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[flang-rt] Use --as-needed for linking flang-rt libraries. #130856

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Merged
merged 2 commits into from
Mar 14, 2025
Merged

[flang-rt] Use --as-needed for linking flang-rt libraries. #130856

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76c2d56
[flang-rt] Use --as-needed for linking flang-rt libraries.
vzakhari Mar 11, 2025
8e21a00
Addressed review comments.
vzakhari Mar 12, 2025
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6 changes: 6 additions & 0 deletions flang-rt/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -245,6 +245,12 @@ check_cxx_compiler_flag("-UTESTFLAG" FLANG_RT_SUPPORTS_UNDEFINE_FLAG)
# Check whether -fno-lto is supported.
check_cxx_compiler_flag(-fno-lto FLANG_RT_HAS_FNO_LTO_FLAG)

# Check whether -Wl,--as-needed is supported.
check_linker_flag(C "LINKER:--as-needed" LINKER_SUPPORTS_AS_NEEDED)
if (LINKER_SUPPORTS_AS_NEEDED)
set(LINKER_AS_NEEDED_OPT "LINKER:--as-needed")
endif()

# Different platform may have different name for the POSIX thread library.
# For example, libpthread.a on AIX. Search for it as it is needed when
# building the shared flang_rt.runtime.so.
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14 changes: 14 additions & 0 deletions flang-rt/cmake/modules/AddFlangRT.cmake
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Expand Up @@ -145,6 +145,20 @@ function (add_flangrt_library name)
if (Threads_FOUND)
target_link_libraries(${name_shared} PUBLIC Threads::Threads)
endif ()

# Special dependencies handling for shared libraries only:
#
# flang-rt libraries must not depend on libc++/libstdc++,
# so set the linker language to C to avoid the unnecessary
# library dependence. Note that libc++/libstdc++ may still
# come through CMAKE_CXX_IMPLICIT_LINK_LIBRARIES.
set_target_properties(${name_shared} PROPERTIES LINKER_LANGUAGE C)
# Use --as-needed to avoid unnecessary dependencies.
if (LINKER_AS_NEEDED_OPT)
target_link_options(${name_shared} BEFORE PRIVATE
"${LINKER_AS_NEEDED_OPT}"
)
endif()
endif ()

if (libtargets)
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