIR}}
-          key: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-${{matrix.threading_model}}-${{github.run_id}}
-          restore-keys: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-${{matrix.threading_model}}-
+          key: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-${{github.run_id}}
+          restore-keys: ${{matrix.os}}-ccache-${{matrix.build_type}}-${{matrix.lib}}-
 
       - name: Setup Environment
         if: matrix.build_type == 'Release'
@@ -67,7 +63,6 @@ jobs:
         run: |
           cmake -S . -B build_${{matrix.build_type}} \
                 -DBUILD_SHARED_LIBS=${{matrix.lib == 'shared'}} \
-                -DCERES_THREADING_MODEL=${{matrix.threading_model}} \
                 -DCMAKE_BUILD_TYPE=${{matrix.build_type}} \
                 -DCMAKE_C_COMPILER_LAUNCHER=$(which ccache) \
                 -DCMAKE_CXX_COMPILER_LAUNCHER=$(which ccache) \
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 01f47b815..c30ae9ffb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -118,24 +118,9 @@ read_ceres_version_from_source(${Ceres_SOURCE_DIR})
 
 enable_testing()
 
-include(CeresThreadingModels)
 include(CMakeDependentOption)
 include(PrettyPrintCMakeList)
 
-find_available_ceres_threading_models(CERES_THREADING_MODELS_AVAILABLE)
-pretty_print_cmake_list(PRETTY_CERES_THREADING_MODELS_AVAILABLE
-  ${CERES_THREADING_MODELS_AVAILABLE})
-message("-- Detected available Ceres threading models: "
-  "${PRETTY_CERES_THREADING_MODELS_AVAILABLE}")
-set(CERES_THREADING_MODEL "${CERES_THREADING_MODEL}" CACHE STRING
-  "Ceres threading back-end" FORCE)
-if (NOT CERES_THREADING_MODEL)
-  list(GET CERES_THREADING_MODELS_AVAILABLE 0 DEFAULT_THREADING_MODEL)
-  update_cache_variable(CERES_THREADING_MODEL ${DEFAULT_THREADING_MODEL})
-endif()
-set_property(CACHE CERES_THREADING_MODEL PROPERTY STRINGS
-  ${CERES_THREADING_MODELS_AVAILABLE})
-
 option(MINIGLOG "Use a stripped down version of glog." OFF)
 option(GFLAGS "Enable Google Flags." ON)
 option(SUITESPARSE "Enable SuiteSparse." ON)
@@ -197,8 +182,8 @@ if (IOS)
   # Apple claims that the BLAS call dsyrk_ is a private API, and will not allow
   # you to submit to the Apple Store if the symbol is present.
   update_cache_variable(LAPACK OFF)
-  message(STATUS "Building for iOS: SuiteSparse, LAPACK, gflags, "
-    "and OpenMP are not available.")
+  message(STATUS "Building for iOS: SuiteSparse, LAPACK, gflags "
+    "are not available.")
 
   update_cache_variable(BUILD_EXAMPLES OFF)
   message(STATUS "Building for iOS: Will not build examples.")
@@ -489,8 +474,6 @@ if (NOT CUSTOM_BLAS)
   message("-- Disabling custom blas")
 endif (NOT CUSTOM_BLAS)
 
-set_ceres_threading_model("${CERES_THREADING_MODEL}")
-
 if (BUILD_BENCHMARKS)
   # Version 1.3 was first to provide import targets
   find_package(benchmark 1.3 QUIET)
diff --git a/cmake/CeresCompileOptionsToComponents.cmake b/cmake/CeresCompileOptionsToComponents.cmake
index 0baf2147b..47b0f2af5 100644
--- a/cmake/CeresCompileOptionsToComponents.cmake
+++ b/cmake/CeresCompileOptionsToComponents.cmake
@@ -80,10 +80,6 @@ function(ceres_compile_options_to_components CURRENT_CERES_COMPILE_OPTIONS CERES
     CERES_NO_ACCELERATE_SPARSE "AccelerateSparse;SparseLinearAlgebraLibrary")
   add_to_output_if_not_found(CURRENT_CERES_COMPILE_OPTIONS ${CERES_COMPONENTS_VAR}
     CERES_RESTRICT_SCHUR_SPECIALIZATION "SchurSpecializations")
-  add_to_output_if_found(CURRENT_CERES_COMPILE_OPTIONS ${CERES_COMPONENTS_VAR}
-    CERES_USE_OPENMP "OpenMP;Multithreading")
-  add_to_output_if_found(CURRENT_CERES_COMPILE_OPTIONS ${CERES_COMPONENTS_VAR}
-    CERES_USE_CXX_THREADS "Multithreading")
   # Remove duplicates of SparseLinearAlgebraLibrary if multiple sparse backends
   # are present.
   list(REMOVE_DUPLICATES ${CERES_COMPONENTS_VAR})
diff --git a/cmake/CeresConfig.cmake.in b/cmake/CeresConfig.cmake.in
index bb6cc43a1..ceb7e269c 100644
--- a/cmake/CeresConfig.cmake.in
+++ b/cmake/CeresConfig.cmake.in
@@ -178,10 +178,9 @@ set(CERES_VERSION @CERES_VERSION@)
 include(CMakeFindDependencyMacro)
 # Optional dependencies
 @METIS_DEPENDENCY@
-@OpenMP_DEPENDENCY@
 @SuiteSparse_DEPENDENCY@
-@Threads_DEPENDENCY@
 @CUDAToolkit_DEPENDENCY@
+@Threads_DEPENDENCY@
 
