[Bug c/108721] New: csmith: really old bug with -O2

[Bug c/108721] New: csmith: really old bug with -O2

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108721

            Bug ID: 108721
           Summary: csmith: really old bug with -O2
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dcb314 at hotmail dot com
  Target Milestone: ---

Created attachment 54436
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54436&action=edit
C source code

For the attached C code, generated from csmith, does this:

$ ../results/bin/gcc -w -O1 bug884.c && ./a.out
checksum = 749D4681
$ ../results/bin/gcc -w -O2 bug884.c && ./a.out
checksum = F8019D24
$ 

The bug seems to exist since sometime before 20220206, about a year ago.

$ ../results.20220206/bin/gcc -w -O2 bug884.c && ./a.out
checksum = F8019D24
$ 

The git hash for 20220206 is g:8eb329e963593342855b6072e5692659107337b7

[Bug middle-end/108721] csmith: really old bug with -O2

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108721

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |middle-end

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There could be aliasing issues here as -fno-strict-aliasing fixes the issue ...

[Bug c/108721] [10/11/12/13 Regression] csmith: really old bug with -O2

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108721

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|csmith: really old bug with |[10/11/12/13 Regression]
                   |-O2                         |csmith: really old bug with
                   |                            |-O2
           Priority|P3                          |P2
          Component|middle-end                  |c
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
   Target Milestone|---                         |10.5

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This one bisected to differences starting with
r0-107468-g605896f5527472717f9fe91ad90f9ce3778cc540

[Bug middle-end/108721] [10/11/12/13 Regression] csmith: really old bug with -O2

[dcb314 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108721

--- Comment #3 from David Binderman <dcb314 at hotmail dot com> ---
Created attachment 54471
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54471&action=edit
C source code

After a short reduction.

[Bug middle-end/108721] [10/11/12/13 Regression] csmith: really old bug with -O2

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108721

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
In the reduced testcase there is certainly an aliasing violation.
struct {
  unsigned f0
} g_95 = {65531};
...
*g_412 = &g_95;
...
  --g_95.f0;
  *g_86 = g_613 || 0;
  *g_412 = 0;
  short *l_873 = &g_95;
  --*l_873;
even after making g_412 unsigned *g_412 = &g_95.f0; it gets different results,
but
while short *l_873 = &g_95.f0; does as well, doing
  typedef short sa __attribute__((may_alias));
  sa *l_873 = &g_95.f0;
one gets the same result.
Now, in the original testcase it actually uses
union U0 {
   uint16_t f0;
   int64_t f1;
   int64_t f2;
   uint32_t f3;
};
static union U0 g_95 = {65531UL};
but then has
static int64_t *g_279 = &g_95.f2;
...
    int64_t *l_939 = &g_95.f2;
...
        uint16_t *l_873 = &g_95.f0;
...
                uint32_t *l_933[8] =
{&g_95.f3,&g_95.f3,&g_95.f3,&g_95.f3,&g_95.f3,&g_95.f3,&g_95.f3,&g_95.f3};
...
        int64_t *l_425 = &g_95.f2;
...
        uint16_t *l_597 = &g_95.f0;
etc.  Now, all of this doesn't necessarily mean it is UB, but it is a strong
indication that it very likely is.  Standard C/C++ doesn't allow type punning
through union, only one of the members can be active at a time, GCC allows it,
but for aliasing requires that accesses have the union type visible to the
compiler on the access.  Taking addresses of different union members means that
most likely that isn't followed, UB would be whenever some union member is
stored and another one read through a pointer without the union in access path
etc.

Now, I think some of the readings of the standard say that whenever say
uint16_t and uint32_t appear in some union anywhere, one basically needs to
treat them as the same alias set, but such reading makes TBAA totally useless. 
So if csmith follows that reading, it is indeed unsafe for TBAA.

[Bug middle-end/108721] [10/11/12/13 Regression] csmith: really old bug with -O2

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108721

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Ok, trying the original testcase compiled with -O0 -g and awatch g_95.f1 in
gdb,
g_95.f0 is read/modified directly, then
(*p_9) = (((**g_412) = ((safe_mod_func_uint8_t_u_u(p_10,
(safe_div_func_uint32_t_u_u(((safe_lshift_func_uint8_t_u_u(((safe_lshift_func_int16_t_s_s((((&g_95
!= &g_95) || ((*g_86) = (g_613[1] || (*g_245)))) | (l_799 < 0UL)), 1)) ^ (p_10
== (0x95L != 247UL))), p_10)) , l_799), 5L)))) >= 0x7BC3L)) < l_799);
stores 0 to it through int64_t * (g_412[0] is &g_95.f1) and is read from it
immediately again.
Then
(*l_871) = (safe_mul_func_int8_t_s_s(((1UL > ((void*)0 == l_871)) <
(--(*l_873))), ((l_876 , ((*l_878) = l_877)) >
(safe_sub_func_int8_t_s_s(((safe_sub_func_uint64_t_u_u(((safe_div_func_int16_t_s_s((((safe_sub_func_uint8_t_u_u((((l_887
!= l_888) != ((safe_add_func_uint64_t_u_u((*l_3), (((--g_118.f0) > ((*l_872) =
(*l_871))) , g_594))) || l_894)) == g_20.f0), (*l_3))) | (*l_3)) >
4294967288UL), g_207[3][7][2])) > 0xCA172E9FL), (*l_3))) < 1L), 0UL)))));
reads it through uint16_t * pointer l_873.  I think that is enough UBs to mark
this as invalid.