| Issue |
91527
|
| Summary |
Opt should know `x/2 <= x - x/2` is true for `udiv`
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
scottmcm
|
Today, this doesn't optimize away <https://llvm.godbolt.org/z/jPjo6M858>
```llvm
define noundef zeroext i1 @demo(i32 noundef %x) unnamed_addr #0 {
start:
%_2 = udiv i32 %x, 2
%_3 = sub i32 %x, %_2
%_0 = icmp ule i32 %_2, %_3
ret i1 %_0
}
```
but that could just be `return true`, as proven in <https://alive2.llvm.org/ce/z/dxHhft>.
Indeed, it's always `true` for any denominator greater than zero. Proof: <https://alive2.llvm.org/ce/z/CmkN_m>
This is important for "half but it needs to be rounded up" cases. `x/2 <= x` is already understood to be true, but when you need the bigger half instead, the optimizer is more confused. (I lost the original non-minimized example, sorry, but it was something like splitting an n-element slice into `n/2` and `n-n/2` slices and ending up with the bounds checking not optimizing out.)
---
Maybe start by getting `sub nuw` for this? Like
```diff
%_2 = udiv i32 %x, 2
- %_3 = sub i32 %x, %_2
+ %_3 = sub nuw i32 %x, %_2
```
Proof for that narrower part: <https://alive2.llvm.org/ce/z/xoCDe5>
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