[llvm] [NVPTX] Add NVPTXIncreaseAligmentPass to improve vectorization (PR #144958)

[Artem Belevich via llvm-commits]

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@@ -0,0 +1,131 @@
+//===-- NVPTXIncreaseAlignment.cpp - Increase alignment for local arrays --===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// A simple pass that looks at local memory arrays that are statically
+// sized and sets an appropriate alignment for them. This enables vectorization
+// of loads/stores to these arrays if not explicitly specified by the client.
+//
+// TODO: Ideally we should do a bin-packing of local arrays to maximize
+// alignments while minimizing holes.
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Artem-B wrote:

With the changes that are likely to affect everyone, the typical approach is to introduce it as an optional feature (or enabled only for clear wins), then allow wider testing with more aggressive settings (I can help with that).
If the changes are deemed to be relatively low risk, aggressive defaults + escape hatch to disable it also works.

I think, in this case we're probably OK with aligning aggressively. 

In fact, I think it will, accidentally, benefit cutlass (https://github.com/NVIDIA/cutlass/issues/2003#issuecomment-2696101576), which does have the code with a known UB, where it uses local variables and then uses vector loads/stores on the, assuming that they are always aligned. It works in optimized builds where the locals are optimized away, but fails in debug builds.

https://github.com/llvm/llvm-project/pull/144958