| Issue |
63868
|
| Summary |
Splat of not could be canonicalised
|
| Labels |
llvm:instcombine
|
| Assignees |
|
| Reporter |
lukel97
|
Alive link: https://alive2.llvm.org/ce/z/4KIIpC
If we have a scalar value that we want negate and then splat, we can either:
1) negate the scalar, then splat
2) splat it, then negate the vector
```llvm
define <2 x i32> @src(i32 %x, <2 x i32> %y) {
%not.x = xor i32 %x, -1
%not.x.head = insertelement <2 x i32> poison, i32 %not.x, i64 0
%not.x.splat = shufflevector <2 x i32> %not.x.head, <2 x i32> poison, <2 x i32> zeroinitializer
%res = and <2 x i32> %not.x.splat, %y
ret <2 x i32> %res
}
define <2 x i32> @tgt(i32 %x, <2 x i32> %y) {
%x.head = insertelement <2 x i32> poison, i32 %x, i64 0
%x.splat = shufflevector <2 x i32> %x.head, <2 x i32> poison, <2 x i32> zeroinitializer
%not.x.splat = xor <2 x i32> %x.splat, shufflevector (<2 x i32> insertelement (<2 x i32> poison, i32 -1, i32 0), <2 x i32> poison, <2 x i32> zeroinitializer)
%res = and <2 x i32> %not.x.splat, %y
ret <2 x i32> %res
}
```
InstCombine doesn't seem to canonicalise it, but I think it could be beneficial to transform to the former.
This would aid in instruction selection for some instructions like [`vandn.vx`](https://github.com/riscv/riscv-crypto/blob/master/doc/vector/insns/vandn.adoc) on RISC-V
(Excuse the gratuitous splatting: this originally came from an example on scalable vectors, but I had to convert it to fixed for it to work on Alive)
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