 # As imported CMake targets are not re-exported when a dependent target is
 # exported, we must invoke find_package(XXX) here to reload the definition
diff --git a/cmake/CeresThreadingModels.cmake b/cmake/CeresThreadingModels.cmake
deleted file mode 100644
index 571dd7d50..000000000
--- a/cmake/CeresThreadingModels.cmake
+++ /dev/null
@@ -1,82 +0,0 @@
-# Ceres Solver - A fast non-linear least squares minimizer
-# Copyright 2018 Google Inc. All rights reserved.
-# http://ceres-solver.org/
-#
-# Redistribution and use in source and binary forms, with or without
-# modification, are permitted provided that the following conditions are met:
-#
-# * Redistributions of source code must retain the above copyright notice,
-#   this list of conditions and the following disclaimer.
-# * Redistributions in binary form must reproduce the above copyright notice,
-#   this list of conditions and the following disclaimer in the documentation
-#   and/or other materials provided with the distribution.
-# * Neither the name of Google Inc. nor the names of its contributors may be
-#   used to endorse or promote products derived from this software without
-#   specific prior written permission.
-#
-# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
-# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-# POSSIBILITY OF SUCH DAMAGE.
-#
-# Author: alexs.mac@gmail.com (Alex Stewart)
-
-# Ordered by expected preference.
-set(CERES_THREADING_MODELS "CXX_THREADS;OPENMP;NO_THREADS")
-
-function(find_available_ceres_threading_models CERES_THREADING_MODELS_AVAILABLE_VAR)
-  set(CERES_THREADING_MODELS_AVAILABLE ${CERES_THREADING_MODELS})
-  # Remove any threading models for which the dependencies are not available.
-  find_package(OpenMP QUIET)
-  if (NOT OPENMP_FOUND)
-    list(REMOVE_ITEM CERES_THREADING_MODELS_AVAILABLE "OPENMP")
-  endif()
-  if (NOT CERES_THREADING_MODELS_AVAILABLE)
-    # At least NO_THREADS should never be removed.  This check is purely
-    # protective against future threading model updates.
-    message(FATAL_ERROR "Ceres bug: Removed all threading models.")
-  endif()
-  set(${CERES_THREADING_MODELS_AVAILABLE_VAR}
-    ${CERES_THREADING_MODELS_AVAILABLE} PARENT_SCOPE)
-endfunction()
-
-macro(set_ceres_threading_model_to_cxx11_threads)
-  list(APPEND CERES_COMPILE_OPTIONS CERES_USE_CXX_THREADS)
-endmacro()
-
-macro(set_ceres_threading_model_to_openmp)
-  find_package(OpenMP REQUIRED)
-  list(APPEND CERES_COMPILE_OPTIONS CERES_USE_OPENMP)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
-  set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
-endmacro()
-
-macro(set_ceres_threading_model_to_no_threads)
-  list(APPEND CERES_COMPILE_OPTIONS CERES_NO_THREADS)
-endmacro()
-
-macro(set_ceres_threading_model CERES_THREADING_MODEL_TO_SET)
-  if ("${CERES_THREADING_MODEL_TO_SET}" STREQUAL "CXX_THREADS")
-    set_ceres_threading_model_to_cxx11_threads()
-  elseif ("${CERES_THREADING_MODEL_TO_SET}" STREQUAL "OPENMP")
-    set_ceres_threading_model_to_openmp()
-  elseif ("${CERES_THREADING_MODEL_TO_SET}" STREQUAL "NO_THREADS")
-    set_ceres_threading_model_to_no_threads()
-  else()
-    include(PrettyPrintCMakeList)
-    find_available_ceres_threading_models(_AVAILABLE_THREADING_MODELS)
-    pretty_print_cmake_list(
-      _AVAILABLE_THREADING_MODELS ${_AVAILABLE_THREADING_MODELS})
-    message(FATAL_ERROR "Unknown threading model specified: "
-      "'${CERES_THREADING_MODEL_TO_SET}'. Available threading models for "
-      "this platform are: ${_AVAILABLE_THREADING_MODELS}")
-  endif()
-  message("-- Using Ceres threading model: ${CERES_THREADING_MODEL_TO_SET}")
-endmacro()
diff --git a/cmake/config.h.in b/cmake/config.h.in
index 40079550f..0e7fd3a9e 100644
--- a/cmake/config.h.in
+++ b/cmake/config.h.in
@@ -70,12 +70,6 @@
 // routines.
 @CERES_NO_CUSTOM_BLAS@
 
-// If defined, Ceres was compiled without multithreading support.
-@CERES_NO_THREADS@
-// If defined Ceres was compiled with OpenMP multithreading.
-@CERES_USE_OPENMP@
-// If defined Ceres was compiled with modern C++ multithreading.
-@CERES_USE_CXX_THREADS@
 // If defined, Ceres was compiled with a version of SuiteSparse/CHOLMOD without
 // the Partition module (requires METIS).
 @CERES_NO_CHOLMOD_PARTITION@
@@ -87,22 +81,6 @@
 // with underscore prefixed versions (e.g. j0() -> _j0()).
 @CERES_MSVC_USE_UNDERSCORE_PREFIXED_BESSEL_FUNCTIONS@
 
-#if defined(CERES_USE_OPENMP)
-#if defined(CERES_USE_CXX_THREADS) || defined(CERES_NO_THREADS)
-#error CERES_USE_OPENMP is mutually exclusive to CERES_USE_CXX_THREADS and CERES_NO_THREADS
-#endif
-#elif defined(CERES_USE_CXX_THREADS)
-#if defined(CERES_USE_OPENMP) || defined(CERES_NO_THREADS)
-#error CERES_USE_CXX_THREADS is mutually exclusive to CERES_USE_OPENMP, CERES_USE_CXX_THREADS and CERES_NO_THREADS
-#endif
-#elif defined(CERES_NO_THREADS)
-#if defined(CERES_USE_OPENMP) || defined(CERES_USE_CXX_THREADS)
-#error CERES_NO_THREADS is mutually exclusive to CERES_USE_OPENMP and CERES_USE_CXX_THREADS
-#endif
-#else
-#  error One of CERES_USE_OPENMP, CERES_USE_CXX_THREADS or CERES_NO_THREADS must be defined.
-#endif
-
 // CERES_NO_SPARSE should be automatically defined by config.h if Ceres was
 // compiled without any sparse back-end.  Verify that it has not subsequently
 // been inconsistently redefined.
diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 06c091497..1e6a69951 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -32,25 +32,10 @@
 
