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[libc] Use -fno-math-errno to for __builtin_fma* to generate fma instructions. #124200

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Merged
merged 1 commit into from
Jan 24, 2025
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[libc] Use -fno-math-errno to for __builtin_fma* to generate fma instructions. #124200

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7 changes: 7 additions & 0 deletions libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,13 @@ function(_get_compile_options_from_flags output_var)
elseif(LIBC_TARGET_ARCHITECTURE_IS_RISCV64)
list(APPEND compile_options "-D__LIBC_RISCV_USE_FMA")
endif()
# For clang, we will build the math functions with `-fno-math-errno` so that
# __builtin_fma* will generate the fused-mutliply-add instructions. We
# don't put the control flag to the public config yet, and see if it makes
# sense to just enable this flag by default.
if(LIBC_ADD_FNO_MATH_ERRNO)
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@petrhosek petrhosek Jan 24, 2025

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Shouldn't this be LIBC_MATH_NO_ERRNO? Where is LIBC_ADD_FNO_MATH_ERRNO being set?

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@lntue lntue Jan 24, 2025

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I was planning to use the flag LIBC_MATH_NO_ERRNO differently, in the sense that our math functions entry points won't set or change errno, regardless of what math_errhandling is set to. And another quite likely possibility that we can just add -fno-math-errno flag by default. So right now I don't add it to our config generator config.json yet. Maybe I should add it to config.json?

list(APPEND compile_options "-fno-math-errno")
endif()
endif()
if(ADD_ROUND_OPT_FLAG)
if(LIBC_TARGET_ARCHITECTURE_IS_X86_64)
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5 changes: 5 additions & 0 deletions libc/cmake/modules/LLVMLibCTestRules.cmake
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Original file line number Diff line number Diff line change
@@ -1,6 +1,11 @@
function(_get_common_test_compile_options output_var c_test flags)
_get_compile_options_from_flags(compile_flags ${flags})

# Remove -fno-math-errno if it was added.
if(LIBC_ADD_FNO_MATH_ERRNO)
list(REMOVE_ITEM compile_options "-fno-math-errno")
endif()

set(compile_options
${LIBC_COMPILE_OPTIONS_DEFAULT}
${LIBC_TEST_COMPILE_OPTIONS_DEFAULT}
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