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enable building with LTO on Windows using clang-cl and lld

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Summary: With clang-cl gaining support for link-time optimization, we can now enable builds using LTO when using clang-cl and lld on Windows. To do this, we must not pass the -flto flag to the linker; lld-link does not understand it, but will perform LTO automatically when it encounters bitcode files. We also don't pass /Brepro when using LTO - the compiler doesn't generate object files for LTO, so passing the flag would only result in a warning about it being unused.

Reviewers: rnk, ruiu, hans

Reviewed By: hans

Subscribers: mgorny, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D30240

llvm-svn: 296658
This commit is contained in:
Bob Haarman 2017-03-01 19:22:18 +00:00
parent 7f74b62b4c
commit a41054c917
1 changed files with 27 additions and 10 deletions
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37
cmake/modules/HandleLLVMOptions.cmake
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@ -12,6 +12,12 @@ include(AddLLVMDefinitions)
include(CheckCCompilerFlag)
include(CheckCXXCompilerFlag)
if(CMAKE_LINKER MATCHES "lld-link.exe" OR (WIN32 AND LLVM_USE_LINKER STREQUAL "lld"))
set(LINKER_IS_LLD_LINK TRUE)
else()
set(LINKER_IS_LLD_LINK FALSE)
endif()
# Ninja Job Pool support
# The following only works with the Ninja generator in CMake >= 3.0.
set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
@ -37,7 +43,7 @@ if(LLVM_PARALLEL_LINK_JOBS)
endif()
if (CMAKE_LINKER MATCHES "lld-link.exe")
if (LINKER_IS_LLD_LINK)
# Pass /MANIFEST:NO so that CMake doesn't run mt.exe on our binaries. Adding
# manifests with mt.exe breaks LLD's symbol tables and takes as much time as
# the link. See PR24476.
@ -407,11 +413,13 @@ if( MSVC )
# "Enforce type conversion rules".
append("/Zc:rvalueCast" CMAKE_CXX_FLAGS)
if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
if (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT LLVM_ENABLE_LTO)
# clang-cl and cl by default produce non-deterministic binaries because
# link.exe /incremental requires a timestamp in the .obj file. clang-cl
# has the flag /Brepro to force deterministic binaries. We want to pass that
# whenever you're building with clang unless you're passing /incremental.
# whenever you're building with clang unless you're passing /incremental
# or using LTO (/Brepro with LTO would result in a warning about the flag
# being unused, because we're not generating object files).
# This checks CMAKE_CXX_COMPILER_ID in addition to check_cxx_compiler_flag()
# because cl.exe does not emit an error on flags it doesn't understand,
# letting check_cxx_compiler_flag() claim it understands all flags.
@ -561,7 +569,7 @@ macro(append_common_sanitizer_flags)
elseif (CLANG_CL)
# Keep frame pointers around.
append("/Oy-" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
if (CMAKE_LINKER MATCHES "lld-link.exe")
if (LINKER_IS_LLD_LINK)
# Use DWARF debug info with LLD.
append("-gdwarf" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
else()
@ -699,20 +707,29 @@ append_if(LLVM_BUILD_INSTRUMENTED_COVERAGE "-fprofile-instr-generate='${LLVM_PRO
set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
if(LLVM_ENABLE_LTO AND LLVM_ON_WIN32 AND NOT LINKER_IS_LLD_LINK)
message(FATAL_ERROR "When compiling for Windows, LLVM_ENABLE_LTO requires using lld as the linker (point CMAKE_LINKER at lld-link.exe)")
endif()
if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
if(NOT LINKER_IS_LLD_LINK)
append("-flto=thin" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
endif()
# On darwin, enable the lto cache. This improves initial build time a little
# since we re-link a lot of the same objects, and significantly improves
# incremental build time.
append_if(APPLE "-Wl,-cache_path_lto,${PROJECT_BINARY_DIR}/lto.cache"
CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
elseif(uppercase_LLVM_ENABLE_LTO STREQUAL "FULL")
append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
if(NOT LINKER_IS_LLD_LINK)
append("-flto=full" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
endif()
elseif(LLVM_ENABLE_LTO)
append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
if(NOT LINKER_IS_LLD_LINK)
append("-flto" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
endif()
endif()
# This option makes utils/extract_symbols.py be used to determine the list of