 # Avoid 'xxx.cc has no symbols' warnings from source files which are 'empty'
 # when their enclosing #ifdefs are disabled.
-if (CERES_THREADING_MODEL STREQUAL "CXX_THREADS")
-  set(CERES_PARALLEL_FOR_SRC parallel_for_cxx.cc thread_pool.cc)
-  find_package(Threads REQUIRED)
-  list(APPEND CERES_LIBRARY_PRIVATE_DEPENDENCIES Threads::Threads)
-
-  # Make dependency visible to the parent CMakeLists.txt
-  set(Threads_DEPENDENCY "find_dependency (Threads)" PARENT_SCOPE)
-elseif (CERES_THREADING_MODEL STREQUAL "OPENMP")
-  # OpenMP requires linking to the corresponding library.
-  find_package(OpenMP REQUIRED COMPONENTS CXX)
-
-  set(CERES_PARALLEL_FOR_SRC parallel_for_openmp.cc)
-  list(APPEND CERES_LIBRARY_PRIVATE_DEPENDENCIES OpenMP::OpenMP_CXX)
-
-  # Make dependency visible to the parent CMakeLists.txt
-  set(OpenMP_DEPENDENCY "find_dependency (OpenMP ${OpenMP_CXX_VERSION} COMPONENTS CXX)" PARENT_SCOPE)
-elseif (CERES_THREADING_MODEL STREQUAL "NO_THREADS")
-  set(CERES_PARALLEL_FOR_SRC parallel_for_nothreads.cc)
-endif()
+find_package(Threads REQUIRED)
+list(APPEND CERES_LIBRARY_PRIVATE_DEPENDENCIES Threads::Threads)
+# Make dependency visible to the parent CMakeLists.txt
+set(Threads_DEPENDENCY "find_dependency (Threads)" PARENT_SCOPE)
 
 # Source files that contain public symbols and live in the ceres namespaces.
 # Such symbols are expected to be marked with CERES_EXPORT and the files below
@@ -171,7 +156,6 @@ endif ()
 # lexicographical order.
 add_library(ceres_internal OBJECT
     ${CERES_INTERNAL_SCHUR_FILES}
-    ${CERES_PARALLEL_FOR_SRC}
     accelerate_sparse.cc
     array_utils.cc
     block_evaluate_preparer.cc
@@ -235,6 +219,7 @@ add_library(ceres_internal OBJECT
     linear_solver.cc
     low_rank_inverse_hessian.cc
     minimizer.cc
+    parallel_for_cxx.cc
     parallel_utils.cc
     parameter_block_ordering.cc
     partitioned_matrix_view.cc
@@ -260,6 +245,7 @@ add_library(ceres_internal OBJECT
     stringprintf.cc
     subset_preconditioner.cc
     suitesparse.cc
+    thread_pool.cc
     thread_token_provider.cc
     triplet_sparse_matrix.cc
     trust_region_minimizer.cc
diff --git a/internal/ceres/concurrent_queue_test.cc b/internal/ceres/concurrent_queue_test.cc
index b0930fe95..08451bdea 100644
--- a/internal/ceres/concurrent_queue_test.cc
+++ b/internal/ceres/concurrent_queue_test.cc
@@ -28,15 +28,12 @@
 //
 // Author: vitus@google.com (Michael Vitus)
 
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#ifdef CERES_USE_CXX_THREADS
+#include "ceres/concurrent_queue.h"
 
 #include <chrono>
 #include <thread>
 
-#include "ceres/concurrent_queue.h"
+#include "ceres/internal/config.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
@@ -300,5 +297,3 @@ TEST(ConcurrentQueue, StopAndEnableWaiters) {
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_USE_CXX_THREADS
diff --git a/internal/ceres/context_impl.cc b/internal/ceres/context_impl.cc
index 56c978557..cc01b5436 100644
--- a/internal/ceres/context_impl.cc
+++ b/internal/ceres/context_impl.cc
@@ -171,9 +171,9 @@ ContextImpl::~ContextImpl() {
   TearDown();
 #endif  // CERES_NO_CUDA
 }
+
 void ContextImpl::EnsureMinimumThreads(int num_threads) {
-#ifdef CERES_USE_CXX_THREADS
   thread_pool.Resize(num_threads);
-#endif  // CERES_USE_CXX_THREADS
 }
+
 }  // namespace ceres::internal
diff --git a/internal/ceres/context_impl.h b/internal/ceres/context_impl.h
index d8f294ace..a18d6388f 100644
--- a/internal/ceres/context_impl.h
+++ b/internal/ceres/context_impl.h
@@ -49,9 +49,7 @@
 #include "cusparse.h"
 #endif  // CERES_NO_CUDA
 
-#ifdef CERES_USE_CXX_THREADS
 #include "ceres/thread_pool.h"
-#endif  // CERES_USE_CXX_THREADS
 
 namespace ceres::internal {
 
@@ -67,9 +65,7 @@ class CERES_NO_EXPORT ContextImpl final : public Context {
   // defined by the hardware.  Otherwise this call is a no-op.
   void EnsureMinimumThreads(int num_threads);
 
-#ifdef CERES_USE_CXX_THREADS
   ThreadPool thread_pool;
-#endif  // CERES_USE_CXX_THREADS
 
 #ifndef CERES_NO_CUDA
   // Note on Ceres' use of CUDA Devices on multi-GPU systems:
diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 9957d1d2d..5d8b87f68 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -65,15 +65,6 @@ using CovarianceBlocks = std::vector<std::pair<const double*, const double*>>;
 
