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JIT: allow forward sub of QMARK nodes #76476

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Oct 1, 2022
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JIT: allow forward sub of QMARK nodes #76476

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42 changes: 36 additions & 6 deletions src/coreclr/jit/forwardsub.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -434,15 +434,13 @@ bool Compiler::fgForwardSubStatement(Statement* stmt)
GenTree* const rhsNode = rootNode->gtGetOp2();
GenTree* fwdSubNode = rhsNode;

// Can't substitute a qmark (unless the use is RHS of an assign... could check for this)
// Can't substitute GT_CATCH_ARG.
// Can't substitute GT_LCLHEAP.
//
// Don't substitute a no return call (trips up morph in some cases).
//
if (fwdSubNode->OperIs(GT_QMARK, GT_CATCH_ARG, GT_LCLHEAP))
if (fwdSubNode->OperIs(GT_CATCH_ARG, GT_LCLHEAP))
{
JITDUMP(" tree to sub is qmark, catch arg, or lcl heap\n");
JITDUMP(" tree to sub is catch arg, or lcl heap\n");
return false;
}

Expand Down Expand Up @@ -511,6 +509,40 @@ bool Compiler::fgForwardSubStatement(Statement* stmt)

JITDUMP(" [%06u] is only use of [%06u] (V%02u) ", dspTreeID(fsv.GetNode()), dspTreeID(lhsNode), lclNum);

// Qmarks must replace top-level uses. Also, restrict to GT_ASG.
// And also to where neither local is normalize on store, otherwise
// something downstream may add a cast over the qmark.
//
GenTree* const nextRootNode = nextStmt->GetRootNode();
if (fwdSubNode->OperIs(GT_QMARK))
{
if ((fsv.GetParentNode() != nextRootNode) || !nextRootNode->OperIs(GT_ASG))
{
JITDUMP(" can't fwd sub qmark as use is not top level ASG\n");
return false;
}

if (varDsc->lvNormalizeOnStore())
{
JITDUMP(" can't fwd sub qmark as V%02u is normalize on store\n", lclNum);
return false;
}

GenTree* const nextRootNodeLHS = nextRootNode->gtGetOp1();

if (nextRootNodeLHS->OperIs(GT_LCL_VAR))
{
const unsigned lhsLclNum = nextRootNodeLHS->AsLclVarCommon()->GetLclNum();
LclVarDsc* const lhsVarDsc = lvaGetDesc(lhsLclNum);

if (lhsVarDsc->lvNormalizeOnStore())
{
JITDUMP(" can't fwd sub qmark as V%02u is normalize on store\n", lhsLclNum);
return false;
}
}
}

// If next statement already has a large tree, hold off
// on making it even larger.
//
Expand All @@ -527,8 +559,6 @@ bool Compiler::fgForwardSubStatement(Statement* stmt)
// Next statement seems suitable.
// See if we can forward sub without changing semantics.
//
GenTree* const nextRootNode = nextStmt->GetRootNode();

// Bail if types disagree.
// Might be able to tolerate these by retyping.
//
Expand Down