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[JumpThreading] Thread through guards

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Summary:
This patch allows JumpThreading also thread through guards.
Virtually, guard(cond) is equivalent to the following construction:

  if (cond) { do something } else {deoptimize}

Yet it is not explicitly converted into IFs before lowering.
This patch enables early threading through guards in simple cases.
Currently it covers the following situation:

  if (cond1) {
    // code A
  } else {
    // code B
  }
  // code C
  guard(cond2)
  // code D

If there is implication cond1 => cond2 or !cond1 => cond2, we can transform
this construction into the following:

  if (cond1) {
    // code A
    // code C
  } else {
    // code B
    // code C
    guard(cond2)
  }
  // code D

Thus, removing the guard from one of execution branches.

Patch by Max Kazantsev!

Reviewers: reames, apilipenko, igor-laevsky, anna, sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29620

llvm-svn: 294617
This commit is contained in:
Sanjoy Das
2017-02-09 19:40:22 +00:00
parent 1677631546
commit 74bda4d591
6 changed files with 382 additions and 14 deletions
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37
llvm/lib/Transforms/Utils/CloneFunction.cpp
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@@ -747,3 +747,40 @@ Loop *llvm::cloneLoopWithPreheader(BasicBlock *Before, BasicBlock *LoopDomBB,
return NewLoop;
}
/// \brief Duplicate non-Phi instructions from the beginning of block up to
/// StopAt instruction into a split block between BB and its predecessor.
BasicBlock *
llvm::DuplicateInstructionsInSplitBetween(BasicBlock *BB, BasicBlock *PredBB,
Instruction *StopAt,
ValueToValueMapTy &ValueMapping) {
// We are going to have to map operands from the original BB block to the new
// copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
// account for entry from PredBB.
BasicBlock::iterator BI = BB->begin();
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
BasicBlock *NewBB = SplitEdge(PredBB, BB);
NewBB->setName(PredBB->getName() + ".split");
Instruction *NewTerm = NewBB->getTerminator();
// Clone the non-phi instructions of BB into NewBB, keeping track of the
// mapping and using it to remap operands in the cloned instructions.
for (; StopAt != &*BI; ++BI) {
Instruction *New = BI->clone();
New->setName(BI->getName());
New->insertBefore(NewTerm);
ValueMapping[&*BI] = New;
// Remap operands to patch up intra-block references.
for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
auto I = ValueMapping.find(Inst);
if (I != ValueMapping.end())
New->setOperand(i, I->second);
}
}
return NewBB;
}