| Issue |
122704
|
| Summary |
Global ISel 64 bit left-shift+add results in 32 bit adds
|
| Labels |
llvm:globalisel,
mlir:amdgpu
|
| Assignees |
|
| Reporter |
tpopp
|
I'm seeing instructions like
```
%i12 = sext i32 %i11 to i64
%i62 = shl i64 %i12, 1
%i64 = add i64 0, %i62
%i65 = add i64 %i64, 1
```
resulting in a sequence of 32 bit adds with global isel, while without results in a 64 bit shl+add. I assume this is worse performance. Is there a reason to prefer this or expect the same performance?
I'm using `llc -O3 -march=amdgcn -mcpu=gfx942 -mtriple amdgcn-amd-hmcsa ./reduced.ll -global-isel={true,false} -o -` in looking at this.
[reduced.gisel.txt](https://github.com/user-attachments/files/18396831/reduced.gisel.txt)
[reduced.sdisel.txt](https://github.com/user-attachments/files/18396830/reduced.sdisel.txt)
[reduced.txt](https://github.com/user-attachments/files/18396829/reduced.txt)
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