https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115132
Bug ID: 115132
Summary: Sibling calls optim should not be performed when
builtin_unwind_init is used
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: akrl at gcc dot gnu.org
Target Milestone: ---
Hi all,
In GNU Emacs we use (in order to have all callee saved registers pushed on the
stack before entering in the garbage collector) '__builtin_unwind_init'.
If sibling calls optimization is performed the call to gc is moved after the
register are popped again making '__builtin_unwind_init' ineffective.
test.c======
void gc ();
void foo (void)
{
__builtin_unwind_init ();
gc ();
}
=======
Expected result:
gcc -fno-optimize-sibling-calls -O2 test.c
=======
foo:
push rbp
xor eax, eax
mov rbp, rsp
push r15
push r14
push r13
push r12
push rbx
sub rsp, 8
call gc
add rsp, 8
pop rbx
pop r12
pop r13
pop r14
pop r15
pop rbp
ret
========
gcc -02 test.c
========
foo:
push rbp
xor eax, eax
mov rbp, rsp
push r15
push r14
push r13
push r12
push rbx
pop rbx
pop r12
pop r13
pop r14
pop r15
pop rbp
jmp gc
========
I understand '__builtin_unwind_init' it's undocumented and this can be
categorized as a non-bug but AFAIU this optimization makes
'__builtin_unwind_init' unusable for its scope.
Another very possible scenario is that I don't understand correctly how
'__builtin_unwind_init' is supposed to be used.
I see this on trunk but this behavior is present since at least GCC13.
Note: Original Emacs bug <https://debbugs.gnu.org/cgi/bugreport.cgi?bug=63365>
Thanks!
Andrea
