https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69986
Bug ID: 69986
Summary: smaller code possible with -Os by using push/pop to
spill/reload
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: minor
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: peter at cordes dot ca
Target Milestone: ---
Target: x86-64-*-*
#include <unistd.h>
int f(int a) { close(a); return a; }
push rbx
mov ebx,edi
call 400490 <close@plt>
mov eax,ebx
pop rbx
ret
with gcc 5.3 -Os.
It could be smaller:
push rbi
call 400490 <close@plt>
pop rax
ret
saving 4 bytes (mov reg,reg is two bytes).
More generally, push/pop are 1 byte each, much smaller than mov [rsp-8], edi or
something.
This might not be a desirable optimization, though, because a round-trip
through memory increases latency. It's one of those code-size optimizations
that will might often have a negative impact on performance in the case where
the function is already hot in L1 I-cache.
It would be nice if there was a way to optimize a bit for code-size without
making bad performance sacrifices, and also another option to optimize for code
size without much regard for performance. -Oss vs. -Os? Or -OS? I assume
tuning these options is a lot of work.
