From 6aa5eb19d53795dd026921859350df89b0169dfa Mon Sep 17 00:00:00 2001 From: Chris Lattner Date: Wed, 29 Nov 2006 07:04:07 +0000 Subject: [PATCH] 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 --- .../Scalar/InstructionCombining.cpp | 34 ++++++++++++++++++- 1 file changed, 33 insertions(+), 1 deletion(-) diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index 92760b60ac1..a5bf60bdeb5 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -5986,7 +5986,39 @@ Instruction *InstCombiner::commonIntCastTransforms(CastInst &CI) { } Instruction *InstCombiner::visitTrunc(CastInst &CI) { - return commonIntCastTransforms(CI); + if (Instruction *Result = commonIntCastTransforms(CI)) + return Result; + + Value *Src = CI.getOperand(0); + const Type *Ty = CI.getType(); + unsigned DestBitWidth = Ty->getPrimitiveSizeInBits(); + + if (Instruction *SrcI = dyn_cast(Src)) { + switch (SrcI->getOpcode()) { + default: break; + case Instruction::LShr: + // We can shrink lshr to something smaller if we know the bits shifted in + // are already zeros. + if (ConstantInt *ShAmtV = dyn_cast(SrcI->getOperand(1))) { + unsigned ShAmt = ShAmtV->getZExtValue(); + + // Get a mask for the bits shifting in. + uint64_t Mask = (~0ULL >> (64-ShAmt)) << DestBitWidth; + if (SrcI->hasOneUse() && MaskedValueIsZero(SrcI->getOperand(0), Mask)) { + if (ShAmt >= DestBitWidth) // All zeros. + return ReplaceInstUsesWith(CI, Constant::getNullValue(Ty)); + + // Okay, we can shrink this. Truncate the input, then return a new + // shift. + Value *V = InsertCastBefore(SrcI->getOperand(0), Ty, CI); + return new ShiftInst(Instruction::LShr, V, SrcI->getOperand(1)); + } + } + break; + } + } + + return 0; } Instruction *InstCombiner::visitZExt(CastInst &CI) { -- 2.34.1