From 9697c664a6bd25a3054fb814a6e5e1b90f7d4fee Mon Sep 17 00:00:00 2001 From: Sanjay Patel Date: Tue, 2 May 2017 14:31:30 +0000 Subject: [PATCH] [InstCombine] don't use DeMorgan's Law on integer constants This is the fold that causes the infinite loop in BoringSSL (https://github.com/google/boringssl/blob/master/crypto/cipher/e_rc2.c) when we fix instcombine demanded bits to prefer 'not' ops as in D32255. There are 2 or 3 problems with dyn_castNotVal, and I don't think we can reinstate D32255 until dyn_castNotVal is completely eliminated. 1. As shown here, it transforms 'not' into random xor. This transform is harmful to SCEV and codegen because 'not' can often be folded while random xor cannot. 2. It does not transform vector constants. This is actually a good thing, but if you don't believe the above argument, then we shouldn't have excluded vectors. 3. It tries to avoid transforming not(not(X)). That's nice, but it doesn't match the greedy nature of instcombine. If we DeMorganize a pattern that has an extra 'not' in it: ~(~(~X) & Y) --> (~X | ~Y) That's just another case of DeMorgan, so we should trust that we'll fold that pattern too: (~X | ~ Y) --> ~(X & Y) Differential Revision: https://reviews.llvm.org/D32665 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301923 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Transforms/InstCombine/InstCombineAndOrXor.cpp | 39 ++++++++++++---------- test/Transforms/InstCombine/assume2.ll | 8 ++--- test/Transforms/InstCombine/demorgan.ll | 12 +++---- 3 files changed, 31 insertions(+), 28 deletions(-) diff --git a/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp index 41ae37ee127..c7092bf3a39 100644 --- a/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp +++ b/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp @@ -2433,29 +2433,32 @@ Instruction *InstCombiner::visitXor(BinaryOperator &I) { if (Value *V = SimplifyBSwap(I)) return replaceInstUsesWith(I, V); + // Apply DeMorgan's Law for 'nand' / 'nor' logic with an inverted operand. + Value *X, *Y; + + // We must eliminate the and/or (one-use) for these transforms to not increase + // the instruction count. + // ~(~X & Y) --> (X | ~Y) + // ~(Y & ~X) --> (X | ~Y) + if (match(&I, m_Not(m_OneUse(m_c_And(m_Not(m_Value(X)), m_Value(Y)))))) { + Value *NotY = Builder->CreateNot(Y, Y->getName() + ".not"); + return BinaryOperator::CreateOr(X, NotY); + } + // ~(~X | Y) --> (X & ~Y) + // ~(Y | ~X) --> (X & ~Y) + if (match(&I, m_Not(m_OneUse(m_c_Or(m_Not(m_Value(X)), m_Value(Y)))))) { + Value *NotY = Builder->CreateNot(Y, Y->getName() + ".not"); + return BinaryOperator::CreateAnd(X, NotY); + } + // Is this a 'not' (~) fed by a binary operator? BinaryOperator *NotOp; if (match(&I, m_Not(m_BinOp(NotOp)))) { if (NotOp->getOpcode() == Instruction::And || NotOp->getOpcode() == Instruction::Or) { - // We must eliminate the and/or for this transform to not increase the - // instruction count. - if (NotOp->hasOneUse()) { - // ~(~X & Y) --> (X | ~Y) - De Morgan's Law - // ~(~X | Y) === (X & ~Y) - De Morgan's Law - if (dyn_castNotVal(NotOp->getOperand(1))) - NotOp->swapOperands(); - if (Value *Op0NotVal = dyn_castNotVal(NotOp->getOperand(0))) { - Value *NotY = Builder->CreateNot( - NotOp->getOperand(1), NotOp->getOperand(1)->getName() + ".not"); - if (NotOp->getOpcode() == Instruction::And) - return BinaryOperator::CreateOr(Op0NotVal, NotY); - return BinaryOperator::CreateAnd(Op0NotVal, NotY); - } - } - - // ~(X & Y) --> (~X | ~Y) - De Morgan's Law - // ~(X | Y) === (~X & ~Y) - De Morgan's Law + // Apply DeMorgan's Law when inverts are free: + // ~(X & Y) --> (~X | ~Y) + // ~(X | Y) --> (~X & ~Y) if (IsFreeToInvert(NotOp->getOperand(0), NotOp->getOperand(0)->hasOneUse()) && IsFreeToInvert(NotOp->getOperand(1), diff --git a/test/Transforms/InstCombine/assume2.ll b/test/Transforms/InstCombine/assume2.ll index e8fbc049f41..8dc8831fffa 100644 --- a/test/Transforms/InstCombine/assume2.ll +++ b/test/Transforms/InstCombine/assume2.ll @@ -21,8 +21,8 @@ define i32 @test1(i32 %a) #0 { define i32 @test2(i32 %a) #0 { ; CHECK-LABEL: @test2( -; CHECK-NEXT: [[A_NOT:%.*]] = or i32 [[A:%.*]], -16 -; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[A_NOT]], -6 +; CHECK-NEXT: [[AND:%.*]] = and i32 [[A:%.*]], 15 +; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[AND]], 10 ; CHECK-NEXT: tail call void @llvm.assume(i1 [[CMP]]) ; CHECK-NEXT: ret i32 2 ; @@ -50,8 +50,8 @@ define i32 @test3(i32 %a) #0 { define i32 @test4(i32 %a) #0 { ; CHECK-LABEL: @test4( -; CHECK-NEXT: [[A_NOT:%.*]] = and i32 [[A:%.*]], 15 -; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[A_NOT]], 10 +; CHECK-NEXT: [[V:%.*]] = or i32 [[A:%.*]], -16 +; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[V]], -6 ; CHECK-NEXT: tail call void @llvm.assume(i1 [[CMP]]) ; CHECK-NEXT: ret i32 2 ; diff --git a/test/Transforms/InstCombine/demorgan.ll b/test/Transforms/InstCombine/demorgan.ll index fc4af3c6dad..26c2270a3fd 100644 --- a/test/Transforms/InstCombine/demorgan.ll +++ b/test/Transforms/InstCombine/demorgan.ll @@ -367,12 +367,12 @@ define i8 @demorgan_nor_use2bc(i8 %A, i8 %B) { ret i8 %r2 } -; FIXME: Do not apply DeMorgan's Law to constants. We prefer 'not' ops. +; Do not apply DeMorgan's Law to constants. We prefer 'not' ops. define i32 @demorganize_constant1(i32 %a) { ; CHECK-LABEL: @demorganize_constant1( -; CHECK-NEXT: [[A_NOT:%.*]] = or i32 %a, -16 -; CHECK-NEXT: [[AND1:%.*]] = xor i32 [[A_NOT]], 15 +; CHECK-NEXT: [[AND:%.*]] = and i32 %a, 15 +; CHECK-NEXT: [[AND1:%.*]] = xor i32 [[AND]], -1 ; CHECK-NEXT: ret i32 [[AND1]] ; %and = and i32 %a, 15 @@ -380,12 +380,12 @@ define i32 @demorganize_constant1(i32 %a) { ret i32 %and1 } -; FIXME: Do not apply DeMorgan's Law to constants. We prefer 'not' ops. +; Do not apply DeMorgan's Law to constants. We prefer 'not' ops. define i32 @demorganize_constant2(i32 %a) { ; CHECK-LABEL: @demorganize_constant2( -; CHECK-NEXT: [[A_NOT:%.*]] = and i32 %a, -16 -; CHECK-NEXT: [[AND1:%.*]] = xor i32 [[A_NOT]], -16 +; CHECK-NEXT: [[AND:%.*]] = or i32 %a, 15 +; CHECK-NEXT: [[AND1:%.*]] = xor i32 [[AND]], -1 ; CHECK-NEXT: ret i32 [[AND1]] ; %and = or i32 %a, 15 -- 2.11.0