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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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37 changes: 26 additions & 11 deletions src/coreclr/jit/fgbasic.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -662,7 +662,8 @@ void Compiler::fgReplaceJumpTarget(BasicBlock* block, BasicBlock* oldTarget, Bas

if (block->FalseEdgeIs(oldEdge))
{
// fgRemoveRefPred returns nullptr for BBJ_COND blocks with two flow edges to target
// Branch was degenerate, simplify it first
//
fgRemoveConditionalJump(block);
assert(block->KindIs(BBJ_ALWAYS));
assert(block->TargetIs(oldTarget));
Expand All @@ -671,10 +672,7 @@ 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 +682,41 @@ 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();

// Already degenerate cases should have taken the true path above.
assert(!block->TrueEdgeIs(oldEdge));

// 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->KindIs(BBJ_COND) && (block->GetFalseEdge() == block->GetTrueEdge()))
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I noticed that we tend to compare the true/false edges with TrueEdgeIs. I'm planning on modifying fgReplaceJumpTarget in an upcoming PR, so I'll fix this then.

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

break;

case BBJ_SWITCH:
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