https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106822
Bug ID: 106822
Summary: "Boolean-or" optimization opportunity not consistently
used
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: benjamin.meier70 at gmail dot com
Target Milestone: ---
Hello
Assume the following code is given:
-----------------------------
#include <stdio.h>
#include <stdbool.h>
typedef struct {
bool a;
bool b;
} MyStruct;
bool f(const MyStruct *s)
{
return s->a || s->b;
}
int main()
{
}
-----------------------------
If the code is compiled with -O3 I would assume it does not include any jumps,
because it should be valid to always read s->b (even if s->a is already true).
However, the function compiles into this:
-----------------------------
f:
movzx eax, BYTE PTR [rdi]
test al, al
jne .L1
movzx eax, BYTE PTR [rdi+1]
.L1:
ret
-----------------------------
(see: https://godbolt.org/z/KdnzWb9Wh)
Interestingly, gcc will produce code without any jumps if MyStruct is slightly
modified:
-----------------------------
#include <stdio.h>
#include <stdbool.h>
typedef struct {
int x;
bool a;
bool b;
} MyStruct;
bool f(const MyStruct *s)
{
return s->a || s ->b;
}
int main()
{
}
-----------------------------
Generated code:
-----------------------------
f:
cmp WORD PTR [rdi+4], 0
setne al
ret
-----------------------------
(see: https://godbolt.org/z/P78daxhhe)
It's not a bug, but I think it's a missed optimization opportunity.
Thanks