 CovarianceImpl::CovarianceImpl(const Covariance::Options& options)
     : options_(options), is_computed_(false), is_valid_(false) {
-#ifdef CERES_NO_THREADS
-  if (options_.num_threads > 1) {
-    LOG(WARNING) << "No threading support is compiled into this binary; "
-                 << "only options.num_threads = 1 is supported. Switching "
-                 << "to single threaded mode.";
-    options_.num_threads = 1;
-  }
-#endif
-
   evaluate_options_.num_threads = options_.num_threads;
   evaluate_options_.apply_loss_function = options_.apply_loss_function;
 }
diff --git a/internal/ceres/covariance_test.cc b/internal/ceres/covariance_test.cc
index caea18c7e..76fcb122e 100644
--- a/internal/ceres/covariance_test.cc
+++ b/internal/ceres/covariance_test.cc
@@ -639,8 +639,6 @@ TEST_F(CovarianceTest, NormalBehavior) {
   ComputeAndCompareCovarianceBlocks(options, expected_covariance);
 }
 
-#ifdef CERES_USE_OPENMP
-
 TEST_F(CovarianceTest, ThreadedNormalBehavior) {
   // J
   //
@@ -691,8 +689,6 @@ TEST_F(CovarianceTest, ThreadedNormalBehavior) {
   ComputeAndCompareCovarianceBlocks(options, expected_covariance);
 }
 
-#endif  // CERES_USE_OPENMP
-
 TEST_F(CovarianceTest, ConstantParameterBlock) {
   problem_.SetParameterBlockConstant(parameters_);
 
@@ -1355,13 +1351,13 @@ class LargeScaleCovarianceTest : public ::testing::Test {
   vector<pair<const double*, const double*>> all_covariance_blocks_;
 };
 
-#if !defined(CERES_NO_SUITESPARSE) && defined(CERES_USE_OPENMP)
+#if !defined(CERES_NO_SUITESPARSE)
 
 TEST_F(LargeScaleCovarianceTest, Parallel) {
   ComputeAndCompare(SPARSE_QR, SUITE_SPARSE, 4);
 }
 
-#endif  // !defined(CERES_NO_SUITESPARSE) && defined(CERES_USE_OPENMP)
+#endif  // !defined(CERES_NO_SUITESPARSE)
 
 }  // namespace internal
 }  // namespace ceres
diff --git a/internal/ceres/generate_bundle_adjustment_tests.py b/internal/ceres/generate_bundle_adjustment_tests.py
index 6e6027085..35865a61c 100644
--- a/internal/ceres/generate_bundle_adjustment_tests.py
+++ b/internal/ceres/generate_bundle_adjustment_tests.py
@@ -221,11 +221,6 @@ def generate_bundle_test(linear_solver,
     preprocessor_conditions_begin.append('#ifndef CERES_NO_CUDA')
     preprocessor_conditions_end.insert(0, '#endif  // CERES_NO_CUDA')
 
-  # Accumulate appropriate #ifdef/#ifndefs for threading conditions.
-  if thread_config == MULTI_THREADED:
-    preprocessor_conditions_begin.append('#ifndef CERES_NO_THREADS')
-    preprocessor_conditions_end.insert(0, '#endif  // CERES_NO_THREADS')
-
   # If there are #ifdefs, put newlines around them.
   if preprocessor_conditions_begin:
     preprocessor_conditions_begin.insert(0, '')
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_auto_threads_test.cc
index d0181e38b..c0020c25b 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_CUDA
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_CUDA
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_user_threads_test.cc
index ecda17c97..042848a21 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_cuda_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_CUDA
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_CUDA
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_auto_threads_test.cc
index a355e851d..3c32212b6 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_auto_threads_test.cc
@@ -38,8 +38,6 @@
 #include "ceres/internal/config.h"
 #include "bundle_adjustment_test_util.h"
 
-#ifndef CERES_NO_THREADS
-
 namespace ceres::internal {
 
 TEST_F(BundleAdjustmentTest,
@@ -59,5 +57,3 @@ TEST_F(BundleAdjustmentTest,
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_user_threads_test.cc
index f005c31d4..9335b8215 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_eigen_user_threads_test.cc
@@ -38,8 +38,6 @@
 #include "ceres/internal/config.h"
 #include "bundle_adjustment_test_util.h"
 
-#ifndef CERES_NO_THREADS
-
 namespace ceres::internal {
 
 TEST_F(BundleAdjustmentTest,
@@ -59,5 +57,3 @@ TEST_F(BundleAdjustmentTest,
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_auto_threads_test.cc
index 4e727dbeb..1acda327c 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_LAPACK
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_LAPACK
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_user_threads_test.cc
index e19734a34..0d39dcd03 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_denseschur_lapack_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_LAPACK
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_LAPACK
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_auto_threads_test.cc
index 93b58ebf1..acb5c1602 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_user_threads_test.cc
index 053e9b236..42570c42c 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clustjacobi_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_auto_threads_test.cc
index 723246a1e..f67030a9c 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_user_threads_test.cc
index 93f5353c2..a638cfed7 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_acceleratesparse_clusttri_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_auto_threads_test.cc
index 1e9dbf05b..5adb25a2b 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_user_threads_test.cc
index 0daf51925..d531c340d 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clustjacobi_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_auto_threads_test.cc
index 1ee128652..c82924cc2 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_user_threads_test.cc
index 6e08b2279..f7de34f25 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_eigensparse_clusttri_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_auto_threads_test.cc
index e0aad4485..4666424cd 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_auto_threads_test.cc
@@ -38,8 +38,6 @@
 #include "ceres/internal/config.h"
 #include "bundle_adjustment_test_util.h"
 
