| Issue |
149073
|
| Summary |
x86-64 optimization: (x <= 0xFFFFF) in '-Os' mode can convert to ((x >> 20) == 0)
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Explorer09
|
Test code
```c
#include <stdbool.h>
#include <stdint.h>
bool func1(uint32_t x) {
return x < 0x100000U;
}
bool func2(uint32_t x) {
return ((x >> 20) == 0);
}
bool func3(uint32_t x) {
return ((x / 0x100000U) == 0);
}
```
The three functions are equivalent. Specifically the threshold constant to be compared with is a power-of-two. If there is no shortage of temporary registers, then the right shift version (that is, `func2`) will likely make the smallest code.
My expected result is like this (assuming `-Os` optimization, not `-O2`)
```x86asm
shrl $20, %edi
sete %al
retq
```
Instead I got this (x86-64 clang 20.1.0 with `-Os` option, tested in Compiler Explorer):
```x86asm
cmpl $1048576, %edi
setb %al
retq
```
(Clang can recognize the three functions are equivalent. It just made suboptimal code among the possible choices.)
Note that in RISC-V targets of Clang, it does use the right shift to reduce code size
```text
srliw a0, a0, 20
seqz a0, a0
ret
```
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