[PATCH] math: Fix asin and acos invalid exception with old gcc

[Wilco Dijkstra]

Hi Szabolcs,

> This works around a gcc issue where it const folded inf/inf into nan,
> preventing the invalid exception to be signalled.
>
> (x-x)/(x-x) is more robust against optimizations and works for all
> out of bounds values including x==nan.
>
> The gcc issue https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95115
> should be fixed on release branches starting from gcc-10, but it is
> better to change the code in case glibc is built with older gcc.

LGTM.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

Cheers,
Wilco


---
 sysdeps/ieee754/dbl-64/e_asin.c | 18 ++----------------
 1 file changed, 2 insertions(+), 16 deletions(-)

diff --git a/sysdeps/ieee754/dbl-64/e_asin.c b/sysdeps/ieee754/dbl-64/e_asin.c
index e7ea0cbe8f..6b7c971e76 100644
--- a/sysdeps/ieee754/dbl-64/e_asin.c
+++ b/sysdeps/ieee754/dbl-64/e_asin.c
@@ -165,14 +165,7 @@ __ieee754_asin(double x){
   /*---------------------------- |x|>=1 -------------------------------*/
   else if (k==0x3ff00000 && u.i[LOW_HALF]==0) return (m>0)?hp0.x:-hp0.x;
   else
-  if (k>0x7ff00000 || (k == 0x7ff00000 && u.i[LOW_HALF] != 0)) return x + x;
-  else {
-    u.i[HIGH_HALF]=0x7ff00000;
-    v.i[HIGH_HALF]=0x7ff00000;
-    u.i[LOW_HALF]=0;
-    v.i[LOW_HALF]=0;
-    return u.x/v.x;  /* NaN */
- }
+    return (x - x) / (x - x);
 }
 #ifndef __ieee754_asin
 libm_alias_finite (__ieee754_asin, __asin)
@@ -334,14 +327,7 @@ __ieee754_acos(double x)
   else
   if (k==0x3ff00000 && u.i[LOW_HALF]==0) return (m>0)?0:2.0*hp0.x;
   else
-  if (k>0x7ff00000 || (k == 0x7ff00000 && u.i[LOW_HALF] != 0)) return x + x;
-  else {
-    u.i[HIGH_HALF]=0x7ff00000;
-    v.i[HIGH_HALF]=0x7ff00000;
-    u.i[LOW_HALF]=0;
-    v.i[LOW_HALF]=0;
-    return u.x/v.x;
-  }
+    return (x - x) / (x - x);
 }
 #ifndef __ieee754_acos
 libm_alias_finite (__ieee754_acos, __acos)