| Issue |
179143
|
| Summary |
[WebAssembly] bitcast defeats dot product pattern
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
folkertdev
|
The dot product recognition is not very robust, see:
https://godbolt.org/z/roKzf5xdK
```llvm
define dso_local <4 x i32> @with_bitcast(<4 x i32> %a, <4 x i32> %b) unnamed_addr {
start:
%_4 = bitcast <4 x i32> %a to <8 x i16>
%_5 = bitcast <4 x i32> %b to <8 x i16>
%0 = sext <8 x i16> %_4 to <8 x i32>
%1 = sext <8 x i16> %_5 to <8 x i32>
%2 = mul nsw <8 x i32> %1, %0
%3 = shufflevector <8 x i32> %2, <8 x i32> poison, <4 x i32> <i32 0, i32 2, i32 4, i32 6>
%4 = shufflevector <8 x i32> %2, <8 x i32> poison, <4 x i32> <i32 1, i32 3, i32 5, i32 7>
%5 = add <4 x i32> %3, %4
ret <4 x i32> %5
}
define dso_local <4 x i32> @without_bitcast(<8 x i16> %a, <8 x i16> %b) unnamed_addr {
start:
%0 = sext <8 x i16> %a to <8 x i32>
%1 = sext <8 x i16> %b to <8 x i32>
%2 = mul nsw <8 x i32> %1, %0
%3 = shufflevector <8 x i32> %2, <8 x i32> poison, <4 x i32> <i32 0, i32 2, i32 4, i32 6>
%4 = shufflevector <8 x i32> %2, <8 x i32> poison, <4 x i32> <i32 1, i32 3, i32 5, i32 7>
%5 = add <4 x i32> %3, %4
ret <4 x i32> %5
}
```
The additional bitcast causes the optimization to be missed
```asm
with_bitcast: # @with_bitcast
local.get 1
local.get 0
i32x4.extmul_low_i16x8_s
local.tee 2
local.get 1
local.get 0
i8x16.shuffle 8, 9, 10, 11, 12, 13, 14, 15, 0, 1, 2, 3, 0, 1, 2, 3
local.get 0
local.get 0
i8x16.shuffle 8, 9, 10, 11, 12, 13, 14, 15, 0, 1, 2, 3, 0, 1, 2, 3
i32x4.extmul_low_i16x8_s
local.tee 0
i8x16.shuffle 0, 1, 2, 3, 8, 9, 10, 11, 16, 17, 18, 19, 24, 25, 26, 27
local.get 2
local.get 0
i8x16.shuffle 4, 5, 6, 7, 12, 13, 14, 15, 20, 21, 22, 23, 28, 29, 30, 31
i32x4.add
end_function
without_bitcast: # @without_bitcast
local.get 1
local.get 0
i32x4.dot_i16x8_s
end_function
```
cc @badumbatish https://github.com/llvm/llvm-project/pull/151775
This is a problem in `rust-lang/stdarch` because usually the input type is an opaque `v128` (represented as `<4 x i32>`), so a bitcast is needed.
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