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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/116357] [12/13/14/15 Regression] The item's address of the
 array is not correct if aligned is used
Date: Tue, 17 Sep 2024 08:36:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D116357

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |c
   Last reconfirmed|                            |2024-09-17
                 CC|                            |jsm28 at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
The optimization issue is because we have an element type that looks like

 <integer_type 0x7ffff69b27e0 spinlock_t volatile unsigned SI
    size <integer_cst 0x7ffff6824198 type <integer_type 0x7ffff68220a8
bitsizetype> constant 32>
    unit-size <integer_cst 0x7ffff68241b0 type <integer_type 0x7ffff6822000
sizetype> constant 4>
    user align:64 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type
0x7ffff69b2738 precision:32 min <integer_cst 0x7ffff68241c8 0> max <integer=
_cst
0x7ffff6824180 4294967295>
    pointer_to_this <pointer_type 0x7ffff69dd540>>

in particular with TYPE_ALIGN > TYPE_SIZE which is a "cannot happen".  Beca=
use
ARRAY_REF records the element size in units of element alignment this ends
up as zero ...

In this case value-numbering makes ARRAY_REF OP2 explicit while keeping it
implicit ends up being "fine" (fine as in still densely packing
spinlock_t and thus giving elements wrong alignment).

I wonder whether the bug is that sizeof (spinlock_t) is 4, but of course
that might be difficult to change.  But I suppose this is also how C works;
possibly the C frontend should build a variant type of the element type
if over-alignment should be ignored - putting in the over-aligned type
most definitely breaks the middle-end with regard to how ARRAY_REF is
defined.

This is a C frontend issue, it produces wrong GENERIC.=