https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114097
Bug ID: 114097
Summary: Missed register optimization in _Noreturn functions
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: pskocik at gmail dot com
Target Milestone: ---
Consider a never-returning functions such as this:
#include<setjmp.h>
#include<stdio.h>
//_Noreturn
void noret(unsigned A, unsigned B, unsigned C, unsigned D, unsigned E, jmp_buf
Jb){
for(;A--;) puts("A");
for(;B--;) puts("B");
for(;C--;) puts("C");
for(;D--;) puts("D");
for(;E--;) puts("E");
longjmp(Jb,1);
}
https://godbolt.org/z/35YjrhjYq
In its prologue, gcc saves the arguments in call-preserved registers to
preserve them around the puts calls, and it does so the usual way: by (1)
pushing the old values of the call-preserved registers to the stack and (2)
actually moving the arguments into the call-preserved registers.
pushq %r15
movq %r9, %r15
pushq %r14
movl %edi, %r14d
pushq %r13
movl %esi, %r13d
pushq %r12
movl %edx, %r12d
pushq %rbp
movl %ecx, %ebp
pushq %rbx
movl %r8d, %ebx
pushq %rax
//...
Since this function demonstrably never returns, step 1 can be entirely elided
as the old values of the call-preserved registers won't ever need to be
restored
(desirably, gcc does not generate the would-be-dead restoration code):
movq %r9, %r15
movl %edi, %r14d
movl %esi, %r13d
movl %edx, %r12d
movl %ecx, %ebp
movl %r8d, %ebx
pushq %rax
//...
(Also desirable would be the unrealized tailoptimization of the longjmp call in
this case: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=10837)
