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JIT: Fix weights in fgOptimizeUncondBranchToSimpleCond #50490

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JIT: Fix weights in fgOptimizeUncondBranchToSimpleCond #50490

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3d258c0
Address feedback
EgorBo Apr 29, 2021
c43a576
Update fgopt.cpp
EgorBo May 24, 2021
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2 changes: 2 additions & 0 deletions src/coreclr/jit/block.h
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Expand Up @@ -1311,6 +1311,8 @@ struct flowList
bool* wbUsedSlop);
void setEdgeWeights(BasicBlock::weight_t newMinWeight, BasicBlock::weight_t newMaxWeight, BasicBlock* bDst);

void scaleEdgeWeights(BasicBlock::weight_t scale, BasicBlock* bDst);

flowList(BasicBlock* block, flowList* rest)
: flNext(rest), m_block(block), flEdgeWeightMin(0), flEdgeWeightMax(0), flDupCount(0)
{
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32 changes: 32 additions & 0 deletions src/coreclr/jit/fgopt.cpp
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Expand Up @@ -3208,6 +3208,16 @@ bool Compiler::fgOptimizeUncondBranchToSimpleCond(BasicBlock* block, BasicBlock*
// backend always calls `fgUpdateFlowGraph` with `doTailDuplication` set to false.
assert(!block->IsLIR());

// See https://github.com/dotnet/runtime/pull/50490#pullrequestreview-625700642
//
// A - block
// X - target
// C - target->bbJumpDest
// D - target->bbNext

const BasicBlock::weight_t scaleA = block->bbWeight / target->bbWeight;
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You probably should watch for division by zero here.

const BasicBlock::weight_t scaleNotA = 1 - scaleA;

Statement* stmt = target->FirstNonPhiDef();
assert(stmt == target->lastStmt());

Expand All @@ -3230,6 +3240,28 @@ bool Compiler::fgOptimizeUncondBranchToSimpleCond(BasicBlock* block, BasicBlock*
fgAddRefPred(next, block);
fgAddRefPred(next->bbJumpDest, next);

// weight(X) = weight(X) * ScaleNotA
target->scaleBBWeight(scaleNotA);

// weight(X->C) = weight(X->C) * ScaleNotA
flowList* xcEdge = fgGetPredForBlock(target->bbJumpDest, target);
xcEdge->scaleEdgeWeights(scaleNotA, target->bbJumpDest);

// weight(X->D) = weight(X->D) * ScaleNotA
flowList* xdEdge = fgGetPredForBlock(target->bbNext, target);
xdEdge->scaleEdgeWeights(scaleNotA, target->bbNext);

// weight(A->C) = weight(X->C) * ScaleA
flowList* acEdge = fgGetPredForBlock(target->bbJumpDest, block);
acEdge->setEdgeWeights(xcEdge->edgeWeightMin() * scaleA, xcEdge->edgeWeightMin() * scaleA, target->bbJumpDest);

// weight(A->T) = weight(X->D) * ScaleA
flowList* atEdge = fgGetPredForBlock(next, block);
atEdge->setEdgeWeights(xdEdge->edgeWeightMin() * scaleA, xdEdge->edgeWeightMin() * scaleA, target->bbNext);

// weight(T) = weight(X->D) * ScaleA
next->setBBProfileWeight(xdEdge->edgeWeightMax() * scaleA);

JITDUMP("fgOptimizeUncondBranchToSimpleCond(from " FMT_BB " to cond " FMT_BB "), created new uncond " FMT_BB "\n",
block->bbNum, target->bbNum, next->bbNum);
JITDUMP(" expecting opts to key off V%02u, added cloned compare [%06u] to " FMT_BB "\n", lclNum,
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16 changes: 16 additions & 0 deletions src/coreclr/jit/fgprofile.cpp
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Expand Up @@ -2750,6 +2750,22 @@ void flowList::setEdgeWeights(BasicBlock::weight_t theMinWeight, BasicBlock::wei
flEdgeWeightMax = theMaxWeight;
}

//------------------------------------------------------------------------
// scaleEdgeWeights: Scale both min and max weights by some factor
//
// Arguments:
// scale - scale to apply
// bDst - the destination block for the edge
//
void flowList::scaleEdgeWeights(BasicBlock::weight_t scale, BasicBlock* bDst)
{
JITDUMP("Scalling edge weights for " FMT_BB " -> " FMT_BB " by " FMT_WT "\n", getBlock()->bbNum, bDst->bbNum,
scale);

flEdgeWeightMin *= scale;
flEdgeWeightMax *= scale;
}

//-------------------------------------------------------------
// fgComputeBlockAndEdgeWeights: determine weights for blocks
// and optionally for edges
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