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Teach instcombine to turn trunc(srl x, c) -> srl (trunc(x), c) when safe.
This implements InstCombine/cast.ll:test34. It fires hundreds of times on 176.gcc. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32009 91177308-0d34-0410-b5e6-96231b3b80d8
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@ -5986,7 +5986,39 @@ Instruction *InstCombiner::commonIntCastTransforms(CastInst &CI) {
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}
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Instruction *InstCombiner::visitTrunc(CastInst &CI) {
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return commonIntCastTransforms(CI);
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if (Instruction *Result = commonIntCastTransforms(CI))
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return Result;
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Value *Src = CI.getOperand(0);
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const Type *Ty = CI.getType();
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unsigned DestBitWidth = Ty->getPrimitiveSizeInBits();
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if (Instruction *SrcI = dyn_cast<Instruction>(Src)) {
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switch (SrcI->getOpcode()) {
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default: break;
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case Instruction::LShr:
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// We can shrink lshr to something smaller if we know the bits shifted in
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// are already zeros.
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if (ConstantInt *ShAmtV = dyn_cast<ConstantInt>(SrcI->getOperand(1))) {
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unsigned ShAmt = ShAmtV->getZExtValue();
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// Get a mask for the bits shifting in.
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uint64_t Mask = (~0ULL >> (64-ShAmt)) << DestBitWidth;
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if (SrcI->hasOneUse() && MaskedValueIsZero(SrcI->getOperand(0), Mask)) {
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if (ShAmt >= DestBitWidth) // All zeros.
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return ReplaceInstUsesWith(CI, Constant::getNullValue(Ty));
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// Okay, we can shrink this. Truncate the input, then return a new
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// shift.
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Value *V = InsertCastBefore(SrcI->getOperand(0), Ty, CI);
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return new ShiftInst(Instruction::LShr, V, SrcI->getOperand(1));
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}
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}
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break;
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}
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}
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return 0;
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}
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Instruction *InstCombiner::visitZExt(CastInst &CI) {
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