From 49064ff7708bcd3b720ce79bc7629c2f3207b8d6 Mon Sep 17 00:00:00 2001 From: Benjamin Kramer Date: Sat, 24 Dec 2011 17:31:53 +0000 Subject: [PATCH] InstCombine: Add a combine that turns (2^n)-1 ^ x back into (2^n)-1 - x iff x is smaller than 2^n and it fuses with a following add. This was intended to undo the sub canonicalization in cases where it's not profitable, but it also finds some cases on it's own. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147256 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/InstCombine/InstCombineAddSub.cpp | 13 +++++++++++++ test/Transforms/InstCombine/sub-xor.ll | 12 ++++++++++++ 2 files changed, 25 insertions(+) diff --git a/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/lib/Transforms/InstCombine/InstCombineAddSub.cpp index 604d9c8bb44..f2be01b73be 100644 --- a/lib/Transforms/InstCombine/InstCombineAddSub.cpp +++ b/lib/Transforms/InstCombine/InstCombineAddSub.cpp @@ -136,6 +136,19 @@ Instruction *InstCombiner::visitAdd(BinaryOperator &I) { Value *NewShl = Builder->CreateShl(XorLHS, ShAmt, "sext"); return BinaryOperator::CreateAShr(NewShl, ShAmt); } + + // If this is a xor that was canonicalized from a sub, turn it back into + // a sub and fuse this add with it. + if (LHS->hasOneUse() && (XorRHS->getValue()+1).isPowerOf2()) { + IntegerType *IT = cast(I.getType()); + APInt Mask = APInt::getAllOnesValue(IT->getBitWidth()); + APInt LHSKnownOne(IT->getBitWidth(), 0); + APInt LHSKnownZero(IT->getBitWidth(), 0); + ComputeMaskedBits(XorLHS, Mask, LHSKnownZero, LHSKnownOne); + if ((XorRHS->getValue() | LHSKnownZero).isAllOnesValue()) + return BinaryOperator::CreateSub(ConstantExpr::getAdd(XorRHS, CI), + XorLHS); + } } } diff --git a/test/Transforms/InstCombine/sub-xor.ll b/test/Transforms/InstCombine/sub-xor.ll index bfa9f408c6f..279e4aca9de 100644 --- a/test/Transforms/InstCombine/sub-xor.ll +++ b/test/Transforms/InstCombine/sub-xor.ll @@ -23,3 +23,15 @@ define i32 @test2(i32 %x) nounwind { ; CHECK-NEXT: xor i32 %count, 31 ; CHECK-NEXT: ret } + +define i32 @test3(i32 %x) nounwind { + %and = and i32 %x, 31 + %sub = xor i32 31, %and + %add = add i32 %sub, 42 + ret i32 %add + +; CHECK: @test3 +; CHECK-NEXT: and i32 %x, 31 +; CHECK-NEXT: sub i32 73, %and +; CHECK-NEXT: ret +} -- 2.34.1