[PATCH v3] Avoid undefined behaviour in ibm128 implementation of llroundl (BZ #29488)

[Aurelien Jarno <aurelien@aurel32.net>]

Detecting an overflow edge case depended on signed overflow of a long
long. Replace the additions and the overflow checks by
__builtin_add_overflow().
---
 sysdeps/ieee754/ldbl-128ibm/s_llroundl.c | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

This patch is based on the original patch from Michael Hudson-Doyle,
using __builtin_add_overflow() as suggested by Florian Weimer

It passes all tests on ppc64el with gcc 12 with both -O2 and -O3.

diff --git a/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c b/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
index d85154e73a..d8c0de1faf 100644
--- a/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
+++ b/sysdeps/ieee754/ldbl-128ibm/s_llroundl.c
@@ -66,38 +66,35 @@ __llroundl (long double x)
       /* Peg at max/min values, assuming that the above conversions do so.
          Strictly speaking, we can return anything for values that overflow,
          but this is more useful.  */
-      res = hi + lo;
-
-      /* This is just sign(hi) == sign(lo) && sign(res) != sign(hi).  */
-      if (__glibc_unlikely (((~(hi ^ lo) & (res ^ hi)) < 0)))
+      if (__glibc_unlikely (__builtin_add_overflow (hi, lo, &res)))
 	goto overflow;
 
       xh -= lo;
       ldbl_canonicalize (&xh, &xl);
 
-      hi = res;
       if (xh > 0.5)
 	{
-	  res += 1;
+	  if (__glibc_unlikely (__builtin_add_overflow (res, 1, &res)))
+	    goto overflow;
 	}
       else if (xh == 0.5)
 	{
 	  if (xl > 0.0 || (xl == 0.0 && res >= 0))
-	    res += 1;
+	    if (__glibc_unlikely (__builtin_add_overflow (res, 1, &res)))
+	      goto overflow;
 	}
       else if (-xh > 0.5)
 	{
-	  res -= 1;
+	  if (__glibc_unlikely (__builtin_add_overflow (res, -1, &res)))
+	    goto overflow;
 	}
       else if (-xh == 0.5)
 	{
 	  if (xl < 0.0 || (xl == 0.0 && res <= 0))
-	    res -= 1;
+	    if (__glibc_unlikely (__builtin_add_overflow (res, -1, &res)))
+	      goto overflow;
 	}
 
-      if (__glibc_unlikely (((~(hi ^ (res - hi)) & (res ^ hi)) < 0)))
-	goto overflow;
-
       return res;
     }
   else
-- 
2.35.1