[all-commits] [llvm/llvm-project] cdaebf: [NVPTX] Fix crash caused by ComputePTXValueVTs (#1...

[Justin Fargnoli via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cdaebf6f0e2be65f55011ccb2f1bc3b9199b6285
      https://github.com/llvm/llvm-project/commit/cdaebf6f0e2be65f55011ccb2f1bc3b9199b6285
  Author: Justin Fargnoli <jfargnoli at nvidia.com>
  Date:   2024-08-29 (Thu, 29 Aug 2024)

  Changed paths:
    M llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp
    A llvm/test/CodeGen/NVPTX/compute-ptx-value-vts.ll
    A llvm/test/CodeGen/NVPTX/vector-returns.ll

  Log Message:
  -----------
  [NVPTX] Fix crash caused by ComputePTXValueVTs (#104524)

When [lowering return
values](https://github.com/llvm/llvm-project/blob/99a10f1fe8a7e4b0fdb4c6dd5e7f24f87e0d3695/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp#L3422)
from LLVM IR to SelectionDAG, we check that [the number of values
`SelectionDAG` tells us to return is equal to the number of values that
`ComputePTXValueVTs()` tells us to
return](https://github.com/llvm/llvm-project/blob/99a10f1fe8a7e4b0fdb4c6dd5e7f24f87e0d3695/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp#L3441).
However, this check can fail on valid IR. For example:

```
define <6 x half> @foo() {
  ret <6 x half> zeroinitializer
}
```

`ComputePTXValueVTs()` tells us to return ***3*** `v2f16` values, while
`SelectionDAG` tells us to return ***6*** `f16` values. Thus, the
compiler will crash.

`ComputePTXValueVTs()` [supports all `half` element vectors with an even
number of
elements](https://github.com/llvm/llvm-project/blob/99a10f1fe8a7e4b0fdb4c6dd5e7f24f87e0d3695/llvm/lib/Target/NVPTX/NVPTXISelLowering.cpp#L213).
Whereas `SelectionDAG` [only supports power-of-2 sized
vectors](https://github.com/llvm/llvm-project/blob/4e078e3797098daa40d254447c499bcf61415308/llvm/lib/CodeGen/TargetLoweringBase.cpp#L1580).
This is the root of the discrepancy.

Assuming that the developers who added the code to
`ComputePTXValueVTs()` overlooked this, I've restricted
`ComputePTXValueVTs()` to compute the same number of return values as
`SelectionDAG`, instead of extending `SelectionDAG` to support
non-power-of-2 sized vectors.



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