[Bug tree-optimization/104341] New: Bogus -Werror=array-bounds since r12-2582-gb9cbf8c9e0bc72f5

[Bug tree-optimization/104341] New: Bogus -Werror=array-bounds since r12-2582-gb9cbf8c9e0bc72f5

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104341

            Bug ID: 104341
           Summary: Bogus -Werror=array-bounds since
                    r12-2582-gb9cbf8c9e0bc72f5
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: marxin at gcc dot gnu.org
                CC: msebor at gcc dot gnu.org
  Target Milestone: ---

Reduced from qemu package:

$ cat acpi.i
int memcpy_src_u_0_0, acpi_sx_zsdt_acpi;
char acpi_sx_zsdt_buf_0_0;
static void *memcpy2(void *dest, void *, int) {
  void *__trans_tmp_2 = dest;
  union {
    int u32[2];
  } *dest_u = __trans_tmp_2;
  dest_u->u32[0] = memcpy_src_u_0_0;
  return 0;
}
void copy_from_user(void *dest) {
  long __trans_tmp_4 = (long)dest, __trans_tmp_3 = __trans_tmp_4;
  memcpy2((void *)__trans_tmp_3, 0, 0);
}
int acpi_sx_zsdt() {
  copy_from_user(&acpi_sx_zsdt_acpi);
  return acpi_sx_zsdt_buf_0_0;
}

$ gcc acpi.i -c  -Werror=array-bounds -O2
In function ‘memcpy2’,
    inlined from ‘copy_from_user’ at acpi.i:13:3,
    inlined from ‘acpi_sx_zsdt’ at acpi.i:16:3:
acpi.i:8:18: error: array subscript ‘union <anonymous>[0]’ is partly outside
array bounds of ‘int[1]’ [-Werror=array-bounds]
    8 |   dest_u->u32[0] = memcpy_src_u_0_0;
      |   ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
acpi.i: In function ‘acpi_sx_zsdt’:
acpi.i:1:23: note: object ‘acpi_sx_zsdt_acpi’ of size 4
    1 | int memcpy_src_u_0_0, acpi_sx_zsdt_acpi;
      |                       ^~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors

[Bug tree-optimization/104341] Bogus -Werror=array-bounds since r12-2582-gb9cbf8c9e0bc72f5

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104341

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |12.0
           Keywords|                            |diagnostic

[Bug tree-optimization/104341] [12 Regression] Bogus -Werror=array-bounds since r12-2582-gb9cbf8c9e0bc72f5

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104341

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-02-02
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  acpi_sx_zsdt_acpi is large enough (but not large enough to hold the
union type so techincally undefined)

[Bug tree-optimization/104341] [12 Regression] Bogus -Werror=array-bounds since r12-2582-gb9cbf8c9e0bc72f5

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104341

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
By using a larger type to access a smaller object the code violates the strict
aliasing rule, and this instance of the warning for it is intended.  The
problem can be reduced to the following test case where GCC points out the
aliasing violation by -Wincompatible-pointer-types, besides also issuing
-Warray-bounds.

The warning could stand to be rephrased to make the problem clearer and moved
under the -Wstrict-alising option (which would also avoid it in code bases like
the Linux kernel that use -fno-strict-aliasing).  I submitted a patch to do all
that last March in an effort to resolve pr98503 of which this effectively a
duplicate, but the reviewer declined to approve it.

$ cat pr104341.c && gcc -O2 -S -Wall pr104341.c
int acpi_sx_zsdt_acpi;

void f (void)
{
  union {
    int u32[2];
  } *d = &acpi_sx_zsdt_acpi;

  d->u32[0] = 0;
}
pr104341.c: In function ‘f’:
pr104341.c:7:10: warning: initialization of ‘union <anonymous> *’ from
incompatible pointer type ‘int *’ [-Wincompatible-pointer-types]
    7 |   } *d = &acpi_sx_zsdt_acpi;
      |          ^
pr104341.c:9:4: warning: array subscript ‘union <anonymous>[0]’ is partly
outside array bounds of ‘int[1]’ [-Warray-bounds]
    9 |   d->u32[0] = 0;
      |    ^~
pr104341.c:1:5: note: object ‘acpi_sx_zsdt_acpi’ of size 4
    1 | int acpi_sx_zsdt_acpi;
      |     ^~~~~~~~~~~~~~~~~

*** This bug has been marked as a duplicate of bug 98503 ***