https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123181
Bug ID: 123181
Summary: Don't optimize to push 7; pop rcx on
x86_64-pc-windows-*
Product: gcc
Version: 15.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: blubban at gmail dot com
Target Milestone: ---
https://learn.microsoft.com/en-us/cpp/build/stack-usage?view=msvc-170
> The stack will always be maintained 16-byte aligned, except within the
> prolog (for example, after the return address is pushed), and except
> where indicated in Function Types for a certain class of frame functions.
> If a frame function does not call another function then it is not required
> to align the stack (referenced in Section Stack Allocation).
Therefore, mov ecx,7 -> push 7; pop rcx is (typically) not a legal optimization
on x86_64-pc-windows-* targets.
void line();
void square(int);
void cube();
void tesseract()
{
line();
square(7);
cube();
}
-Oz, with a mingw gcc build
Expected:
tesseract:
sub rsp, 40
call line
mov ecx, 7
call square
nop
add rsp, 40
jmp cube
Actual:
tesseract:
sub rsp, 40
call line
push 7
pop rcx
call square
nop
add rsp, 40
jmp cube
https://godbolt.org/z/c1zK94ae1
(not sure what's up with the nop either, but both gcc and clang do that, so
I'll assume there's a good reason. Even if not, it's missed-optimization at
worst, not worth caring about.)