-#ifndef CERES_NO_THREADS
-
 namespace ceres::internal {
 
 TEST_F(BundleAdjustmentTest,
@@ -59,5 +57,3 @@ TEST_F(BundleAdjustmentTest,
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_user_threads_test.cc
index c68f6f052..09d5bd034 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_jacobi_user_threads_test.cc
@@ -38,8 +38,6 @@
 #include "ceres/internal/config.h"
 #include "bundle_adjustment_test_util.h"
 
-#ifndef CERES_NO_THREADS
-
 namespace ceres::internal {
 
 TEST_F(BundleAdjustmentTest,
@@ -59,5 +57,3 @@ TEST_F(BundleAdjustmentTest,
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_auto_threads_test.cc
index 6d97ebab0..beaa92a47 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_auto_threads_test.cc
@@ -38,8 +38,6 @@
 #include "ceres/internal/config.h"
 #include "bundle_adjustment_test_util.h"
 
-#ifndef CERES_NO_THREADS
-
 namespace ceres::internal {
 
 TEST_F(BundleAdjustmentTest,
@@ -59,5 +57,3 @@ TEST_F(BundleAdjustmentTest,
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_user_threads_test.cc
index 71220f74a..762f8810c 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_schurjacobi_user_threads_test.cc
@@ -38,8 +38,6 @@
 #include "ceres/internal/config.h"
 #include "bundle_adjustment_test_util.h"
 
-#ifndef CERES_NO_THREADS
-
 namespace ceres::internal {
 
 TEST_F(BundleAdjustmentTest,
@@ -59,5 +57,3 @@ TEST_F(BundleAdjustmentTest,
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_auto_threads_test.cc
index 874168db3..e31446a69 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_user_threads_test.cc
index ae07667ae..d26cc333e 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clustjacobi_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_auto_threads_test.cc
index aee5e5a31..35df772e0 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_user_threads_test.cc
index 9bc2d3fb7..e71b4e5c9 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_iterschur_suitesparse_clusttri_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_auto_threads_test.cc
index 79769b435..38731ecd6 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_user_threads_test.cc
index 6131f1dc3..27e32d122 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_acceleratesparse_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_auto_threads_test.cc
index 46e5dac97..6b9372782 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_user_threads_test.cc
index 68f3be7f5..a92ccfbaa 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_eigensparse_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_auto_threads_test.cc
index ef1100577..f8aa16ff9 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_user_threads_test.cc
index 862142a25..09ac24e5a 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparsecholesky_suitesparse_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_auto_threads_test.cc
index 595fc155e..b6b53ce39 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_user_threads_test.cc
index c1302f731..da4c9df49 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_acceleratesparse_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_ACCELERATE_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_ACCELERATE_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_auto_threads_test.cc
index 137725bad..7a8bd90c4 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_user_threads_test.cc
index 14dcabf31..35c73ca8f 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_eigensparse_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifdef CERES_USE_EIGEN_SPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_USE_EIGEN_SPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_auto_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_auto_threads_test.cc
index cf015b87b..2cdb888f4 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_auto_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_auto_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_user_threads_test.cc b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_user_threads_test.cc
index b13915e70..04f36337b 100644
--- a/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_user_threads_test.cc
+++ b/internal/ceres/generated_bundle_adjustment_tests/ba_sparseschur_suitesparse_user_threads_test.cc
@@ -39,7 +39,6 @@
 #include "bundle_adjustment_test_util.h"
 
 #ifndef CERES_NO_SUITESPARSE
-#ifndef CERES_NO_THREADS
 
 namespace ceres::internal {
 
@@ -61,5 +60,4 @@ TEST_F(BundleAdjustmentTest,
 
 }  // namespace ceres::internal
 
-#endif  // CERES_NO_THREADS
 #endif  // CERES_NO_SUITESPARSE
diff --git a/internal/ceres/parallel_for.h b/internal/ceres/parallel_for.h
index fe5d242a4..d91a195a6 100644
--- a/internal/ceres/parallel_for.h
+++ b/internal/ceres/parallel_for.h
@@ -338,21 +338,7 @@ void ParallelFor(ContextImpl* context,
 }  // namespace ceres::internal
 
 // Backend-specific implementations of ParallelInvoke
-#include "ceres/parallel_for_cxx.h"
-#include "ceres/parallel_for_openmp.h"
-#ifdef CERES_NO_THREADS
-namespace ceres::internal {
-template <typename F>
-void ParallelInvoke(ContextImpl* context,
-                    int start,
-                    int end,
-                    int num_threads,
-                    const F& function) {
-  ParallelFor(context, start, end, 1, function);
-}
-}  // namespace ceres::internal
-#endif
-
 #include "ceres/internal/disable_warnings.h"
+#include "ceres/parallel_for_cxx.h"
 
 #endif  // CERES_INTERNAL_PARALLEL_FOR_H_
diff --git a/internal/ceres/parallel_for_cxx.cc b/internal/ceres/parallel_for_cxx.cc
index 13cabf90b..cdf9966f5 100644
--- a/internal/ceres/parallel_for_cxx.cc
+++ b/internal/ceres/parallel_for_cxx.cc
@@ -28,17 +28,13 @@
 //
 // Author: vitus@google.com (Michael Vitus)
 
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#ifdef CERES_USE_CXX_THREADS
-
 #include <atomic>
 #include <cmath>
 #include <condition_variable>
 #include <memory>
 #include <mutex>
 
+#include "ceres/internal/config.h"
 #include "ceres/parallel_for.h"
 #include "glog/logging.h"
 
@@ -79,5 +75,3 @@ ThreadPoolState::ThreadPoolState(int start,
 int MaxNumThreadsAvailable() { return ThreadPool::MaxNumThreadsAvailable(); }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_USE_CXX_THREADS
diff --git a/internal/ceres/parallel_for_cxx.h b/internal/ceres/parallel_for_cxx.h
index b745a6407..073160992 100644
--- a/internal/ceres/parallel_for_cxx.h
+++ b/internal/ceres/parallel_for_cxx.h
@@ -29,21 +29,16 @@
 // Authors: vitus@google.com (Michael Vitus),
 //          dmitriy.korchemkin@gmail.com (Dmitriy Korchemkin)
 
