| Issue |
184131
|
| Summary |
Select of lshr not converted to lshr of select with one multi-use operand
|
| Labels |
llvm:instcombine,
missed-optimization
|
| Assignees |
|
| Reporter |
nikic
|
From https://github.com/rust-lang/rust/pull/151780.
https://alive2.llvm.org/ce/z/AHRhE2
```llvm
define i16 @or(i16 %0, i16 %1) {
start:
%.sroa.0.0.extract.shift = lshr i16 %0, 8
%2 = trunc i16 %.sroa.0.0.extract.shift to i1
%_0.sroa.0.0.extract.shift = lshr i16 %1, 8
%_0.sroa.0.0.v = select i1 %2, i16 %.sroa.0.0.extract.shift, i16 %_0.sroa.0.0.extract.shift
%_0.sroa.3.0.v = select i1 %2, i16 %0, i16 %1
%_0.sroa.3.0.insert.ext = and i16 %_0.sroa.3.0.v, 255
%_0.sroa.0.0.insert.shift = shl nuw i16 %_0.sroa.0.0.v, 8
%_0.sroa.0.0.insert.insert = or disjoint i16 %_0.sroa.0.0.insert.shift, %_0.sroa.3.0.insert.ext
ret i16 %_0.sroa.0.0.insert.insert
}
define i16 @tgt(i16 %0, i16 %1) {
start:
%2 = and i16 %0, 256
%.not = icmp eq i16 %2, 0
%_0.sroa.3.0.v = select i1 %.not, i16 %1, i16 %0
ret i16 %_0.sroa.3.0.v
}
```
This would simplify if the select of lshr would be converted into lshr of select, but this doesn't happen because one of the operands is multi-use.
Possibly we can relax this fold to allow one multi-use operand, not sure if that will cause other issues.
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