Hi!
On Sun, Feb 14, 2021 at 12:38:26PM +0100, Stefan Ring via Gcc-help wrote:
> I recently noticed that gcc 9 introduced a strange push/pop pair in a
> function that does nothing other than shift all arguments by one
> position and transfer control to another function:
>
> int func(int, int, int, int, int, int);
> int caller(int a, int b, int c, int d, int e) { return func(0, a, b, c, d, e); }
>
> pushq %r12
> movl %r8d, %r9d
> popq %r12
> movl %ecx, %r8d
> movl %edx, %ecx
> movl %esi, %edx
> movl %edi, %esi
> xorl %edi, %edi
> jmp func
>
> Obviously, pushing and popping r12 serves no useful purpose, and gcc 8
> does not produce it. It also disappears when a is used instead of the
> constant 0 as the first argument. Where does this come from?
The pop was emitted right before the jump, but it was moved to earlier
by the instruction scheduled (sched2).
The prologue/epilogue push and pop r12 because that is a non-volatile
("callee-saved") register. At the point the prologue and expilogue code
is generated r12 is used in the code, to shuffle these registers through.
It is essentially
r10 := edi
r11 := esi
r12 := edx
r9 := r8
r8 := ecx
ecx := r12
edx := r11
esi := r10
edi := 0
which then by cprop_hardreg is simplified to
r9 := r8
r8 := ecx
ecx := edx
edx := esi
esi := edi
edi := 0
but by then it is too late to omit the push and pop.
Please open a PR (see https://gcc.gnu.org/bugs.html for how). Thanks!
Segher
