From 1bee6e7df8ae835aa539c69bdbaad90be66160f1 Mon Sep 17 00:00:00 2001 From: Jason Rhinelander Date: Tue, 17 Jan 2017 02:13:11 -0500 Subject: [PATCH] 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. --- tests/CMakeLists.txt | 2 +- tools/pybind11Tools.cmake | 123 +++++++++++++++++++++++--------------- 2 files changed, 77 insertions(+), 48 deletions(-) diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt index acb67fca0..05d03b274 100644 --- a/tests/CMakeLists.txt +++ b/tests/CMakeLists.txt @@ -65,7 +65,7 @@ if(PYBIND11_TEST_FILES_EIGEN_I GREATER -1) 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} ${PYBIND11_HEADERS}) pybind11_enable_warnings(pybind11_tests) diff --git a/tools/pybind11Tools.cmake b/tools/pybind11Tools.cmake index 20935e867..dd3fadf4f 100644 --- a/tools/pybind11Tools.cmake +++ b/tools/pybind11Tools.cmake @@ -36,27 +36,63 @@ function(select_cxx_standard) endif() endfunction() -# Internal: find the appropriate LTO flag for this compiler -macro(_pybind11_find_lto_flag output_var prefer_thin_lto) - if(${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: ${cxxflags}") + return() endif() endmacro() +# Internal: find the appropriate link time optimization flags for this compiler +function(_pybind11_find_lto_flags cxxflags_out linkerflags_out prefer_thin_lto) + if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang") + set(cxx_append "") + set(linker_append "") + if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT APPLE) + # Clang Gold plugin does not support -Os; append -O3 to MinSizeRel builds to override it + set(linker_append "$<$: -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(${linkerflags_out} "" PARENT_SCOPE) + message(STATUS "LTO disabled (not supported by the compiler and/or linker)") +endfunction() + # Build a Python extension module: # pybind11_add_module( [MODULE | SHARED] [EXCLUDE_FROM_ALL] # [NO_EXTRAS] [THIN_LTO] source1 [source2 ...]) @@ -124,42 +160,35 @@ function(pybind11_add_module target_name) return() endif() - if(NOT MSVC) - # Enable link time optimization and set the default symbol - # visibility to hidden (very important to obtain small binaries) - string(TOUPPER "${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 "$<$>:${lto_cxx_flags}>") + target_link_libraries(${target_name} PRIVATE "$<$>:${lto_linker_flags}>") + endif() - # Default symbol visibility - target_compile_options(${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 $) - else() - add_custom_command(TARGET ${target_name} POST_BUILD - COMMAND ${CMAKE_STRIP} $) - 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 $) + else() + add_custom_command(TARGET ${target_name} POST_BUILD + COMMAND ${CMAKE_STRIP} $) endif() endif() - elseif(MSVC) + endif() + + if(MSVC) # /MP enables multithreaded builds (relevant when there are many files), /bigobj is # needed for bigger binding projects due to the limit to 64k addressable sections target_compile_options(${target_name} PRIVATE /MP /bigobj) - - # Enforce link time code generation on MSVC, except in debug mode - target_compile_options(${target_name} PRIVATE $<$>:/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()