| Issue |
55394
|
| Summary |
constant propagation can pessimize libcall optimizations.
|
| Labels |
llvm:optimizations,
missed-optimization
|
| Assignees |
|
| Reporter |
nickdesaulniers
|
This case just came up in the context of the Linux kernel.
https://lore.kernel.org/llvm/[email protected]/
Consider the following code:
```c
#define CONST 0x01000000
// try removing static
static unsigned ffs (int x) {
int r;
asm("bsfl %1,%0"
: "=r" (r)
: "rm" (x), "0" (-1));
return r;
}
unsigned int foo(void) {
return ffs(CONST);
}
unsigned int bar(void) {
return __builtin_ffs(CONST);
}
```
It looks like constprop sinks `CONST` into ffs when ffs is static, but if it wasn't then constprop wouldn't sink CONST. instcombine will fail to optimize `ffs(CONST)` into 25 since dead arg elimination will remove the parameter after const prop.
cc @fhahn @efriedma-quic
We're fixing this in the kernel by changing our ffs to dispatch to __builtin_ffs after a __builtin_constant_p check.
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