| Issue |
153808
|
| Summary |
Type legalizing emits invalid `extract_subvector` node
|
| Labels |
bug,
backend:AMDGPU,
llvm:SelectionDAG
|
| Assignees |
|
| Reporter |
lialan
|
# Problem
Hitting assertion:
```bash
llvm-project/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:25712: llvm::SDValue narrowExtractedVectorBinOp(llvm::EVT, llvm::SDValue, unsigned int, const llvm::SDLoc &, llvm::SelectionDAG &, bool): Assertion `(Index % VT.getVectorNumElements()) == 0 && "Extract index is not a multiple of the vector length."' failed.
```
When compiling the attached program.
[bug.txt](https://github.com/user-attachments/files/21797144/bug.txt)
# To reproduce:
```
llc --march=amdgcn ./bug.txt
```
confirmed that at commit `5d115e49ba70` the problem still exists.
# Initial triage
Initial triage posted here: https://github.com/iree-org/iree/issues/21646#issuecomment-3190415870
> Problem is that when doing dagcombine we hit this assertion: https://github.com/llvm/llvm-project/pull/138279/files#diff-d0eb75096db76ab253fc7f8ae6343c4b4516fc619d851898cbdac1a5bf481941R25186-L25197
> The culprit is this node:
```
t544: v32f32 = fdiv # D:1 t541, t464
...
t557: v3f32 = extract_subvector # D:1 t544, Constant:i32<16>
```
> where index `16` cannot be divided by vector length `3` -- which is a hard requirement for `extract_subvector`: the index must be a multiple of the minimum number of elements in the result type. Should look for the reason why `v3f32` is a legalized type at this stage.
# Simple problem analysis
The input program is trying to do reduction of a vector using `@llvm.vector.reduce.fadd.v3f32` in a loop. The loop is unrolled. And then type legalizer turns the residual extraction:
```
t370: v51f32 = fdiv # D:1 t369, t12 // both operands are of type v51f32
...
t431: v3f32 = extract_subvector # D:1 t370, Constant:i32<48>
```
into:
```
t544: v32f32 = fdiv # D:1 t541, t464 // both operands are of type v32f32
...
t557: v3f32 = extract_subvector # D:1 t544, Constant:i32<16>
```
without checking if it is possible to form a valid `extract_subvector`.
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