Overhaul LTO flag detection
Clang on linux currently fails to run cmake:
$ CC=clang CXX=clang++ cmake ..
...
-- Configuring done
CMake Error at tools/pybind11Tools.cmake:135 (target_compile_options):
Error evaluating generator expression:
$<:-flto>
Expression did not evaluate to a known generator expression
Call Stack (most recent call first):
tests/CMakeLists.txt:68 (pybind11_add_module)
But investigating this led to various other -flto detection problems;
this commit thus overhauls LTO flag detection:
- -flto needs to be passed to the linker as well
- Also compile with -fno-fat-lto-objects under GCC
- Pass the equivalent flags to MSVC
- Enable LTO flags for via generator expressions (for non-debug builds
only), so that multi-config builds (like on Windows) still work
properly. This seems reasonable, however, even on single-config
builds (and simplifies the cmake code a bit).
- clang's lto linker plugins don't accept '-Os', so replace it with
'-O3' when doing a MINSIZEREL build
- Enable trying ThinLTO by default for test suite (only affects clang)
- Match Clang$ rather than ^Clang$ because, for cmake with 3.0+
policies in effect, the compiler ID will be AppleClang on macOS.
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index acb67fc..05d03b2 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -65,7 +65,7 @@
endif()
# Create the binding library
-pybind11_add_module(pybind11_tests pybind11_tests.cpp
+pybind11_add_module(pybind11_tests THIN_LTO pybind11_tests.cpp
${PYBIND11_TEST_FILES} ${PYBIND11_HEADERS})
pybind11_enable_warnings(pybind11_tests)
diff --git a/tools/pybind11Tools.cmake b/tools/pybind11Tools.cmake
index 20935e8..dd3fadf 100644
--- a/tools/pybind11Tools.cmake
+++ b/tools/pybind11Tools.cmake
@@ -36,27 +36,63 @@
endif()
endfunction()
-# Internal: find the appropriate LTO flag for this compiler
-macro(_pybind11_find_lto_flag output_var prefer_thin_lto)
- if(${prefer_thin_lto})
- # Check for ThinLTO support (Clang)
- check_cxx_compiler_flag("-flto=thin" HAS_THIN_LTO_FLAG)
- set(${output_var} $<${HAS_THIN_LTO_FLAG}:-flto=thin>)
- endif()
+# Internal: try compiling with the given c++ and linker flags; set success_out to 1 if it works
+function(_pybind11_check_cxx_and_linker_flags cxxflags linkerflags success_out)
+ set(CMAKE_REQUIRED_QUIET 1)
+ set(CMAKE_REQUIRED_LIBRARIES ${CMAKE_REQUIRED_LIBRARIES} ${linkerflags})
+ check_cxx_compiler_flag("${cxxflags}" has_lto)
+ set(${success_out} ${has_lto} PARENT_SCOPE)
+endfunction()
- if(NOT ${prefer_thin_lto} OR NOT HAS_THIN_LTO_FLAG)
- if(NOT CMAKE_CXX_COMPILER_ID MATCHES "Intel")
- # Check for Link Time Optimization support (GCC/Clang)
- check_cxx_compiler_flag("-flto" HAS_LTO_FLAG)
- set(${output_var} $<${HAS_LTO_FLAG}:-flto>)
- else()
- # Intel equivalent to LTO is called IPO
- check_cxx_compiler_flag("-ipo" HAS_IPO_FLAG)
- set(${output_var} $<${HAS_IPO_FLAG}:-ipo>)
- endif()
+# Wraps the above in a macro that sets the flags in the parent scope and returns if the check
+# succeeded (since this is a macro, not a function, the PARENT_SCOPE set and return is from the
+# caller's POV)
+macro(_pybind11_return_if_cxx_and_linker_flags_work feature cxxflags linkerflags cxx_out linker_out)
+ _pybind11_check_cxx_and_linker_flags(${cxxflags} ${linkerflags} check_success)
+ if(check_success)
+ set(${cxx_out} ${cxxflags} PARENT_SCOPE)
+ set(${linker_out} ${linkerflags} PARENT_SCOPE)
+ message(STATUS "Found ${feature} flags: ${cxxflags}")
+ return()
endif()
endmacro()
+# Internal: find the appropriate link time optimization flags for this compiler
+function(_pybind11_find_lto_flags cxxflags_out linkerflags_out prefer_thin_lto)
+ if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
+ set(cxx_append "")
+ set(linker_append "")
+ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT APPLE)
+ # Clang Gold plugin does not support -Os; append -O3 to MinSizeRel builds to override it
+ set(linker_append "$<$<CONFIG:MinSizeRel>: -O3>")
+ elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+ set(cxx_append " -fno-fat-lto-objects")
+ endif()
+
+ if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND ${prefer_thin_lto})
+ _pybind11_return_if_cxx_and_linker_flags_work("LTO"
+ "-flto=thin${cxx_append}" "-flto=thin${linker_append}" ${cxxflags_out} ${linkerflags_out})
+ endif()
+
+ _pybind11_return_if_cxx_and_linker_flags_work("LTO"
+ "-flto${cxx_append}" "-flto${linker_append}" ${cxxflags_out} ${linkerflags_out})
+ elseif (CMAKE_CXX_COMPILER_ID MATCHES "Intel")
+ # Intel equivalent to LTO is called IPO
+ _pybind11_return_if_cxx_and_linker_flags_work("LTO"
+ "-ipo" "-ipo" ${cxxflags_out} ${linkerflags_out})
+ elseif(MSVC)
+ # cmake only interprets libraries as linker flags when they start with a - (otherwise it
+ # converts /LTCG to \LTCG as if it was a Windows path). Luckily MSVC supports passing flags
+ # with - instead of /, even if it is a bit non-standard:
+ _pybind11_return_if_cxx_and_linker_flags_work("LTO"
+ "/GL" "-LTCG" ${cxxflags_out} ${linkerflags_out})
+ endif()
+
+ set(${cxxflags_out} "" PARENT_SCOPE)
+ set(${linkerflags_out} "" PARENT_SCOPE)
+ message(STATUS "LTO disabled (not supported by the compiler and/or linker)")
+endfunction()
