[Bug tree-optimization/111715] New: Missed optimization in FRE because of weak TBAA

["rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111715

            Bug ID: 111715
           Summary: Missed optimization in FRE because of weak TBAA
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

Jakub reports a spurious diagnostic with tree-affine.cc with the wide-int
patches:

> ./cc1plus  -quiet -O2 -Wall -fno-exceptions /tmp/t.ii
In file included from ../../gcc/coretypes.h:465,
                 from ../../gcc/tree-affine.cc:22:
In member function 'void widest_int_storage<N>::set_len(unsigned int, bool)
[with int N = 576]',
    inlined from 'void wi::copy(T1&, const T2&) [with T1 =
widest_int_storage<576>; T2 = generic_wide_int<wide_int_ref_storage<true, true>
>]' at ../../gcc/wide-int.h:2407:13,
    inlined from 'widest_int_storage<N>& widest_int_storage<N>::operator=(const
T&) [with T = int; int N = 576]' at ../../gcc/wide-int.h:1805:12,
    inlined from 'generic_wide_int<storage>&
generic_wide_int<T>::operator=(const T&) [with T = int; storage =
widest_int_storage<576>]' at ../../gcc/wide-int.h:1018:23,
    inlined from 'void aff_combination_remove_elt(aff_tree*, unsigned int)' at
../../gcc/tree-affine.cc:596:34:
../../gcc/wide-int.h:1856:14: warning: 'void* memcpy(void*, const void*,
size_t)' offset [0, 7] is out of the bounds [0, 0] [-Warray-bounds=]
../../gcc/wide-int.h:1857:12: warning: 'void free(void*)' called on a pointer
to an unallocated object '1' [-Wfree-nonheap-object]
In member function 'void widest_int_storage<N>::set_len(unsigned int, bool)
[with int N = 576]',
    inlined from 'void wi::copy(T1&, const T2&) [with T1 =
widest_int_storage<576>; T2 = generic_wide_int<wide_int_ref_storage<true, true>
>]' at ../../gcc/wide-int.h:2407:13,
    inlined from 'widest_int_storage<N>& widest_int_storage<N>::operator=(const
T&) [with T = int; int N = 576]' at ../../gcc/wide-int.h:1805:12,
    inlined from 'generic_wide_int<storage>&
generic_wide_int<T>::operator=(const T&) [with T = int; storage =
widest_int_storage<576>]' at ../../gcc/wide-int.h:1018:23,
    inlined from 'void aff_combination_convert(aff_tree*, tree)' at
../../gcc/tree-affine.cc:256:34:
../../gcc/wide-int.h:1856:14: warning: 'void* memcpy(void*, const void*,
size_t)' offset [0, 7] is out of the bounds [0, 0] [-Warray-bounds=]
../../gcc/wide-int.h:1857:12: warning: 'void free(void*)' called on a pointer
to an unallocated object '1' [-Wfree-nonheap-object]

that's caused by failing to CSE in

  MEM <struct aff_tree> [(struct widest_int_storage
*)comb_13(D)].elts[_6].coef.D.132464.len = 1;
  _49 = (sizetype) _6;
  _50 = _49 * 88;
  _101 = _50 + 104;
  _74 = comb_13(D) + _101;
  *_74 = 1;
  _80 = MEM <struct aff_tree> [(struct widest_int_storage
*)comb_13(D)].elts[_6].coef.D.132464.len;

where the view-convert pun in the MEM_REF makes it get the alias set of
wide-int storage which conflicts with the *_74 store of a 'long'.  We
fail to pick up the store to an 'int' (the len) and possible disambiguation
with TBAA.

That's somewhat by design since we are faced with a lot of address-taking
and dereferencing of pointers as part of C++ abstraction.  But the
"tail" is still a valid access.