[Bug c/109979] New: [12 Regression] -Wformat-overflow false positive for %d and non-basic expression

[vincent-gcc at vinc17 dot net via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109979

            Bug ID: 109979
           Summary: [12 Regression] -Wformat-overflow false positive for
                    %d and non-basic expression
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

Consider

#include <stdio.h>

void f (int *);

void g (void)
{
  int e;
  char s[4];

  f (&e);
  sprintf (s, "%d", e);
  sprintf (s, "%d", e - 1);
}

I get on my Linux/x86_64 machine with gcc-12 (Debian 12.2.0-14) 12.2.0:

zira:~> gcc-12 -Wformat-overflow -c tst.c
tst.c: In function ‘g’:
tst.c:12:16: warning: ‘%d’ directive writing between 1 and 11 bytes into a
region of size 4 [-Wformat-overflow=]
   12 |   sprintf (s, "%d", e - 1);
      |                ^~
tst.c:12:15: note: directive argument in the range [-2147483648, 2147483646]
   12 |   sprintf (s, "%d", e - 1);
      |               ^~~~
tst.c:12:3: note: ‘sprintf’ output between 2 and 12 bytes into a destination of
size 4
   12 |   sprintf (s, "%d", e - 1);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~

Note that the warning occurs for "e - 1" but not for "e".

This bug was found when compiling GNU MPFR 4.2.0 with "-std=c90 -Werror=format
-m32" (compilation failure for get_d64.c).