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From: "rth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107389] New: Alignment not inferred from type at -O0
Date: Tue, 25 Oct 2022 07:57:14 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107389

            Bug ID: 107389
           Summary: Alignment not inferred from type at -O0
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rth at gcc dot gnu.org
  Target Milestone: ---

Consider

typedef __uint128_t aligned_type __attribute__((aligned(16)));
_Static_assert(__alignof(aligned_type) =3D=3D 16);
__uint128_t foo(aligned_type *p) { return __atomic_load_n(p, 0); }

For s390x, atomic_loadti should expand this to LPQ.

For my purposes, it must also do this at -O0, not just with
optimization.  But the alignment seen by gen_atomic_loadti
is only 8, so it FAILs the expansion and falls back to libatomic.

The following appears to solve the problem:

--- a/gcc/builtins.cc
+++ b/gcc/builtins.cc
@@ -468,8 +468,11 @@ get_pointer_alignment_1
        }
       else
        {
+         /* Assume alignment from the type. */
+         tree ptr_type =3D TREE_TYPE (exp);
+         tree obj_type =3D TREE_TYPE (ptr_type);
+         *alignp =3D TYPE_ALIGN (obj_type);
          *bitposp =3D 0;
-         *alignp =3D BITS_PER_UNIT;
          return false;
        }
     }

but I have an inkling that would have undesired effects
for other usages.  If so, perhaps a special case could be
made for the usage in get_builtin_sync_mem.=