[Bug c++/106074] New: Spurious Wstringop-overflow for int-to-string with SSE4

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106074

            Bug ID: 106074
           Summary: Spurious Wstringop-overflow for int-to-string with
                    SSE4
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Code adapted from
https://github.com/capnproto/capnproto/blob/be7a80b4add706ddaeb41689221146b86c3e0f5f/c%2B%2B/src/kj/string.c%2B%2B#L157-L203
:

auto f(short i) {
  struct R { char data[6]; } result;
  bool negative = i < 0;
  unsigned u = i;
  if (negative)
    u = -u;
  unsigned char reverse[5];
  unsigned char* p = reverse;
  if (u == 0)
    *p++ = 0;
  else
    for (; u > 0; u /= 10)
      *p++ = u % 10;
  char* p2 = result.data;
  if (negative)
    *p2++ = '-';
  while (p > reverse) {
#ifdef ASSUME
    if (p2 >= (&result.data)[1]) __builtin_unreachable();
#endif
    *p2++ = '0' + *--p;
  }
  if (p2 < (&result.data)[1])
    *p2 = '\0';
  return result;
}

When compiled with -O3 -msse4, from 12.1.0 through 13.0.0 20220619:

<source>: In function 'auto f(short int)':
<source>:21:11: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
   21 |     *p2++ = '0' + *--p;
      |     ~~~~~~^~~~~~~~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
    2 |   struct R { char data[6]; } result;
      |                   ^~~~
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
    2 |   struct R { char data[6]; } result;
      |                              ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
    2 |   struct R { char data[6]; } result;
      |                   ^~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
    2 |   struct R { char data[6]; } result;
      |                              ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
    2 |   struct R { char data[6]; } result;
      |                   ^~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
    2 |   struct R { char data[6]; } result;
      |                              ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
    2 |   struct R { char data[6]; } result;
      |                   ^~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
    2 |   struct R { char data[6]; } result;
      |                              ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
    2 |   struct R { char data[6]; } result;
      |                   ^~~~

Adding the assumption at line 19 (-DASSUME) works around this.