[PATCH] D19073: [ValueTracking] Improve isImpliedCondition for conditions with matching LHS operands and Immediate RHS operands.

[Haicheng Wu via llvm-commits]

haicheng added a subscriber: haicheng.
haicheng added a comment.

Hi Chad,

I think LazyValueInfo can figure out the branch direction in your test cases and then JumpThreading can optimize the CFG.

The related JumpThreading code is

JumpThreading.cpp

bool JumpThreading::ProcessBlock(BasicBlock *BB)

  if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
    // If we're branching on a conditional, LVI might be able to determine
    // it's value at the branch instruction.  We only handle comparisons
    // against a constant at this time.
    // TODO: This should be extended to handle switches as well.
    BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
    Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
    if (CondBr && CondConst && CondBr->isConditional()) {
      LazyValueInfo::Tristate Ret =
        LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0),
                            CondConst, CondBr);
      if (Ret != LazyValueInfo::Unknown) {
        unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0;
        unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1;
        CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
        BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
        CondBr->eraseFromParent();
        if (CondCmp->use_empty())
          CondCmp->eraseFromParent();
        else if (CondCmp->getParent() == BB) {
          // If the fact we just learned is true for all uses of the
          // condition, replace it with a constant value
          auto *CI = Ret == LazyValueInfo::True ?
            ConstantInt::getTrue(CondCmp->getType()) :
            ConstantInt::getFalse(CondCmp->getType());
          CondCmp->replaceAllUsesWith(CI);
          CondCmp->eraseFromParent();
        }
        return true;
      }
    }
  
    if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
      return true;
  }

Best,

Haicheng


http://reviews.llvm.org/D19073