https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91091
Bug ID: 91091
Summary: [missed optimization] Missing aliasing optimization
Product: gcc
Version: 5.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: aleksey.covacevice at gmail dot com
Target Milestone: ---
Consider the following:
struct s { int x; };
struct t { int x; };
void swap(struct s* p, struct t* q) {
p->x = q->x;
q->x = p->x;
}
Aliasing rules forbid `p` and `q` to point to the same object; yet, GCC 5.4 and
most subsequent versions produce (-O3):
swap(s*, t*):
mov eax, DWORD PTR [rsi]
mov DWORD PTR [rdi], eax
mov DWORD PTR [rsi], eax // Possible alias between p and q
ret
whereas GCC versions 4.5.3 to 5.3 and versions 8.1 to 8.2 correctly produce:
swap(s*, t*):
mov eax, DWORD PTR [rsi]
mov DWORD PTR [rdi], eax
ret
All versions produce the correct code if __restrict__ is used on any pointer.
This behavior can be verified on Godbolt: https://godbolt.org/z/WYMoFI
