https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
Bug ID: 98236
Summary: x plus/minus y cmp 0 produces unoptimal code
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: denis.campredon at gmail dot com
Target Milestone: ---
Created attachment 49733
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49733&action=edit
input file
Compiling test.c on x86 with -O2 leads to unoptimal code generation.
f1 to f4 could be optimized to `add + set(g|ge|l|le)`
f5 to f8 could be optimized to `sud|cmp + set(g|ge|l|le)`
For f2, f4, f6 and f8 no pattern is recognized.
For f1, f3, f5 and f7, the optimizers are not aware that, at least on x86,
`add` and `sub` can set flags.
This can also be seen with the following function. gcc will produce `cmp + sub`
although only `sub` could be used
-----------
void foo();
int bar(int x, int y) {
if (x - y)
foo();
return x - y;
}
-----------
produces
-----------
bar(int, int):
push rbp
mov ebp, esi
push rbx
mov ebx, edi
sub rsp, 8
cmp edi, esi
je .L2
call foo()
.L2:
mov eax, ebx
add rsp, 8
sub eax, ebp
pop rbx
pop rbp
ret
-----------
expected
-----------
bar(int, int):
push rbx
mov ebx, edi
sub ebx, esi
je .L2
call foo()
.L2:
mov eax, ebx
pop rbx
ret
-----------
