https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125344
Bug ID: 125344
Summary: returned struct and inline asm with multiple
alternative constraints compiles incorrectly on x86_64
Product: gcc
Version: 16.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: chris+gcc at lulabs dot net
Target Milestone: ---
The following C code compiles incorrectly on x86-64. bad() "loses" the value of
x and returns uninitialized memory on the stack:
struct S {
float x;
float y;
};
static struct S f() {
struct S x = {42.0f, 1337.0f};
return x;
}
float bad() {
float x = f().x;
asm volatile ("" : "+m,r"(x));
return x;
}
Compile with GCC 16.1, -O1:
"bad":
movss xmm0, DWORD PTR [rsp-4]
ret
(https://godbolt.org/z/485e5rjMK)
>From what I can tell, the "+m,r" constraint should be equivalent to "+mr" in
this case, which produces a correct result.
The bug appears to reproduce in all GCC versions since 4.8.1. 4.7.4 does not
have the issue.