+
# Build a Python extension module:
# pybind11_add_module(<name> [MODULE | SHARED] [EXCLUDE_FROM_ALL]
# [NO_EXTRAS] [THIN_LTO] source1 [source2 ...])
@@ -124,42 +160,35 @@
return()
endif()
- if(NOT MSVC)
- # Enable link time optimization and set the default symbol
- # visibility to hidden (very important to obtain small binaries)
- string(TOUPPER "${CMAKE_BUILD_TYPE}" U_CMAKE_BUILD_TYPE)
- if (NOT ${U_CMAKE_BUILD_TYPE} MATCHES DEBUG)
- # Link Time Optimization
- if(NOT CYGWIN)
- _pybind11_find_lto_flag(lto_flag ARG_THIN_LTO)
- target_compile_options(${target_name} PRIVATE ${lto_flag})
- endif()
+ # Find LTO flags
+ _pybind11_find_lto_flags(lto_cxx_flags lto_linker_flags ARG_THIN_LTO)
+ if (lto_cxx_flags OR lto_linker_flags)
+ # Enable LTO flags if found, except for Debug builds
+ separate_arguments(lto_cxx_flags)
+ separate_arguments(lto_linker_flags)
+ target_compile_options(${target_name} PRIVATE "$<$<NOT:$<CONFIG:Debug>>:${lto_cxx_flags}>")
+ target_link_libraries(${target_name} PRIVATE "$<$<NOT:$<CONFIG:Debug>>:${lto_linker_flags}>")
+ endif()
- # Default symbol visibility
- target_compile_options(${target_name} PRIVATE "-fvisibility=hidden")
+ # Set the default symbol visibility to hidden (very important to obtain small binaries)
+ if (NOT MSVC)
+ target_compile_options(${target_name} PRIVATE "-fvisibility=hidden")
- # Strip unnecessary sections of the binary on Linux/Mac OS
- if(CMAKE_STRIP)
- if(APPLE)
- add_custom_command(TARGET ${target_name} POST_BUILD
- COMMAND ${CMAKE_STRIP} -x $<TARGET_FILE:${target_name}>)
- else()
- add_custom_command(TARGET ${target_name} POST_BUILD
- COMMAND ${CMAKE_STRIP} $<TARGET_FILE:${target_name}>)
- endif()
+ # Strip unnecessary sections of the binary on Linux/Mac OS
+ if(CMAKE_STRIP)
+ if(APPLE)
+ add_custom_command(TARGET ${target_name} POST_BUILD
+ COMMAND ${CMAKE_STRIP} -x $<TARGET_FILE:${target_name}>)
+ else()
+ add_custom_command(TARGET ${target_name} POST_BUILD
+ COMMAND ${CMAKE_STRIP} $<TARGET_FILE:${target_name}>)
endif()
endif()
- elseif(MSVC)
+ endif()
+
+ if(MSVC)
# /MP enables multithreaded builds (relevant when there are many files), /bigobj is
# needed for bigger binding projects due to the limit to 64k addressable sections
target_compile_options(${target_name} PRIVATE /MP /bigobj)
-
- # Enforce link time code generation on MSVC, except in debug mode
- target_compile_options(${target_name} PRIVATE $<$<NOT:$<CONFIG:Debug>>:/GL>)
-
- # Fancy generator expressions don't work with linker flags, for reasons unknown
- set_property(TARGET ${target_name} APPEND_STRING PROPERTY LINK_FLAGS_RELEASE /LTCG)
- set_property(TARGET ${target_name} APPEND_STRING PROPERTY LINK_FLAGS_MINSIZEREL /LTCG)
- set_property(TARGET ${target_name} APPEND_STRING PROPERTY LINK_FLAGS_RELWITHDEBINFO /LTCG)
endif()
endfunction()