[Bug c/89699] New: [8 regression] false warning -Wstringop-overflow and memcmp

[chantry.xavier at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89699

            Bug ID: 89699
           Summary: [8 regression] false warning -Wstringop-overflow and
                    memcmp
           Product: gcc
           Version: 8.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: chantry.xavier at gmail dot com
  Target Milestone: ---

Created attachment 45958
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=45958&action=edit
reduced test case

I am using gcc 8.3.1 20190228

I got a strange warning on a memcmp call, it looks like gcc is tricked to
believe the length is negative and that it overflows, or something like that ?
Not sure.

With "gcc -O -c memcmp-short.c" :

memcmp-short.c: In function ‘f’:
memcmp-short.c:9:12: warning: ‘memcmp’ specified size between
18446744071562067968 and 18446744073709551615 exceeds maximum object size
9223372036854775807 [-Wstringop-overflow=]
     return memcmp(s1, s2, min);
            ^~~~~~~~~~~~~~~~~~~

This is only triggered with optimizations (-O).
And only with gcc >= 8 according to godbolt.

I think I will use a workaround similar to the one suggested in 89689 :
if (min < 0) { __builtin_unreachable(); }

Cheers !