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JIT: don't create degenerate flow in fgReplaceJumpTarget #99509

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merged 2 commits into from
Mar 11, 2024
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JIT: don't create degenerate flow in fgReplaceJumpTarget #99509

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JIT: don't create degenerate flow in `fgReplaceJumpTarget`
AndyAyersMS Mar 11, 2024
a59c6f2
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AndyAyersMS Mar 11, 2024
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43 changes: 33 additions & 10 deletions src/coreclr/jit/fgbasic.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -662,6 +662,8 @@ void Compiler::fgReplaceJumpTarget(BasicBlock* block, BasicBlock* oldTarget, Bas

if (block->FalseEdgeIs(oldEdge))
{
// Branch was degenerate, simplify it first
//
// fgRemoveRefPred returns nullptr for BBJ_COND blocks with two flow edges to target
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Since you're here, could you please remove this comment about fgRemoveRefPred returning nullptr? The new version of it that takes in a flow edge doesn't return anything. Thanks!

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Yep.

fgRemoveConditionalJump(block);
assert(block->KindIs(BBJ_ALWAYS));
Expand All @@ -671,10 +673,7 @@ void Compiler::fgReplaceJumpTarget(BasicBlock* block, BasicBlock* oldTarget, Bas
// fgRemoveRefPred should have removed the flow edge
fgRemoveRefPred(oldEdge);
assert(oldEdge->getDupCount() == 0);

// TODO-NoFallThrough: Proliferate weight from oldEdge
// (as a quirk, we avoid doing so for the true target to reduce diffs for now)
FlowEdge* const newEdge = fgAddRefPred(newTarget, block);
FlowEdge* const newEdge = fgAddRefPred(newTarget, block, oldEdge);

if (block->KindIs(BBJ_ALWAYS))
{
Expand All @@ -684,24 +683,48 @@ void Compiler::fgReplaceJumpTarget(BasicBlock* block, BasicBlock* oldTarget, Bas
{
assert(block->KindIs(BBJ_COND));
block->SetTrueEdge(newEdge);

if (oldEdge->hasLikelihood())
{
newEdge->setLikelihood(oldEdge->getLikelihood());
}
}
}
else
{
assert(block->FalseTargetIs(oldTarget));
FlowEdge* const oldEdge = block->GetFalseEdge();

if (block->TrueEdgeIs(oldEdge))
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I'm a bit confused by this check here. If a block's true and false edges are the same, then shouldn't the condition if (block->TrueTargetIs(oldTarget)) above be true, making this code unreachable?

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Yeah, good point.

{
// Branch was degenerate
//
// fgRemoveRefPred returns nullptr for BBJ_COND blocks with two flow edges to target
fgRemoveConditionalJump(block);
assert(block->KindIs(BBJ_ALWAYS));
assert(block->TargetIs(oldTarget));
}

// fgRemoveRefPred should have removed the flow edge
fgRemoveRefPred(oldEdge);
assert(oldEdge->getDupCount() == 0);
FlowEdge* const newEdge = fgAddRefPred(newTarget, block, oldEdge);
block->SetFalseEdge(newEdge);

if (block->KindIs(BBJ_ALWAYS))
{
block->SetTargetEdge(newEdge);
}
else
{
assert(block->KindIs(BBJ_COND));
block->SetFalseEdge(newEdge);
}
}

if (block->GetFalseEdge() == block->GetTrueEdge())
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I think we need to check if block is still a BBJ_COND here before calling GetFalseEdge/GetTrueEdge. If we converted it to a BBJ_ALWAYS above, this will assert.

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Added the check.

{
// Block became degenerate, simplify
//
fgRemoveConditionalJump(block);
assert(block->KindIs(BBJ_ALWAYS));
assert(block->TargetIs(newTarget));
}

break;

case BBJ_SWITCH:
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