[llvm] Issues found PVS studio static analyzer. LoopUnrollAndJam.cpp

[Bhuvan1527]

Minor misoptimization. The PVS-Studio warning: V791 The initial value of the index in the nested loop equals 'I'. Perhaps, 'I + 1' should be used instead. LoopUnrollAndJam.cpp 793

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[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Bala_Bhuvan_Varma (Bhuvan1527)

Changes

Minor misoptimization. The PVS-Studio warning: V791 The initial value of the index in the nested loop equals 'I'. Perhaps, 'I + 1' should be used instead. LoopUnrollAndJam.cpp 793

---

Full diff: https://github.com/llvm/llvm-project/pull/170162.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp (+1-1)

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp b/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp index 1e614bd29ee6e..663e45e89c5da 100644 --- a/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp +++ b/llvm/lib/Transforms/Utils/LoopUnrollAndJam.cpp @@ -789,7 +789,7 @@ checkDependencies(Loop &Root, const BasicBlockSet &SubLoopBlocks, size_t NumInsts = CurrentLoadsAndStores.size(); for (size_t I = 0; I < NumInsts; ++I) { - for (size_t J = I; J < NumInsts; ++J) { + for (size_t J = I + 1; J < NumInsts; ++J) { if (!checkDependency(CurrentLoadsAndStores[I], CurrentLoadsAndStores[J], LoopDepth, CurLoopDepth, true, DI)) return false; 

[boomanaiden154]

How does this preserve the existing semantics or serve as an optimization in any way?

[Bhuvan1527]

How does this preserve the existing semantics or serve as an optimization in any way?

Hi @boomanaiden154
Well, I went with the logic that,

When var J = I, then we are checking the dependency of an instruction with itself, is that check even necessary?

So, this will save the existing semantics. I even ran all the llvm tests before raising the pr.

And the optimization can be reducing an iteration of J loop I believe.

Would love to discuss more, if you have question. I'm new to LLVM. So lets learn.

Thanks