| Issue |
102875
|
| Summary |
LLVM fails to optimize out an arithmetic right shift of an I64 by 63 followed by a fshl of an I64 by 1
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
johnplatts
|
LLVM fails to optimize out the `@llvm.fshl.i64(i64 %shifted_hi, i64 %1, i64 1)` below (as all of the bits of `%shifted_hi` are known to be equal to the MSB of `%1` in the code below):
```
define { i64, i64 } @src(i64 %0, i64 %1) local_unnamed_addr #0 {
%shifted_hi = ashr i64 %1, 63
; shifted_hi = ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss
; %1 = sxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
%shifted_lo = tail call i64 @llvm.fshl.i64(i64 %shifted_hi, i64 %1, i64 1)
; %shifted_lo = ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss
%result0 = insertvalue { i64, i64 } poison, i64 %shifted_lo, 0
%result1 = insertvalue { i64, i64 } %result0, i64 %shifted_hi, 1
ret { i64, i64 } %result1
}
define { i64, i64 } @tgt(i64 %0, i64 %1) local_unnamed_addr #0 {
%shr_result = ashr i64 %1, 63
; shifted_hi = ssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssss
; %1 = sxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
%result0 = insertvalue { i64, i64 } poison, i64 %shr_result, 0
%result1 = insertvalue { i64, i64 } %result0, i64 %shr_result, 1
ret { i64, i64 } %result1
}
attributes #0 = { mustprogress nofree norecurse nosync nounwind willreturn memory(none) uwtable }
attributes #1 = { nocallback nofree nosync nounwind speculatable willreturn memory(none) }
```
The above snippet run through alive-tv and opt can be found at https://alive2.llvm.org/ce/z/dew45A.
`%shifted_lo = tail call i64 @llvm.fshl.i64(i64 %shifted_hi, i64 %1, i64 1)` above can be optimized down to `%shifted_hi` in the case where `%shifted_hi` is equal to the result of `ashr i64 %1, 63` as all of the bits of `%shifted_hi` are equal to the MSB of `%1`.
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