-// This include must come before any #ifndef check on Ceres compile options.
 #ifndef CERES_INTERNAL_PARALLEL_FOR_CXX_H_
 #define CERES_INTERNAL_PARALLEL_FOR_CXX_H_
 
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#ifdef CERES_USE_CXX_THREADS
-
 #include <atomic>
 #include <cmath>
 #include <condition_variable>
 #include <memory>
 #include <mutex>
 
+#include "ceres/internal/config.h"
 #include "glog/logging.h"
 
 namespace ceres::internal {
@@ -243,6 +238,7 @@ void ParallelInvoke(ContextImpl* context,
   // Wait until all tasks have finished.
   shared_state->block_until_finished.Block();
 }
+
 }  // namespace ceres::internal
-#endif
-#endif
+
+#endif  // CERES_INTERNAL_PARALLEL_FOR_CXX_H_
diff --git a/internal/ceres/parallel_for_nothreads.cc b/internal/ceres/parallel_for_nothreads.cc
deleted file mode 100644
index 8d3611dbc..000000000
--- a/internal/ceres/parallel_for_nothreads.cc
+++ /dev/null
@@ -1,47 +0,0 @@
-// Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2018 Google Inc. All rights reserved.
-// http://ceres-solver.org/
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are met:
-//
-// * Redistributions of source code must retain the above copyright notice,
-//   this list of conditions and the following disclaimer.
-// * Redistributions in binary form must reproduce the above copyright notice,
-//   this list of conditions and the following disclaimer in the documentation
-//   and/or other materials provided with the distribution.
-// * Neither the name of Google Inc. nor the names of its contributors may be
-//   used to endorse or promote products derived from this software without
-//   specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
-// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-// POSSIBILITY OF SUCH DAMAGE.
-//
-// Author: alexs.mac@gmail.com (Alex Stewart)
-
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#ifdef CERES_NO_THREADS
-
-#include "ceres/parallel_for.h"
-#include "glog/logging.h"
-
-namespace ceres {
-namespace internal {
-
-int MaxNumThreadsAvailable() { return 1; }
-
-}  // namespace internal
-}  // namespace ceres
-
-#endif  // CERES_NO_THREADS
diff --git a/internal/ceres/parallel_for_openmp.cc b/internal/ceres/parallel_for_openmp.cc
deleted file mode 100644
index 02690f310..000000000
--- a/internal/ceres/parallel_for_openmp.cc
+++ /dev/null
@@ -1,48 +0,0 @@
-// Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2018 Google Inc. All rights reserved.
-// http://ceres-solver.org/
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are met:
-//
-// * Redistributions of source code must retain the above copyright notice,
-//   this list of conditions and the following disclaimer.
-// * Redistributions in binary form must reproduce the above copyright notice,
-//   this list of conditions and the following disclaimer in the documentation
-//   and/or other materials provided with the distribution.
-// * Neither the name of Google Inc. nor the names of its contributors may be
-//   used to endorse or promote products derived from this software without
-//   specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
-// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-// POSSIBILITY OF SUCH DAMAGE.
-//
-// Author: vitus@google.com (Michael Vitus)
-
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#if defined(CERES_USE_OPENMP)
-
-#include "ceres/parallel_for.h"
-#include "glog/logging.h"
-#include "omp.h"
-
-namespace ceres {
-namespace internal {
-
-int MaxNumThreadsAvailable() { return omp_get_max_threads(); }
-
-}  // namespace internal
-}  // namespace ceres
-
-#endif  // defined(CERES_USE_OPENMP)
diff --git a/internal/ceres/parallel_for_openmp.h b/internal/ceres/parallel_for_openmp.h
deleted file mode 100644
index 94254c455..000000000
--- a/internal/ceres/parallel_for_openmp.h
+++ /dev/null
@@ -1,70 +0,0 @@
-// Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2018 Google Inc. All rights reserved.
-// http://ceres-solver.org/
-//
-// Redistribution and use in source and binary forms, with or without
-// modification, are permitted provided that the following conditions are met:
-//
-// * Redistributions of source code must retain the above copyright notice,
-//   this list of conditions and the following disclaimer.
-// * Redistributions in binary form must reproduce the above copyright notice,
-//   this list of conditions and the following disclaimer in the documentation
-//   and/or other materials provided with the distribution.
-// * Neither the name of Google Inc. nor the names of its contributors may be
-//   used to endorse or promote products derived from this software without
-//   specific prior written permission.
-//
-// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
-// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
-// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
-// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
-// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
-// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
-// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
-// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
-// POSSIBILITY OF SUCH DAMAGE.
-//
-// Authors: vitus@google.com (Michael Vitus),
-//          dmitriy.korchemkin@gmail.com (Dmitriy Korchemkin)
-
-// This include must come before any #ifndef check on Ceres compile options.
-#ifndef CERES_INTERNAL_PARALLEL_FOR_OPENMP_H_
-#define CERES_INTERNAL_PARALLEL_FOR_OPENMP_H_
-
-#include "ceres/internal/config.h"
-
-#if defined(CERES_USE_OPENMP)
-
-#include "ceres/parallel_for.h"
-#include "ceres/scoped_thread_token.h"
-#include "ceres/thread_token_provider.h"
-#include "glog/logging.h"
-#include "omp.h"
-
-namespace ceres::internal {
-
-template <typename F>
-void ParallelInvoke(ContextImpl* context,
-                    int start,
-                    int end,
-                    int num_threads,
-                    const F& function) {
-  using namespace parallel_for_details;
-  ThreadTokenProvider token_provider(num_threads);
-#pragma omp parallel num_threads(num_threads)
-  {
-    const ScopedThreadToken scoped_thread_token(&token_provider);
-    const int thread_id = scoped_thread_token.token();
-#pragma omp for schedule(guided)
-    for (int i = start; i < end; ++i) {
-      Invoke<F>(thread_id, i, function);
-    }
-  }
-}
-
-}  // namespace ceres::internal
-
-#endif
-#endif
diff --git a/internal/ceres/parallel_for_test.cc b/internal/ceres/parallel_for_test.cc
index 54ca63683..6974ab903 100644
--- a/internal/ceres/parallel_for_test.cc
+++ b/internal/ceres/parallel_for_test.cc
@@ -28,11 +28,6 @@
 //
 // Author: vitus@google.com (Michael Vitus)
 
