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[OpenMP] Disable LTO build of libomptarget and plugins by default. #79387

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merged 1 commit into from
Jan 26, 2024
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[OpenMP] Disable LTO build of libomptarget and plugins by default. #79387

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25 changes: 15 additions & 10 deletions openmp/libomptarget/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -81,17 +81,22 @@ if(NOT LLVM_ENABLE_RTTI)
set(offload_compile_flags ${offload_compile_flags} -fno-rtti)
endif()

# If LTO is not explicitly disabled we check if we can enable it and do so.
set(LIBOMPTARGET_USE_LTO TRUE CACHE BOOL "Use LTO for the offload runtimes if available")
# TODO: Consider enabling LTO by default if supported.
# https://cmake.org/cmake/help/latest/module/CheckIPOSupported.html can be used
# to test for working LTO. However, before CMake 3.24 this will test the
# default linker and ignore options such as LLVM_ENABLE_LLD. As a result, CMake
# would test whether LTO works with the default linker but build with another one.
# In a typical scenario, libomptarget is compiled with the in-tree Clang, but
# linked with ld.gold, which requires the LLVMgold plugin, when it actually
# would work with the lld linker (or also fail because the system lld is too old
# to understand opaque pointers). Using gcc as the compiler would pass the test, but fail
# when linking with lld since does not understand gcc's LTO format.
set(LIBOMPTARGET_USE_LTO FALSE CACHE BOOL "Use LTO for the offload runtimes if available")
if (LIBOMPTARGET_USE_LTO)
include(CheckIPOSupported)
check_ipo_supported(RESULT use_lto OUTPUT output)
if(use_lto)
set(offload_compile_flags ${offload_compile_flags} -flto)
set(offload_link_flags ${offload_link_flags} -flto)
else()
message(WARNING "LTO is not supported: ${output}")
endif()
# CMake sets CMAKE_CXX_COMPILE_OPTIONS_IPO depending on the compiler and is
# also what CheckIPOSupported uses to test support.
list(APPEND offload_compile_flags ${CMAKE_CXX_COMPILE_OPTIONS_IPO})
list(APPEND offload_link_flags ${CMAKE_CXX_COMPILE_OPTIONS_IPO})
endif()

# OMPT support for libomptarget
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