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From: "borysp at invisiblethingslab dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/99258] New: volatile struct access optimized
 away
Date: Wed, 24 Feb 2021 18:17:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99258

            Bug ID: 99258
           Summary: volatile struct access optimized away
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: borysp at invisiblethingslab dot com
  Target Milestone: ---

Access to a volatile struct is optimized away, if the size of the struct is
>16B (at least on x64 cpu with sse). Small repro:
[borys@mowmiwuju test]$ cat a.c
struct A {
    long a;
    long b;
};

struct B {
    long a;
    long b;
    long c;
};

void f(volatile struct A* x) {
    *x;
}

void g(volatile struct B* x) {
    *x;
}
[borys@mowmiwuju test]$ gcc --version=20
gcc (GCC) 10.2.0
[borys@mowmiwuju test]$ gcc -Wall -Wextra -std=3Dc11 -O2 -c a.c
[borys@mowmiwuju test]$ objdump -d -Mintel a.o

a.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <f>:
   0:   f3 0f 6f 07             movdqu xmm0,XMMWORD PTR [rdi]
   4:   c3                      ret=20=20=20=20
   5:   66 66 2e 0f 1f 84 00    data16 cs nop WORD PTR [rax+rax*1+0x0]
   c:   00 00 00 00=20

0000000000000010 <g>:
  10:   c3                      ret=20


Testing on https://godbolt.org/ indicates this issue is present in all gcc
versions. clang emits correct code for this test case.=