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JIT: Renable SSA-aware PHI jump threading #127103

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AndyAyersMS merged 1 commit into from
Apr 20, 2026
+29 −7
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JIT: Renable SSA-aware PHI jump threading #127103

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36 changes: 29 additions & 7 deletions src/coreclr/jit/redundantbranchopts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1764,7 +1764,7 @@ Compiler::JumpThreadCheckResult Compiler::optJumpThreadCheck(BasicBlock* const b
{
JITDUMP(FMT_BB " has global phi for V%02u.%u; no phi-based threading\n", block->bbNum, lclNum,
ssaNum);
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return JumpThreadCheckResult::CannotThread;
hasGlobalPhiUses = true;
}
}

Expand Down Expand Up @@ -2303,23 +2303,45 @@ bool Compiler::optJumpThreadCore(JumpThreadInfo& jti)

GenTreeLclVarCommon* const use = phiUse.m_use;
unsigned const oldSsaNum = use->GetSsaNum();
unsigned const lclNum = use->GetLclNum();

if (oldSsaNum == phiUse.m_replacementSsaNum)
{
continue;
}
assert(oldSsaNum != phiUse.m_replacementSsaNum);
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JITDUMP("Updating [%06u] in " FMT_BB " from u:%u to u:%u\n", dspTreeID(use), phiUse.m_block->bbNum, oldSsaNum,
phiUse.m_replacementSsaNum);

LclSsaVarDsc* const replacementSsaDef = lvaGetDesc(use->GetLclNum())->GetPerSsaData(phiUse.m_replacementSsaNum);
LclSsaVarDsc* const replacementSsaDef = lvaGetDesc(lclNum)->GetPerSsaData(phiUse.m_replacementSsaNum);

use->SetSsaNum(phiUse.m_replacementSsaNum);

// Keep the use's value number in sync with the rewritten SSA def.
//
if (use->gtVNPair != replacementSsaDef->m_vnPair)
{
use->SetVNs(replacementSsaDef->m_vnPair);
ValueNumPair newVNPair = replacementSsaDef->m_vnPair;

// If this is a field use, get the proper field VN.

if (use->OperIs(GT_LCL_FLD))
{
GenTreeLclFld* const lclFld = use->AsLclFld();

newVNPair = vnStore->VNPairForLoad(replacementSsaDef->m_vnPair, lvaLclValueSize(lclNum),
lclFld->TypeGet(), lclFld->GetLclOffs(), lclFld->GetValueSize());
}
else
{
assert(use->OperIs(GT_LCL_VAR));
}

JITDUMP("Updating [%06u] VN from ", dspTreeID(use));
JITDUMPEXEC(vnpPrint(use->gtVNPair, 1));
JITDUMP(" to ");
JITDUMPEXEC(vnpPrint(newVNPair, 1));
JITDUMP("\n");

use->SetVNs(newVNPair);

GenTree* node = use;
GenTree* parent = node->gtGetParent(nullptr);

Expand Down
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