| Issue |
178940
|
| Summary |
[WebAssembly] manual splat not recognized
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
folkertdev
|
The wasm backend does not recognize a manual splat, while other backends do. For e.g. `s390x` vectors are not part of the baseline, and the pattern is only recognized when the `vector` target feature is enabled. For wasm, enabling `+simd128` still does not recognize the splat.
https://llvm.godbolt.org/z/z3Ma9fTja
For instance this IR
```llvm
define dso_local void @foo(i16 noundef %v, ptr noalias noundef writeonly align 1 captures(none) dereferenceable(16) initializes((0, 16)) %p) unnamed_addr #0 {
start:
store i16 %v, ptr %p, align 1
%_5.sroa.4.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 2
store i16 %v, ptr %_5.sroa.4.0.p.sroa_idx, align 1
%_5.sroa.5.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 4
store i16 %v, ptr %_5.sroa.5.0.p.sroa_idx, align 1
%_5.sroa.6.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 6
store i16 %v, ptr %_5.sroa.6.0.p.sroa_idx, align 1
%_5.sroa.7.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 8
store i16 %v, ptr %_5.sroa.7.0.p.sroa_idx, align 1
%_5.sroa.8.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 10
store i16 %v, ptr %_5.sroa.8.0.p.sroa_idx, align 1
%_5.sroa.9.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 12
store i16 %v, ptr %_5.sroa.9.0.p.sroa_idx, align 1
%_5.sroa.10.0.p.sroa_idx = getelementptr inbounds nuw i8, ptr %p, i32 14
store i16 %v, ptr %_5.sroa.10.0.p.sroa_idx, align 1
ret void
}
```
should optimize to
```asm
define dso_local void @foo(i16 noundef %v, ptr noalias noundef writeonly align 1 captures(none) dereferenceable(16) initializes((0, 16)) %p) unnamed_addr #0 {
start:
%0 = insertelement <8 x i16> poison, i16 %v, i64 0
%1 = shufflevector <8 x i16> %0, <8 x i16> poison, <8 x i32> zeroinitializer
store <8 x i16> %1, ptr %p, align 1
ret void
}
```
LLVM does just that for most targets, but not on webassembly.
This manual splat pattern is not uncommon in rust when array expressions are used, see https://github.com/rust-lang/rust/issues/97804. Previously other backends also did not optimize this pattern, that was tracked in https://github.com/llvm/llvm-project/issues/55994 but the issue was silently resolved.
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