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From: "zhan3299 at purdue dot edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/99207] New: #pragma pack(1) and __int128 lead to bad
 optimization under O2 and higher optimization
Date: Mon, 22 Feb 2021 17:03:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99207

            Bug ID: 99207
           Summary: #pragma pack(1) and __int128 lead to bad optimization
                    under O2 and higher optimization
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhan3299 at purdue dot edu
  Target Milestone: ---

Following code behaves differently with different optimizations.


$ cat test.c
#pragma pack(1)

struct {
  char a;
  __int128 b;
} c;

__int128 *d =3D &c.b;

int main() { *d =3D 0; }



In short, with O2 and higher optimization, gcc tries to use xmm register to
initialize *d. However, as #pragma pack(1) enforces a different alignments,=
 xmm
operation will trigger a segment fault.

O0 (program returns 0): https://godbolt.org/z/nc997z
O1 (program returns 0): https://godbolt.org/z/GqGfnf
O2 (program returns 139): https://godbolt.org/z/4PWKMo
O3 (program returns 139): https://godbolt.org/z/Y81sKK
Os (program returns 139): https://godbolt.org/z/M3KGnh

With asan, we can get: https://godbolt.org/z/34oW7e
AddressSanitizer:DEADLYSIGNAL
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
=3D=3D1=3D=3DERROR: AddressSanitizer: SEGV on unknown address (pc 0x0000004=
010ae bp
0x000000000000 sp 0x7ffd361619b8 T0)
=3D=3D1=3D=3DThe signal is caused by a READ memory access.
=3D=3D1=3D=3DHint: this fault was caused by a dereference of a high value a=
ddress (see
register values below).  Dissassemble the provided pc to learn which regist=
er
was used.
    #0 0x4010ae in main example.c:10
    #1 0x7f8c4b3b60b2 in __libc_start_main
(/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #2 0x40111d in _start (/app/output.s+0x40111d)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV example.c:10 in main
=3D=3D1=3D=3DABORTING=