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[DFAJumpThreading] Prevent pass from using too much memory. #145482

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[DFAJumpThreading] Prevent pass from using too much memory. #145482

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[DFAJumpThreading] Prevent pass from using too much memory.
dybv-sc Jun 23, 2025
f0703ad
Fix formatting
dybv-sc Jun 24, 2025
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8 changes: 8 additions & 0 deletions llvm/lib/Transforms/Scalar/DFAJumpThreading.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -618,6 +618,8 @@ struct AllSwitchPaths {

VisitedBlocks UniqueBlocks;
for (auto *IncomingBB : Phi->blocks()) {
if (Res.size() >= MaxNumPaths)
break;
if (!UniqueBlocks.insert(IncomingBB).second)
continue;
if (!SwitchOuterLoop->contains(IncomingBB))
Expand Down Expand Up @@ -657,6 +659,8 @@ struct AllSwitchPaths {
getPathsFromStateDefMap(StateDef, IncomingPhi, VB);
for (ThreadingPath &Path : PredPaths) {
Path.push_back(PhiBB);
if (Res.size() >= MaxNumPaths)
break;
Res.push_back(std::move(Path));
}
continue;
Expand All @@ -679,6 +683,10 @@ struct AllSwitchPaths {
ThreadingPath NewPath(Path);
NewPath.appendExcludingFirst(IPath);
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IntermediatePaths are appended to the list of PredPaths (here, and also in the run() function), so the total number of paths is the multiplication of the sizes of the two lists. IntermediatePaths are already calculated so we are wasting compute cycles here by doing wasteful computation and by breaking the loop later than we should.

Since getPathsFromStateDefMap() and paths() are called multiple times and from different places they don't see the global picture of the number of paths we already have.

My suggestion is to add an argument to these two functions and update every call to these functions with a more appropriate limit instead of MaxNumPaths.

NewPath.push_back(PhiBB);
if (Res.size() >= MaxNumPaths) {
VB.erase(PhiBB);
return Res;
}
Res.push_back(NewPath);
}
}
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