| Issue |
164436
|
| Summary |
[InstCombine] Failure to recognise CTTZ pattern using CTLZ
|
| Labels |
good first issue,
llvm:instcombine,
missed-optimization
|
| Assignees |
|
| Reporter |
RKSimon
|
While we correctly fold this hackers delight ctpop pattern into ctz:
```ll
define i4 @src_ctpop(i4 %a0) {
%dec = add i4 %a0, -1
%not = xor i4 %a0, -1
%and = and i4 %dec, %not
%clz = tail call i4 @llvm.ctpop.i4(i4 %and)
ret i4 %clz
}
define i4 @tgt_ctpop(i4 %a0) {
%ctz = tail call i4 @llvm.cttz.i4(i4 %a0, i1 false)
ret i4 %ctz
}
```
we fail to recognise the related clz pattern:
```ll
define i4 @src_lz(i4 %a0) {
%dec = add i4 %a0, -1
%not = xor i4 %a0, -1
%and = and i4 %dec, %not
%clz = tail call i4 @llvm.ctlz.i4(i4 %and, i1 false) ; undef = true is ok as well
%ctz = sub i4 4, %clz
ret i4 %ctz
}
define i4 @tgt_lz(i4 %a0) {
%ctz = tail call i4 @llvm.cttz.i4(i4 %a0, i1 false)
ret i4 %ctz
}
```
Alive2: https://alive2.llvm.org/ce/z/AdPVa5
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