From e9cdbf68e542bbb79597d6233dd2a339c89862a2 Mon Sep 17 00:00:00 2001 From: Benjamin Kramer Date: Sat, 16 Nov 2013 16:00:48 +0000 Subject: InstCombine: fold (A >> C) == (B >> C) --> (A^B) < (1 << C) for constant Cs. This is common in bitfield code. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194925 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/InstCombine/InstCombineCompares.cpp | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) (limited to 'lib') diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp index e624572..226126b 100644 --- a/lib/Transforms/InstCombine/InstCombineCompares.cpp +++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp @@ -2948,6 +2948,24 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) { Builder->CreateTrunc(B, A->getType())); } + // (A >> C) == (B >> C) --> (A^B) u< (1 << C) + // For lshr and ashr pairs. + if ((match(Op0, m_OneUse(m_LShr(m_Value(A), m_ConstantInt(Cst1)))) && + match(Op1, m_OneUse(m_LShr(m_Value(B), m_Specific(Cst1))))) || + (match(Op0, m_OneUse(m_AShr(m_Value(A), m_ConstantInt(Cst1)))) && + match(Op1, m_OneUse(m_AShr(m_Value(B), m_Specific(Cst1)))))) { + unsigned TypeBits = Cst1->getBitWidth(); + unsigned ShAmt = (unsigned)Cst1->getLimitedValue(TypeBits); + if (ShAmt < TypeBits && ShAmt != 0) { + ICmpInst::Predicate Pred = I.getPredicate() == ICmpInst::ICMP_NE + ? ICmpInst::ICMP_UGE + : ICmpInst::ICMP_ULT; + Value *Xor = Builder->CreateXor(A, B, I.getName() + ".unshifted"); + APInt CmpVal = APInt::getOneBitSet(TypeBits, ShAmt); + return new ICmpInst(Pred, Xor, Builder->getInt(CmpVal)); + } + } + // Transform "icmp eq (trunc (lshr(X, cst1)), cst" to // "icmp (and X, mask), cst" uint64_t ShAmt = 0; -- cgit v1.1