From 7159a307b9a0a53f9a33e416d24b9308ef547d97 Mon Sep 17 00:00:00 2001 From: Benjamin Kramer Date: Fri, 10 May 2013 16:26:37 +0000 Subject: InstCombine: Don't claim to be able to evaluate any shl in a zexted type. The shift amount may be larger than the type leading to undefined behavior. Limit the transform to constant shift amounts. While there update the bits to clear in the result which may enable additional optimizations. PR15959. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181604 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/InstCombine/InstCombineCasts.cpp | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) (limited to 'lib/Transforms') diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp index 2ee1278d23..361acdde81 100644 --- a/lib/Transforms/InstCombine/InstCombineCasts.cpp +++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp @@ -677,7 +677,6 @@ static bool CanEvaluateZExtd(Value *V, Type *Ty, unsigned &BitsToClear) { case Instruction::Add: case Instruction::Sub: case Instruction::Mul: - case Instruction::Shl: if (!CanEvaluateZExtd(I->getOperand(0), Ty, BitsToClear) || !CanEvaluateZExtd(I->getOperand(1), Ty, Tmp)) return false; @@ -701,6 +700,17 @@ static bool CanEvaluateZExtd(Value *V, Type *Ty, unsigned &BitsToClear) { // Otherwise, we don't know how to analyze this BitsToClear case yet. return false; + case Instruction::Shl: + // We can promote shl(x, cst) if we can promote x. Since shl overwrites the + // upper bits we can reduce BitsToClear by the shift amount. + if (ConstantInt *Amt = dyn_cast(I->getOperand(1))) { + if (!CanEvaluateZExtd(I->getOperand(0), Ty, BitsToClear)) + return false; + uint64_t ShiftAmt = Amt->getZExtValue(); + BitsToClear = ShiftAmt < BitsToClear ? BitsToClear - ShiftAmt : 0; + return true; + } + return false; case Instruction::LShr: // We can promote lshr(x, cst) if we can promote x. This requires the // ultimate 'and' to clear out the high zero bits we're clearing out though. -- cgit v1.2.3-70-g09d2