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JIT: Detect and handle imprecision in 3-opt layout cost computation #110069

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JIT: Detect and handle imprecision in 3-opt layout cost computation #110069

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20 changes: 16 additions & 4 deletions src/coreclr/jit/fgopt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4927,21 +4927,33 @@ Compiler::ThreeOptLayout::ThreeOptLayout(Compiler* comp)
// endPos - The inclusive ending index of the region
//
// Returns:
// The region's layout cost
// The region's layout cost, or 'BB_MAX_WEIGHT' for pathalogically costly layouts
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// The region's layout cost, or 'BB_MAX_WEIGHT' for pathalogically costly layouts
// The region's layout cost, or 'BB_MAX_WEIGHT' for pathologically costly layouts

//
weight_t Compiler::ThreeOptLayout::GetLayoutCost(unsigned startPos, unsigned endPos)
{
assert(startPos <= endPos);
assert(endPos < numCandidateBlocks);
weight_t layoutCost = BB_ZERO_WEIGHT;

// As 'layoutCost' grows, we may begin to incur noticeable floating-point imprecision,
// which complicates comparing layout costs.
// Detect this, and return 'BB_MAX_WEIGHT'.
auto sumAndCheckForImprecision = [&layoutCost](weight_t cost) -> bool {
assert(cost >= BB_ZERO_WEIGHT);
const weight_t oldLayoutCost = layoutCost;
layoutCost += cost;
return !Compiler::fgProfileWeightsEqual(cost, layoutCost - oldLayoutCost, 0.001);
};

for (unsigned position = startPos; position < endPos; position++)
{
layoutCost += GetCost(blockOrder[position], blockOrder[position + 1]);
if (sumAndCheckForImprecision(GetCost(blockOrder[position], blockOrder[position + 1])))
{
return BB_MAX_WEIGHT;
}
}

layoutCost += blockOrder[endPos]->bbWeight;
return layoutCost;
return sumAndCheckForImprecision(blockOrder[endPos]->bbWeight) ? BB_MAX_WEIGHT : layoutCost;
}
#endif // DEBUG

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