[libc-commits] [libc] [libc] Make LIBC_MATH_NO_ERRNO imply `-fno-math-errno` (PR #125794)

[Petr Hosek via libc-commits]

https://github.com/petrhosek updated https://github.com/llvm/llvm-project/pull/125794

>From 6d9dac2d228dff80cb413ae8dd2b6508f8d286ae Mon Sep 17 00:00:00 2001
From: Petr Hosek <phosek at google.com>
Date: Wed, 5 Feb 2025 01:52:10 +0000
Subject: [PATCH 1/2] [libc] Make LIBC_MATH_NO_ERRNO imply `-fno-math-errno`

This partially reverts #124200. Rather than using a CMake option to
control whether to enable `-fno-math-errno`, use LIBC_MATH_NO_ERRNO
configuration option. While there might be other cases when we want
to set `-fno-math-errno`, having LIBC_MATH_NO_ERRNO imply it should
be always safe and represents a reasonable starting point.
---
 libc/cmake/modules/LLVMLibCCompileOptionRules.cmake | 10 +++-------
 libc/cmake/modules/LLVMLibCTestRules.cmake          |  5 -----
 libc/config/baremetal/config.json                   |  2 +-
 3 files changed, 4 insertions(+), 13 deletions(-)

diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index 0facb0b9be0c134..f5f638bef5c1d8e 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -28,13 +28,6 @@ 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)
-        list(APPEND compile_options "-fno-math-errno")
-      endif()
     endif()
     if(ADD_ROUND_OPT_FLAG)
       if(LIBC_TARGET_ARCHITECTURE_IS_X86_64)
@@ -71,6 +64,9 @@ function(_get_compile_options_from_flags output_var)
              "SHELL:-Xclang -target-feature -Xclang +fullfp16")
       endif()
     endif()
+    if(LIBC_CONF_MATH_OPTIMIZATIONS MATCHES "LIBC_MATH_NO_ERRNO")
+      list(APPEND compile_options "-fno-math-errno")
+    endif()
   elseif(MSVC)
     if(ADD_FMA_FLAG)
       list(APPEND compile_options "/arch:AVX2")
diff --git a/libc/cmake/modules/LLVMLibCTestRules.cmake b/libc/cmake/modules/LLVMLibCTestRules.cmake
index 10bb9c9487d6367..96fa6c3a707e47a 100644
--- a/libc/cmake/modules/LLVMLibCTestRules.cmake
+++ b/libc/cmake/modules/LLVMLibCTestRules.cmake
@@ -1,11 +1,6 @@
 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}
diff --git a/libc/config/baremetal/config.json b/libc/config/baremetal/config.json
index 08c581d1c682268..d29fb64e370ddbb 100644
--- a/libc/config/baremetal/config.json
+++ b/libc/config/baremetal/config.json
@@ -28,7 +28,7 @@
   },
   "math": {
     "LIBC_CONF_MATH_OPTIMIZATIONS": {
-      "value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES)"
+      "value": "(LIBC_MATH_SKIP_ACCURATE_PASS | LIBC_MATH_SMALL_TABLES | LIBC_MATH_NO_ERRNO)"
     }
   }
 }

>From 92a0fd5c869aff07407e734213824ea9f097ae4c Mon Sep 17 00:00:00 2001
From: Petr Hosek <phosek at google.com>
Date: Wed, 5 Feb 2025 08:01:06 +0000
Subject: [PATCH 2/2] Move the check

---
 libc/cmake/modules/LLVMLibCCompileOptionRules.cmake | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index f5f638bef5c1d8e..1ab5dbeb5cae064 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -64,9 +64,6 @@ function(_get_compile_options_from_flags output_var)
              "SHELL:-Xclang -target-feature -Xclang +fullfp16")
       endif()
     endif()
-    if(LIBC_CONF_MATH_OPTIMIZATIONS MATCHES "LIBC_MATH_NO_ERRNO")
-      list(APPEND compile_options "-fno-math-errno")
-    endif()
   elseif(MSVC)
     if(ADD_FMA_FLAG)
       list(APPEND compile_options "/arch:AVX2")
@@ -100,6 +97,9 @@ function(_get_compile_options_from_config output_var)
 
   if(LIBC_CONF_MATH_OPTIMIZATIONS)
     list(APPEND config_options "-DLIBC_MATH=${LIBC_CONF_MATH_OPTIMIZATIONS}")
+    if(LIBC_CONF_MATH_OPTIMIZATIONS MATCHES "LIBC_MATH_NO_ERRNO")
+      list(APPEND config_options "-fno-math-errno")
+    endif()
   endif()
 
   set(${output_var} ${config_options} PARENT_SCOPE)