https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99258
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
gcc (GCC) 10.2.0
[borys@mowmiwuju test]$ gcc -Wall -Wextra -std=c11 -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
5: 66 66 2e 0f 1f 84 00 data16 cs nop WORD PTR [rax+rax*1+0x0]
c: 00 00 00 00
0000000000000010 <g>:
10: c3 ret
Testing on https://godbolt.org/ indicates this issue is present in all gcc
versions. clang emits correct code for this test case.