-// This include must come before any #ifndef check on Ceres compile options.
-// clang-format off
-#include "ceres/internal/config.h"
-// clang-format on
-
 #include "ceres/parallel_for.h"
 
 #include <cmath>
@@ -44,6 +39,7 @@
 #include <vector>
 
 #include "ceres/context_impl.h"
+#include "ceres/internal/config.h"
 #include "glog/logging.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -133,8 +129,6 @@ TEST(ParallelForWithThreadId, NestedParallelForDeadlock) {
   }
 }
 
-// This test is only valid when multithreading support is enabled.
-#ifndef CERES_NO_THREADS
 TEST(ParallelForWithThreadId, UniqueThreadIds) {
   // Ensure the hardware supports more than 1 thread to ensure the test will
   // pass.
@@ -166,7 +160,6 @@ TEST(ParallelForWithThreadId, UniqueThreadIds) {
 
   EXPECT_THAT(x, UnorderedElementsAreArray({0, 1}));
 }
-#endif  // CERES_NO_THREADS
 
 // Helper function for partition tests
 bool BruteForcePartition(
diff --git a/internal/ceres/parallel_utils_test.cc b/internal/ceres/parallel_utils_test.cc
index fd2d79246..1facb3e25 100644
--- a/internal/ceres/parallel_utils_test.cc
+++ b/internal/ceres/parallel_utils_test.cc
@@ -28,13 +28,9 @@
 //
 // Author: wjr@google.com (William Rucklidge)
 
-// This include must come before any #ifndef check on Ceres compile options.
-// clang-format off
-#include "ceres/internal/config.h"
-// clang-format on
-
 #include "ceres/parallel_utils.h"
 
+#include "ceres/internal/config.h"
 #include "glog/logging.h"
 #include "gtest/gtest.h"
 
diff --git a/internal/ceres/problem_impl.cc b/internal/ceres/problem_impl.cc
index 326416649..4440526fb 100644
--- a/internal/ceres/problem_impl.cc
+++ b/internal/ceres/problem_impl.cc
@@ -672,17 +672,7 @@ bool ProblemImpl::Evaluate(const Problem::EvaluateOptions& evaluate_options,
   // the Evaluator decides the storage for the Jacobian based on the
   // type of linear solver being used.
   evaluator_options.linear_solver_type = SPARSE_NORMAL_CHOLESKY;
-#ifdef CERES_NO_THREADS
-  if (evaluate_options.num_threads > 1) {
-    LOG(WARNING)
-        << "No threading support is compiled into this binary; "
-        << "only evaluate_options.num_threads = 1 is supported. Switching "
-        << "to single threaded mode.";
-  }
-  evaluator_options.num_threads = 1;
-#else
   evaluator_options.num_threads = evaluate_options.num_threads;
-#endif  // CERES_NO_THREADS
 
   // The main thread also does work so we only need to launch num_threads - 1.
   context_impl_->EnsureMinimumThreads(evaluator_options.num_threads - 1);
diff --git a/internal/ceres/program_evaluator.h b/internal/ceres/program_evaluator.h
index 1176bb860..251a17704 100644
--- a/internal/ceres/program_evaluator.h
+++ b/internal/ceres/program_evaluator.h
@@ -119,15 +119,6 @@ class ProgramEvaluator final : public Evaluator {
         jacobian_writer_(options, program),
         evaluate_preparers_(std::move(
             jacobian_writer_.CreateEvaluatePreparers(options.num_threads))) {
-#ifdef CERES_NO_THREADS
-    if (options_.num_threads > 1) {
-      LOG(WARNING) << "No threading support is compiled into this binary; "
-                   << "only options.num_threads = 1 is supported. Switching "
-                   << "to single threaded mode.";
-      options_.num_threads = 1;
-    }
-#endif  // CERES_NO_THREADS
-
     BuildResidualLayout(*program, &residual_layout_);
     evaluate_scratch_ = std::move(CreateEvaluatorScratch(
         *program, static_cast<unsigned>(options.num_threads)));
diff --git a/internal/ceres/solver_utils.cc b/internal/ceres/solver_utils.cc
index 4c4a354ae..de1c38b47 100644
--- a/internal/ceres/solver_utils.cc
+++ b/internal/ceres/solver_utils.cc
@@ -78,12 +78,6 @@ constexpr char kVersion[] =
   "-no_schur_specializations"
 #endif
 
-#ifdef CERES_USE_OPENMP
-  "-openmp"
-#else
-  "-no_openmp"
-#endif
-
 #ifdef CERES_NO_CUSTOM_BLAS
   "-no_custom_blas"
 #endif
diff --git a/internal/ceres/thread_pool.cc b/internal/ceres/thread_pool.cc
index 1fd1a4caf..16b86e1f8 100644
--- a/internal/ceres/thread_pool.cc
+++ b/internal/ceres/thread_pool.cc
@@ -28,15 +28,12 @@
 //
 // Author: vitus@google.com (Michael Vitus)
 
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#ifdef CERES_USE_CXX_THREADS
+#include "ceres/thread_pool.h"
 
