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Re: [PATCH] Improve math_errhandling

Joseph Myers wrote:
> The header definitely needs to check for __FAST_MATH__.  There is a case 
> for defining math_errhandling to 0 in the __FAST_MATH__ case (a 
> nonconforming value, but -ffast-math is a nonconforming mode, and 0 
> accurately reflects the situation that neither errno nor exceptions can be 
> relied upon in that case) rather than the existing state of being 
> undefined (also nonconforming).  Note that GCC versions before GCC 5 don't 
> define __NO_MATH_ERRNO__ at all.

OK, setting to 0 for __FAST_MATH__ seems reasonable. Here is the revised patch:

Currently math_errhandling is always set to MATH_ERRNO | MATH_ERREXCEPT
even if -fno-math-errno is used.  It is not defined at all when fast-math
is used.  Set it to 0 with fast-math - this is noncomforming but more
useful than not define math_errhandling at all. Also take __NO_MATH_ERRNO__
into account and update comment.

2017-12-21  Wilco Dijkstra  <>

	* math/math.h (math_errhandling): Add __NO_MATH_ERRNO__ check.

diff --git a/math/math.h b/math/math.h
index 72d7dc59391af71c4db9634998463b4f947b9f09..9bf0c66e862d15277f8e4f31f3b35b9d4a3ad958 100644
--- a/math/math.h
+++ b/math/math.h
@@ -665,10 +665,16 @@ enum
 # define MATH_ERRNO	1	/* errno set by math functions.  */
 # define MATH_ERREXCEPT	2	/* Exceptions raised by math functions.  */
-/* By default all functions support both errno and exception handling.
-   In gcc's fast math mode and if inline functions are defined this
-   might not be true.  */
-# ifndef __FAST_MATH__
+/* By default all math functions support both errno and exception handling
+   (except for soft floating point implementations which may only support
+   errno handling).  If errno handling is disabled, exceptions are still
+   supported by GLIBC.  Set math_errhandling to 0 with -ffast-math (this is
+   nonconforming but it is more useful than leaving it undefined).  */
+# ifdef __FAST_MATH__
+#  define math_errhandling	0
+# elif defined __NO_MATH_ERRNO__
+#  define math_errhandling	(MATH_ERREXCEPT)
+# else
 #  define math_errhandling	(MATH_ERRNO | MATH_ERREXCEPT)
 # endif

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