| Issue |
75752
|
| Summary |
Missed optimization: invert nested branches on comparisons
|
| Labels |
missed-optimization
|
| Assignees |
|
| Reporter |
Kmeakin
|
Reporting from https://github.com/rust-lang/rust/issues/117970
SimplifyCFG could invert nested branches to reduce code size and number of comparisons
# Rust example
[Compiler explorer](https://godbolt.org/z/zzYzvz8qq)
```rs
extern "Rust" {
fn foo() -> u32;
fn bar() -> u32;
fn baz() -> u32;
}
pub fn src(x: u32, y: u32) -> u32 {
unsafe {
match (x, y) {
(1, 10) => foo(),
(2, 10) => bar(),
(3, 10) => baz(),
_ => 0,
}
}
}
pub fn tgt(x: u32, y: u32) -> u32 {
unsafe {
match (y, x) {
(10, 1) => foo(),
(10, 2) => bar(),
(10, 3) => baz(),
_ => 0,
}
}
}
```
# C equivalent
[Compiler explorer](url)
```c
#include <stdint.h>
uint32_t foo();
uint32_t bar();
uint32_t baz();
uint32_t src(uint32_t x, uint32_t y) {
switch (x) {
case 1:
switch (y) {
case 10:
return foo();
default:
return 0;
}
case 2:
switch (y) {
case 10:
return bar();
default:
return 0;
}
case 3:
switch (y) {
case 10:
return baz();
default:
return 0;
}
default:
return 0;
}
}
uint32_t tgt(uint32_t x, uint32_t y) {
switch (y) {
case 10:
switch (x) {
case 1:
return foo();
case 2:
return bar();
case 3:
return baz();
default:
return 0;
}
default:
return 0;
}
}
```
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