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Revert "[LoopUnroll] Clamp PartialThreshold for large LoopMicroOpBuff…
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…erSize (llvm#67657)"

This reverts commit f0b3654.

This commit triggers UB by reading an uninitialized variable.

`UP.PartialThreshold` is used uninitialized in `getUnrollingPreferences()` when
it is called from `LoopVectorizationPlanner::executePlan()`. In this case the
`UP` variable is created on the stack and its fields are not initialized.

```
==8802==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x557c0b081b99 in llvm::BasicTTIImplBase<llvm::X86TTIImpl>::getUnrollingPreferences(llvm::Loop*, llvm::ScalarEvolution&, llvm::TargetTransformInfo::UnrollingPreferences&, llvm::OptimizationRemarkEmitter*) llvm-project/llvm/include/llvm/CodeGen/BasicTTIImpl.h
    #1 0x557c0b07a40c in llvm::TargetTransformInfo::Model<llvm::X86TTIImpl>::getUnrollingPreferences(llvm::Loop*, llvm::ScalarEvolution&, llvm::TargetTransformInfo::UnrollingPreferences&, llvm::OptimizationRemarkEmitter*) llvm-project/llvm/include/llvm/Analysis/TargetTransformInfo.h:2277:17
    rust-lang#2 0x557c0f5d69ee in llvm::TargetTransformInfo::getUnrollingPreferences(llvm::Loop*, llvm::ScalarEvolution&, llvm::TargetTransformInfo::UnrollingPreferences&, llvm::OptimizationRemarkEmitter*) const llvm-project/llvm/lib/Analysis/TargetTransformInfo.cpp:387:19
    rust-lang#3 0x557c0e6b96a0 in llvm::LoopVectorizationPlanner::executePlan(llvm::ElementCount, unsigned int, llvm::VPlan&, llvm::InnerLoopVectorizer&, llvm::DominatorTree*, bool, llvm::DenseMap<llvm::SCEV const*, llvm::Value*, llvm::DenseMapInfo<llvm::SCEV const*, void>, llvm::detail::DenseMapPair<llvm::SCEV const*, llvm::Value*>> const*) llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7624:7
    rust-lang#4 0x557c0e6e4b63 in llvm::LoopVectorizePass::processLoop(llvm::Loop*) llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:10253:13
    rust-lang#5 0x557c0e6f2429 in llvm::LoopVectorizePass::runImpl(llvm::Function&, llvm::ScalarEvolution&, llvm::LoopInfo&, llvm::TargetTransformInfo&, llvm::DominatorTree&, llvm::BlockFrequencyInfo*, llvm::TargetLibraryInfo*, llvm::DemandedBits&, llvm::AssumptionCache&, llvm::LoopAccessInfoManager&, llvm::OptimizationRemarkEmitter&, llvm::ProfileSummaryInfo*) llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:10344:30
    rust-lang#6 0x557c0e6f2f97 in llvm::LoopVectorizePass::run(llvm::Function&, llvm::AnalysisManager<llvm::Function>&) llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:10383:9

[...]

  Uninitialized value was created by an allocation of 'UP' in the stack frame
    #0 0x557c0e6b961e in llvm::LoopVectorizationPlanner::executePlan(llvm::ElementCount, unsigned int, llvm::VPlan&, llvm::InnerLoopVectorizer&, llvm::DominatorTree*, bool, llvm::DenseMap<llvm::SCEV const*, llvm::Value*, llvm::DenseMapInfo<llvm::SCEV const*, void>, llvm::detail::DenseMapPair<llvm::SCEV const*, llvm::Value*>> const*) llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7623:3
```
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gribozavr committed May 16, 2024
1 parent 5ffd154 commit 83974a4
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Showing 2 changed files with 742 additions and 30 deletions.
8 changes: 1 addition & 7 deletions llvm/include/llvm/CodeGen/BasicTTIImpl.h
Original file line number Diff line number Diff line change
Expand Up @@ -612,13 +612,7 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
if (PartialUnrollingThreshold.getNumOccurrences() > 0)
MaxOps = PartialUnrollingThreshold;
else if (ST->getSchedModel().LoopMicroOpBufferSize > 0)
// Upper bound by the default PartialThreshold, which is the same as
// the default full-unroll Threshold. Even if the loop micro-op buffer
// is very large, this does not mean that we want to unroll all loops
// to that length, as it would increase code size beyond the limits of
// what unrolling normally allows.
MaxOps = std::min(ST->getSchedModel().LoopMicroOpBufferSize,
UP.PartialThreshold);
MaxOps = ST->getSchedModel().LoopMicroOpBufferSize;
else
return;

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