| Issue |
58137
|
| Summary |
Missing optimization for reducing common factors in integer division.
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tstanisl
|
The optimizer fails to reduce a common factor in nominator and denominator producing efficient code.
```
int foo(int a, int b) {
return (2 * a * b) / (2 * a);
}
int bar(int a, int b) {
return (2 * b * a) / (2 * a);
}
```
Compiled with CLANG 15.0 and options "-O3" produces assembly:
```
foo(int, int):
mov eax,esi
ret
bar(int, int):
mov eax,esi
add edi,edi
imul eax,edi
cdq
idiv edi
ret
```
The `idiv` instruction is about 10+ times slower than `imul`. Using `int` guarantees no overflow due to invoking UB so the order of operand of `*` can be safely reordered allowing reduction of common factors in nominator and denominator. This issue appears when working with VLA when the number of elements is computed with `sizeof` operator. For example:
```
int A[n][m];
int rows = sizeof A / sizeof *A;
```
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