[Bug c/94337] New: Incorrect "dereferencing type-punned pointer will break strict-aliasing rules" warning

[Bug c/94337] New: Incorrect "dereferencing type-punned pointer will break strict-aliasing rules" warning

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94337

            Bug ID: 94337
           Summary: Incorrect "dereferencing type-punned pointer will
                    break strict-aliasing rules" warning
           Product: gcc
           Version: 10.0
            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 the following example.

#include <stdio.h>

struct s1
{
  int a;
};

struct s2
{
  int a, b;
};

int main (void)
{
  union {
    struct s1 m1[1];
    struct s2 m2[1];
  } u;

  (u.m2)->b = 17;
  printf ("%d\n", ((struct s2 *) (struct s1 *) u.m2)->b);
  printf ("%d\n", ((struct s2 *) u.m1)->b);
  return 0;
}

zira:~> gcc-10 tst.c -o tst -O2 -Wstrict-aliasing
tst.c: In function ‘main’:
tst.c:22:20: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
   22 |   printf ("%d\n", ((struct s2 *) u.m1)->b);
      |                   ~^~~~~~~~~~~~~~~~~~~

But there is no type-punning here. All accesses are done via struct s2.
Everything else is pointer conversions, which are not related to the aliasing
rules.

[Bug c/94337] Incorrect "dereferencing type-punned pointer will break strict-aliasing rules" warning

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94337

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The warning only looks at the single expression it quotes which isn't really
enough to discover you are doing right.  It tries to be helpful - if you know
better then disable the warning.

[Bug c/94337] Incorrect "dereferencing type-punned pointer will break strict-aliasing rules" warning

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94337

--- Comment #2 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
Why not having a level with no false positives? This would avoid to disable the
warning globally.

IMHO, using it when a union is involved is likely to generate false positives.