| Issue |
64729
|
| Summary |
Negative effects of subtraction on optimization
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
ZY546
|
Hello, I've noticed that there are often missed optimizations that occur when there is a subtraction or a minus sign in an _expression_. For example, in the following example, `var_4 + var_1` should ideally be computed only once:
https://godbolt.org/z/dPa75Enaq
```c
extern int var_23;
extern int var_24;
void test(int var_1, int var_3, int var_4) {
var_23 = var_4 + var_1;
var_24 = -var_3 + var_4 + var_1;
}
```
clang16 -O3:
```asm
test(int, int, int): # @test(int, int, int)
lea eax, [rdx + rdi]
mov rcx, qword ptr [rip + var_23@GOTPCREL]
mov dword ptr [rcx], eax
sub edi, esi
add edi, edx
mov rax, qword ptr [rip + var_24@GOTPCREL]
mov dword ptr [rax], edi
ret
```
```
define dso_local void @test(int, int, int)(i32 noundef %var_1, i32 noundef %var_3, i32 noundef %var_4) local_unnamed_addr {
entry:
%add = add nsw i32 %var_4, %var_1
store i32 %add, ptr @var_23, align 4
%add1 = sub i32 %var_1, %var_3
%add2 = add i32 %add1, %var_4
store i32 %add2, ptr @var_24, align 4
ret void
}
```
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