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JIT: fix exit edge likelihood computation for capped loops #84817

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merged 2 commits into from
Apr 14, 2023
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JIT: fix exit edge likelihood computation for capped loops #84817

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ad07cf4
JIT: fix exit edge likelihood computation for capped loops
AndyAyersMS Apr 13, 2023
9d8e665
adjust to avoid cases where exit weight is near or above 1.0 already
AndyAyersMS Apr 14, 2023
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120 changes: 88 additions & 32 deletions src/coreclr/jit/fgprofilesynthesis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1140,12 +1140,16 @@ void ProfileSynthesis::ComputeCyclicProbabilities(SimpleLoop* loop)
//
if (cyclicWeight > cappedLikelihood)
{
JITDUMP("Cyclic weight " FMT_WT " > " FMT_WT "(cap) -- will reduce to cap\n", cyclicWeight, cappedLikelihood);
capped = true;
cyclicWeight = cappedLikelihood;
m_cappedCyclicProbabilities++;
}

weight_t cyclicProbability = 1.0 / (1.0 - cyclicWeight);
// Note this value is not actually a probability; it is the expected
// iteration count of the loop.
//
weight_t const cyclicProbability = 1.0 / (1.0 - cyclicWeight);

JITDUMP("For loop at " FMT_BB " cyclic weight is " FMT_WT " cyclic probability is " FMT_WT "%s\n",
loop->m_head->bbNum, cyclicWeight, cyclicProbability, capped ? " [capped]" : "");
Expand All @@ -1154,50 +1158,102 @@ void ProfileSynthesis::ComputeCyclicProbabilities(SimpleLoop* loop)

// Try and adjust loop exit likelihood to reflect capping.
// If there are multiple exits we just adjust the first one we can. This is somewhat arbitrary.
// If there are no exits, there's nothing we can do.
//
if (capped)
if (capped && (loop->m_exitEdges.size() > 0))
{
bool adjustedExit = false;
// Figure out how much flow exits the loop with the capped probablility
// and current block frequencies and exit likelihoods.
//
weight_t cappedExitWeight = 0.0;

for (FlowEdge* const exitEdge : loop->m_exitEdges)
{
BasicBlock* const exitBlock = exitEdge->getSourceBlock();
BasicBlock* const exitBlock = exitEdge->getSourceBlock();
weight_t const exitBlockFrequency = exitBlock->bbWeight;
weight_t const exitBlockWeight = exitBlockFrequency * cyclicProbability;
weight_t const exitWeight = exitEdge->getLikelihood() * exitBlockWeight;
cappedExitWeight += exitWeight;
JITDUMP("Exit from " FMT_BB " has weight " FMT_WT "\n", exitBlock->bbNum, exitWeight);
}

JITDUMP("Total exit weight " FMT_WT "\n", cappedExitWeight);

// We should end up with a value less than one since we input one unit of flow into the
// loop and are artificially capping the iteration count of the loop, so less weight is
// now flowing out than in. However because of rounding we might end up near or a bit over 1.0.
//
if ((cappedExitWeight + epsilon) < 1.0)
{
// We want to increase the exit likelihood of one exit block to create
// additional flow out of the loop. Figure out how much we need.
//
weight_t const missingExitWeight = 1.0 - cappedExitWeight;
JITDUMP("Loop exit flow deficit from capping is " FMT_WT "\n", missingExitWeight);

if ((exitBlock->bbJumpKind == BBJ_COND) &&
!Compiler::fgProfileWeightsEqual(exitBlock->bbWeight, 0.0, epsilon))
bool adjustedExit = false;

for (FlowEdge* const exitEdge : loop->m_exitEdges)
{
JITDUMP("Will adjust likelihood of the exit edge from loop exit block " FMT_BB
" to reflect capping; current likelihood is " FMT_WT "\n",
exitBlock->bbNum, exitEdge->getLikelihood());

BasicBlock* const jump = exitBlock->bbJumpDest;
BasicBlock* const next = exitBlock->bbNext;
FlowEdge* const jumpEdge = m_comp->fgGetPredForBlock(jump, exitBlock);
FlowEdge* const nextEdge = m_comp->fgGetPredForBlock(next, exitBlock);
weight_t const continueLikelihood = cappedLikelihood / exitBlock->bbWeight;
weight_t const breakLikelihood = 1 - continueLikelihood;

if (jumpEdge == exitEdge)
{
jumpEdge->setLikelihood(breakLikelihood);
nextEdge->setLikelihood(continueLikelihood);
}
else
// Does this block have enough weight that it can supply all the missing weight?
//
BasicBlock* const exitBlock = exitEdge->getSourceBlock();
weight_t const exitBlockFrequency = exitBlock->bbWeight;
weight_t const exitBlockWeight = exitBlockFrequency * cyclicProbability;
weight_t const currentExitWeight = exitEdge->getLikelihood() * exitBlockWeight;

// TODO: we might also want to exclude edges that are exiting from child loops here,
// or think harder about what might be appropriate in those cases. Seems like we ought
// to adjust an edge's likelihoods at most once.
//
// Currently we don't know which edges do this.
//
if ((exitBlock->bbJumpKind == BBJ_COND) && (exitBlockWeight > (missingExitWeight + currentExitWeight)))
{
assert(nextEdge == exitEdge);
jumpEdge->setLikelihood(continueLikelihood);
nextEdge->setLikelihood(breakLikelihood);
JITDUMP("Will adjust likelihood of the exit edge from loop exit block " FMT_BB
" to reflect capping; current likelihood is " FMT_WT "\n",
exitBlock->bbNum, exitEdge->getLikelihood());

BasicBlock* const jump = exitBlock->bbJumpDest;
BasicBlock* const next = exitBlock->bbNext;
FlowEdge* const jumpEdge = m_comp->fgGetPredForBlock(jump, exitBlock);
FlowEdge* const nextEdge = m_comp->fgGetPredForBlock(next, exitBlock);
weight_t const exitLikelihood = (missingExitWeight + currentExitWeight) / exitBlockWeight;
weight_t const continueLikelihood = 1.0 - exitLikelihood;

// We are making it more likely that the loop exits, so the new exit likelihood
// should be greater than the old.
//
assert(exitLikelihood > exitEdge->getLikelihood());

if (jumpEdge == exitEdge)
{
jumpEdge->setLikelihood(exitLikelihood);
nextEdge->setLikelihood(continueLikelihood);
}
else
{
assert(nextEdge == exitEdge);
jumpEdge->setLikelihood(continueLikelihood);
nextEdge->setLikelihood(exitLikelihood);
}
adjustedExit = true;

JITDUMP("New likelihood is " FMT_WT "\n", exitEdge->getLikelihood());
break;
}
adjustedExit = true;
}

JITDUMP("New likelihood is " FMT_WT "\n", exitEdge->getLikelihood());
break;
if (!adjustedExit)
{
// Possibly we could have fixed things up by adjusting more than one exit?
//
JITDUMP("Unable to find suitable exit to carry off capped flow\n");
}
}

if (!adjustedExit)
else
{
JITDUMP("Unable to find suitable exit to carry off capped flow\n");
JITDUMP("Exit weight comparable or above 1.0, leaving as is\n");
}
}
}
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