| Issue |
150844
|
| Summary |
Optimization when 'while' loop can be converted to 'do-while'
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Explorer09
|
This missed optimization issue is more like a feature request than a bug.
When the body of a loop contains only counter increments and the loop condition is determined externally, a `while` loop construct can sometimes be converted to a `do`-`while`.
Clang can do the optimization when the loop counter change is addition or subtraction. Now I wish the optimization can also be performed when the counter change is multiplication (by a constant) or bit shift (shift left or shift right by a constant).
(I'm not requesting for general division cases except for division by a power of two.)
Example code:
```c
#include <stdbool.h>
#include <stdint.h>
extern bool cond(uint32_t x);
uint32_t func2_a(uint32_t x) {
uint32_t i = 10;
while (cond(i)) {
i *= 2;
}
return i;
}
uint32_t func2_b(uint32_t x) {
uint32_t i = 5;
do {
i *= 2;
} while (cond(i));
return i;
}
```
`func2_a` and `func2_b` should be equivalent. By turning `func2_a` to `func2_b` it can usually save a `jmp` instruction.
[Related GCC bug report](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121258)
The example code can be tested in Compiler Explorer ([link](https://godbolt.org/z/Kqnr1o8zh)).
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