https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67809
Bug ID: 67809
Summary: Empty pointer-chasing loops aren't optimized out
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: matt at godbolt dot org
Target Milestone: ---
The following code:
```
struct Foo {
Foo *next;
void release() {
Foo *tmp = 0;
for (Foo *it = next; it; it = tmp) {
tmp = it->next;
}
}
};
void test(Foo &f) { f.release(); }
```
Results in pointer-chasing code when compiled at -O3:
```
test(Foo&):
mov rax, QWORD PTR [rdi]
test rax, rax
je .L1
.L3:
mov rax, QWORD PTR [rax]
test rax, rax
jne .L3
.L1:
rep ret
```
clang and icc both optimize this to a single ret. e.g. https://goo.gl/saN4XC vs
https://goo.gl/LeUGn0
Would be nice for this loop to go away completely. For context, this was in
code I added to use ASAN_POISON_MEMORY_REGION() around a pooled allocator upon
free of a list (each node was poisoned individually). Without
-fsanitize=address I was expecting the loop to entirely vanish, but the
pointer-chase was still done.
