[libc] Make LIBC_MATH_NO_ERRNO imply -fno-math-errno

This partially reverts llvm#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.

Author: petrhosek

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")

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}

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)"
     }
   }
 }