https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68369
Bug ID: 68369
Summary: Unnecessary copying esp to ebp with regparm={1,2,3} on
x86 at -Os
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: technosaurus at yahoo dot com
Target Milestone: ---
This occurs on all versions tested for this simple test case:
> int nop(int x){return x;}
with gcc 5.2+ @ -O1,-O2 and -O3 this correctly compiles to:
> nop(int):
> rep ret
however with -Os, it sub-optimally compiles to:
> nop(int):
> pushl %ebp
> movl %esp, %ebp
> popl %ebp
> ret
At -O0, ebp is actually used which may hint why -Os doesn't pick it up
> nop(int):
> pushl %ebp
> movl %esp, %ebp
> subl $4, %esp
> movl %eax, -4(%ebp)
> movl -4(%ebp), %eax
> leave
> ret
Backtracking other optimization levels, the same behavior was fixed for -O1,
-O2 and -O3 somewhere between gcc-4.5.3 and 4.6.4 but remains suboptimal for
-Os up to 5.2.0
