https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105328
Bug ID: 105328
Summary: [x86] Failure to optimize out test instruction after
add
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
void f1();
void f2();
void f3();
void g(int b, int c)
{
int a = b + c;
if (a > 0)
f1();
else if (a < 0)
f2();
else
f3();
}
With -O3, GCC outputs this:
g(int, int):
add edi, esi
test edi, edi
jg .L5
je .L3
jmp f2()
.L3:
jmp f3()
.L5:
jmp f1()
LLVM instead outputs this:
g(int, int):
add edi, esi
jle .LBB0_1
jmp f1()@PLT # TAILCALL
.LBB0_1:
js .LBB0_4
jmp f3()@PLT # TAILCALL
.LBB0_4:
jmp f2()@PLT # TAILCALL
It appears like the `test` instruction can be removed (I assume without having
to change anything about which functions get called on branchless paths and
things like that). I'm not completely sure about this, considering how complex
x86 performance can sometimes be, but I'd think removing that instruction
should be beneficial everywhere.
