[Bug c/110645] New: False positive -Warray-bounds warning

["andres at anarazel dot de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110645

            Bug ID: 110645
           Summary: False positive -Warray-bounds warning
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andres at anarazel dot de
  Target Milestone: ---

Created attachment 55531
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=55531&action=edit
reproducer

Hi,

The attached minimized reproducer (from postgres code) triggers a clearly bogus
-Warray-bounds warning, even with -Warray-bounds=1.

$ gcc -O1 -Wall -Warray-bounds=1 -c -o /dev/null -c repro.i 
In function ‘formTextDatum’,
    inlined from ‘spgist_name_compress’ at /tmp/cvise2/repro.i:44:9:
/tmp/cvise2/repro.i:35:17: warning: ‘memcpy’ forming offset [2, 254] is out of
the bounds [0, 2] [-Warray-bounds]
   35 |                 memcpy(att->data, src, srclen);
      |                 ^~~~~~~~~~~~

This is despite srclen being strlen(src), which gcc does know is <= 1.

It seems that the known range of srclen is "widened" due to the if (srclen <
0xff), causing the warning.

It is somewhat interesting that removing -Wall removes the spurious warning,
despite -Warray-bounds=1.


This triggers for gcc-11 - HEAD at 3b007164b3e, but not in gcc-10.


I looked through quite a few bugs about spurious -Warray-bounds warnings to see
if this is a duplicate - quite possible, but I didn't find a real match for
this.

Regards,

Andres