From 9004c8afd4bf7b3f27f4a4f8fd069379afa97c83 Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Thu, 21 May 2009 02:28:33 +0000 Subject: [PATCH] Teach ValueTracking a new way to analyze PHI nodes, and and teach Instcombine to be more aggressive about using SimplifyDemandedBits on shift nodes. This allows a shift to be simplified to zero in the included test case. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@72204 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Analysis/ValueTracking.cpp | 29 +++++++++++++-- .../Scalar/InstructionCombining.cpp | 6 ++-- test/Transforms/InstCombine/lshr-phi.ll | 35 +++++++++++++++++++ 3 files changed, 66 insertions(+), 4 deletions(-) create mode 100644 test/Transforms/InstCombine/lshr-phi.ll diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp index c4f6faf612f..3c21e2039c1 100644 --- a/lib/Analysis/ValueTracking.cpp +++ b/lib/Analysis/ValueTracking.cpp @@ -48,8 +48,9 @@ static unsigned getOpcode(const Value *V) { void llvm::ComputeMaskedBits(Value *V, const APInt &Mask, APInt &KnownZero, APInt &KnownOne, TargetData *TD, unsigned Depth) { + const unsigned MaxDepth = 6; assert(V && "No Value?"); - assert(Depth <= 6 && "Limit Search Depth"); + assert(Depth <= MaxDepth && "Limit Search Depth"); unsigned BitWidth = Mask.getBitWidth(); assert((V->getType()->isInteger() || isa(V->getType())) && "Not integer or pointer type!"); @@ -88,7 +89,7 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask, KnownZero.clear(); KnownOne.clear(); // Start out not knowing anything. - if (Depth == 6 || Mask == 0) + if (Depth == MaxDepth || Mask == 0) return; // Limit search depth. User *I = dyn_cast(V); @@ -522,6 +523,30 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask, } } } + + // Otherwise take the unions of the known bit sets of the operands, + // taking conservative care to avoid excessive recursion. + if (Depth < MaxDepth - 1 && !KnownZero && !KnownOne) { + KnownZero = APInt::getAllOnesValue(BitWidth); + KnownOne = APInt::getAllOnesValue(BitWidth); + for (unsigned i = 0, e = P->getNumIncomingValues(); i != e; ++i) { + // Skip direct self references. + if (P->getIncomingValue(i) == P) continue; + + KnownZero2 = APInt(BitWidth, 0); + KnownOne2 = APInt(BitWidth, 0); + // Recurse, but cap the recursion to one level, because we don't + // want to waste time spinning around in loops. + ComputeMaskedBits(P->getIncomingValue(i), KnownZero | KnownOne, + KnownZero2, KnownOne2, TD, MaxDepth-1); + KnownZero &= KnownZero2; + KnownOne &= KnownOne2; + // If all bits have been ruled out, there's no need to check + // more operands. + if (!KnownZero && !KnownOne) + break; + } + } break; } case Instruction::Call: diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp index 527ed430106..df5145da2a0 100644 --- a/lib/Transforms/Scalar/InstructionCombining.cpp +++ b/lib/Transforms/Scalar/InstructionCombining.cpp @@ -7152,6 +7152,10 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) { return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType())); } + // See if we can fold away this shift. + if (!isa(I.getType()) && SimplifyDemandedInstructionBits(I)) + return &I; + // Try to fold constant and into select arguments. if (isa(Op0)) if (SelectInst *SI = dyn_cast(Op1)) @@ -7171,8 +7175,6 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, ConstantInt *Op1, // See if we can simplify any instructions used by the instruction whose sole // purpose is to compute bits we don't care about. uint32_t TypeBits = Op0->getType()->getPrimitiveSizeInBits(); - if (SimplifyDemandedInstructionBits(I)) - return &I; // shl uint X, 32 = 0 and shr ubyte Y, 9 = 0, ... just don't eliminate shr // of a signed value. diff --git a/test/Transforms/InstCombine/lshr-phi.ll b/test/Transforms/InstCombine/lshr-phi.ll new file mode 100644 index 00000000000..683eb87931e --- /dev/null +++ b/test/Transforms/InstCombine/lshr-phi.ll @@ -0,0 +1,35 @@ +; RUN: llvm-as < %s | opt -instcombine | llvm-dis > %t +; RUN: not grep lshr %t +; RUN: grep add %t | count 1 + +; Instcombine should be able to eliminate the lshr, because only +; bits in the operand which might be non-zero will be shifted +; off the end. + +define i32 @hash_string(i8* nocapture %key) nounwind readonly { +entry: + %t0 = load i8* %key, align 1 ; [#uses=1] + %t1 = icmp eq i8 %t0, 0 ; [#uses=1] + br i1 %t1, label %bb2, label %bb + +bb: ; preds = %bb, %entry + %indvar = phi i64 [ 0, %entry ], [ %tmp, %bb ] ; [#uses=2] + %k.04 = phi i32 [ 0, %entry ], [ %t8, %bb ] ; [#uses=2] + %cp.05 = getelementptr i8* %key, i64 %indvar ; [#uses=1] + %t2 = shl i32 %k.04, 1 ; [#uses=1] + %t3 = lshr i32 %k.04, 14 ; [#uses=1] + %t4 = add i32 %t2, %t3 ; [#uses=1] + %t5 = load i8* %cp.05, align 1 ; [#uses=1] + %t6 = sext i8 %t5 to i32 ; [#uses=1] + %t7 = xor i32 %t6, %t4 ; [#uses=1] + %t8 = and i32 %t7, 16383 ; [#uses=2] + %tmp = add i64 %indvar, 1 ; [#uses=2] + %scevgep = getelementptr i8* %key, i64 %tmp ; [#uses=1] + %t9 = load i8* %scevgep, align 1 ; [#uses=1] + %t10 = icmp eq i8 %t9, 0 ; [#uses=1] + br i1 %t10, label %bb2, label %bb + +bb2: ; preds = %bb, %entry + %k.0.lcssa = phi i32 [ 0, %entry ], [ %t8, %bb ] ; [#uses=1] + ret i32 %k.0.lcssa +}