 #include <cmath>
 #include <limits>
 
-#include "ceres/thread_pool.h"
+#include "ceres/internal/config.h"
 
 namespace ceres::internal {
 namespace {
@@ -105,5 +102,3 @@ void ThreadPool::ThreadMainLoop() {
 void ThreadPool::Stop() { task_queue_.StopWaiters(); }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_USE_CXX_THREADS
diff --git a/internal/ceres/thread_pool_test.cc b/internal/ceres/thread_pool_test.cc
index e64011db8..63b05d08b 100644
--- a/internal/ceres/thread_pool_test.cc
+++ b/internal/ceres/thread_pool_test.cc
@@ -28,17 +28,14 @@
 //
 // Author: vitus@google.com (Michael Vitus)
 
-// This include must come before any #ifndef check on Ceres compile options.
-#include "ceres/internal/config.h"
-
-#ifdef CERES_USE_CXX_THREADS
+#include "ceres/thread_pool.h"
 
 #include <chrono>
 #include <condition_variable>
 #include <mutex>
 #include <thread>
 
-#include "ceres/thread_pool.h"
+#include "ceres/internal/config.h"
 #include "glog/logging.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
@@ -193,5 +190,3 @@ TEST(ThreadPool, Resize) {
 }
 
 }  // namespace ceres::internal
-
-#endif  // CERES_USE_CXX_THREADS
diff --git a/internal/ceres/thread_token_provider.cc b/internal/ceres/thread_token_provider.cc
index e48964c95..c9bb3fc00 100644
--- a/internal/ceres/thread_token_provider.cc
+++ b/internal/ceres/thread_token_provider.cc
@@ -30,42 +30,20 @@
 
 #include "ceres/thread_token_provider.h"
 
-#ifdef CERES_USE_OPENMP
-#include <omp.h>
-#endif
-
 namespace ceres::internal {
 
 ThreadTokenProvider::ThreadTokenProvider(int num_threads) {
-  (void)num_threads;
-#ifdef CERES_USE_CXX_THREADS
   for (int i = 0; i < num_threads; i++) {
     pool_.Push(i);
   }
-#endif
 }
 
 int ThreadTokenProvider::Acquire() {
-#ifdef CERES_USE_OPENMP
-  return omp_get_thread_num();
-#endif
-
-#ifdef CERES_NO_THREADS
-  return 0;
-#endif
-
-#ifdef CERES_USE_CXX_THREADS
   int thread_id;
   CHECK(pool_.Wait(&thread_id));
   return thread_id;
-#endif
 }
 
-void ThreadTokenProvider::Release(int thread_id) {
-  (void)thread_id;
-#ifdef CERES_USE_CXX_THREADS
-  pool_.Push(thread_id);
-#endif
-}
+void ThreadTokenProvider::Release(int thread_id) { pool_.Push(thread_id); }
 
 }  // namespace ceres::internal
diff --git a/internal/ceres/thread_token_provider.h b/internal/ceres/thread_token_provider.h
index d8640df2d..b14df0e92 100644
--- a/internal/ceres/thread_token_provider.h
+++ b/internal/ceres/thread_token_provider.h
@@ -31,13 +31,10 @@
 #ifndef CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_
 #define CERES_INTERNAL_THREAD_TOKEN_PROVIDER_H_
 
+#include "ceres/concurrent_queue.h"
 #include "ceres/internal/config.h"
 #include "ceres/internal/export.h"
 
-#ifdef CERES_USE_CXX_THREADS
-#include "ceres/concurrent_queue.h"
-#endif
-
 namespace ceres::internal {
 
 // Helper for C++ thread number identification that is similar to
@@ -47,12 +44,6 @@ namespace ceres::internal {
 // 0 to num_threads - 1 that can be acquired to identify the thread in a thread
 // pool.
 //
-// If CERES_NO_THREADS is defined, Acquire() always returns 0 and Release()
-// takes no action.
-//
-// If CERES_USE_OPENMP, omp_get_thread_num() is used to Acquire() with no action
-// in Release()
-//
 //
 // Example usage pseudocode:
 //
@@ -77,15 +68,12 @@ class CERES_NO_EXPORT ThreadTokenProvider {
   void Release(int thread_id);
 
  private:
-#ifdef CERES_USE_CXX_THREADS
   // This queue initially holds a sequence from 0..num_threads-1. Every
   // Acquire() call the first number is removed from here. When the token is not
   // needed anymore it shall be given back with corresponding Release()
   // call. This concurrent queue is more expensive than TBB's version, so you
   // should not acquire the thread ID on every for loop iteration.
   ConcurrentQueue<int> pool_;
-#endif
-
   ThreadTokenProvider(ThreadTokenProvider&) = delete;
   ThreadTokenProvider& operator=(ThreadTokenProvider&) = delete;
 };
diff --git a/internal/ceres/wall_time.cc b/internal/ceres/wall_time.cc
index c36092a8e..f57da318c 100644
--- a/internal/ceres/wall_time.cc
+++ b/internal/ceres/wall_time.cc
@@ -30,13 +30,9 @@
 
 #include "ceres/wall_time.h"
 
-#include "ceres/internal/config.h"
-
-#ifdef CERES_USE_OPENMP
-#include <omp.h>
-#else
 #include <ctime>
-#endif
+
+#include "ceres/internal/config.h"
 
 #ifdef _WIN32
 #include <windows.h>
@@ -47,9 +43,6 @@
 namespace ceres::internal {
 
 double WallTimeInSeconds() {
-#ifdef CERES_USE_OPENMP
-  return omp_get_wtime();
-#else
 #ifdef _WIN32
   LARGE_INTEGER count;
   LARGE_INTEGER frequency;
@@ -62,7 +55,6 @@ double WallTimeInSeconds() {
   gettimeofday(&time_val, nullptr);
   return (time_val.tv_sec + time_val.tv_usec * 1e-6);
 #endif
-#endif
 }
 
 EventLogger::EventLogger(const std::string& logger_name) {