[llvm] [LAA] Use diff checks for read-modify-write pointers (PR #226816)

[via llvm-commits]

llvmorg-github-actions[bot] wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Dávid Bolvanský (davidbolvansky)

<details>
<summary>Changes</summary>

Consider this loop:

```c
void update(int *dst, const int *src) {
  for (int i = 0; i != 256; ++i)
    dst[i + 128] += src[i];
}
```

Runtime pointer checking currently falls back to a full-range overlap check because `dst` is both read and written.

Deduplicate runtime-check pointers while forming checking groups, then allow a difference check when one pointer is read-modify-write and the other is read-only. Keep the existing fallback for other read/write combinations and when the difference check would not be invariant in the parent loop.

This is sound because the extra dependence is read/read. The difference check still covers the only potentially conflicting write/read pair.


---
Full diff: https://github.com/llvm/llvm-project/pull/226816.diff


13 Files Affected:

- (modified) llvm/lib/Analysis/LoopAccessAnalysis.cpp (+26-6) 
- (modified) llvm/test/Transforms/LoopVectorize/dead_instructions.ll (+17-22) 
- (modified) llvm/test/Transforms/LoopVectorize/if-conversion-nest.ll (+16-20) 
- (modified) llvm/test/Transforms/LoopVectorize/if-conversion.ll (+14-23) 
- (modified) llvm/test/Transforms/LoopVectorize/if-pred-non-void.ll (+62-62) 
- (modified) llvm/test/Transforms/LoopVectorize/induction.ll (+260-295) 
- (modified) llvm/test/Transforms/LoopVectorize/opaque-ptr.ll (+36-46) 
- (modified) llvm/test/Transforms/LoopVectorize/runtime-check.ll (+20-27) 
- (modified) llvm/test/Transforms/LoopVectorize/runtime-checks-difference.ll (+60) 
- (modified) llvm/test/Transforms/LoopVectorize/runtime-checks-hoist.ll (+59-64) 
- (modified) llvm/test/Transforms/LoopVectorize/select-cmp-multiuse.ll (+94-112) 
- (modified) llvm/test/Transforms/LoopVectorize/unroll-novec-memcheck-metadata.ll (+1) 
- (modified) llvm/test/Transforms/LoopVectorize/widen-intrinsic.ll (+14-14) 


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</details>


https://github.com/llvm/llvm-project/pull/226816