diff options
Diffstat (limited to 'llvm/lib')
| -rw-r--r-- | llvm/lib/Analysis/ScalarEvolution.cpp | 149 |
1 files changed, 149 insertions, 0 deletions
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp index 9df31ed9e3a..e0658c21050 100644 --- a/llvm/lib/Analysis/ScalarEvolution.cpp +++ b/llvm/lib/Analysis/ScalarEvolution.cpp @@ -83,6 +83,7 @@ #include "llvm/IR/LLVMContext.h" #include "llvm/IR/Metadata.h" #include "llvm/IR/Operator.h" +#include "llvm/IR/PatternMatch.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/ErrorHandling.h" @@ -5398,6 +5399,11 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L, return ItCnt; } + ExitLimit ShiftEL = computeShiftCompareExitLimit( + ExitCond->getOperand(0), ExitCond->getOperand(1), L, Cond); + if (ShiftEL.hasAnyInfo()) + return ShiftEL; + const SCEV *LHS = getSCEV(ExitCond->getOperand(0)); const SCEV *RHS = getSCEV(ExitCond->getOperand(1)); @@ -5576,6 +5582,149 @@ ScalarEvolution::computeLoadConstantCompareExitLimit( return getCouldNotCompute(); } +ScalarEvolution::ExitLimit ScalarEvolution::computeShiftCompareExitLimit( + Value *LHS, Value *RHSV, const Loop *L, ICmpInst::Predicate Pred) { + ConstantInt *RHS = dyn_cast<ConstantInt>(RHSV); + if (!RHS) + return getCouldNotCompute(); + + const BasicBlock *Latch = L->getLoopLatch(); + if (!Latch) + return getCouldNotCompute(); + + const BasicBlock *Predecessor = L->getLoopPredecessor(); + if (!Predecessor) + return getCouldNotCompute(); + + // Return true if V is of the form "LHS `shift_op` <positive constant>". + // Return LHS in OutLHS and shift_opt in OutOpCode. + auto MatchPositiveShift = + [](Value *V, Value *&OutLHS, Instruction::BinaryOps &OutOpCode) { + + using namespace PatternMatch; + + ConstantInt *ShiftAmt; + if (match(V, m_LShr(m_Value(OutLHS), m_ConstantInt(ShiftAmt)))) + OutOpCode = Instruction::LShr; + else if (match(V, m_AShr(m_Value(OutLHS), m_ConstantInt(ShiftAmt)))) + OutOpCode = Instruction::AShr; + else if (match(V, m_Shl(m_Value(OutLHS), m_ConstantInt(ShiftAmt)))) + OutOpCode = Instruction::Shl; + else + return false; + + return ShiftAmt->getValue().isStrictlyPositive(); + }; + + // Recognize a "shift recurrence" either of the form %iv or of %iv.shifted in + // + // loop: + // %iv = phi i32 [ %iv.shifted, %loop ], [ %val, %preheader ] + // %iv.shifted = lshr i32 %iv, <positive constant> + // + // Return true on a succesful match. Return the corresponding PHI node (%iv + // above) in PNOut and the opcode of the shift operation in OpCodeOut. + auto MatchShiftRecurrence = + [&](Value *V, PHINode *&PNOut, Instruction::BinaryOps &OpCodeOut) { + Optional<Instruction::BinaryOps> PostShiftOpCode; + + { + Instruction::BinaryOps OpC; + Value *V; + + // If we encounter a shift instruction, "peel off" the shift operation, + // and remember that we did so. Later when we inspect %iv's backedge + // value, we will make sure that the backedge value uses the same + // operation. + // + // Note: the peeled shift operation does not have to be the same + // instruction as the one feeding into the PHI's backedge value. We only + // really care about it being the same *kind* of shift instruction -- + // that's all that is required for our later inferences to hold. + if (MatchPositiveShift(LHS, V, OpC)) { + PostShiftOpCode = OpC; + LHS = V; + } + } + + PNOut = dyn_cast<PHINode>(LHS); + if (!PNOut || PNOut->getParent() != L->getHeader()) + return false; + + Value *BEValue = PNOut->getIncomingValueForBlock(Latch); + Value *OpLHS; + + return + // The backedge value for the PHI node must be a shift by a positive + // amount + MatchPositiveShift(BEValue, OpLHS, OpCodeOut) && + + // of the PHI node itself + OpLHS == PNOut && + + // and the kind of shift should be match the kind of shift we peeled + // off, if any. + (!PostShiftOpCode.hasValue() || *PostShiftOpCode == OpCodeOut); + }; + + PHINode *PN; + Instruction::BinaryOps OpCode; + if (!MatchShiftRecurrence(LHS, PN, OpCode)) + return getCouldNotCompute(); + + const DataLayout &DL = getDataLayout(); + + // The key rationale for this optimization is that for some kinds of shift + // recurrences, the value of the recurrence "stabilizes" to either 0 or -1 + // within a finite number of iterations. If the condition guarding the + // backedge (in the sense that the backedge is taken if the condition is true) + // is false for the value the shift recurrence stabilizes to, then we know + // that the backedge is taken only a finite number of times. + + ConstantInt *StableValue = nullptr; + switch (OpCode) { + default: + llvm_unreachable("Impossible case!"); + + case Instruction::AShr: { + // {K,ashr,<positive-constant>} stabilizes to signum(K) in at most + // bitwidth(K) iterations. + Value *FirstValue = PN->getIncomingValueForBlock(Predecessor); + bool KnownZero, KnownOne; + ComputeSignBit(FirstValue, KnownZero, KnownOne, DL, 0, nullptr, + Predecessor->getTerminator(), &DT); + auto *Ty = cast<IntegerType>(RHS->getType()); + if (KnownZero) + StableValue = ConstantInt::get(Ty, 0); + else if (KnownOne) + StableValue = ConstantInt::get(Ty, -1, true); + else + return getCouldNotCompute(); + + break; + } + case Instruction::LShr: + case Instruction::Shl: + // Both {K,lshr,<positive-constant>} and {K,shl,<positive-constant>} + // stabilize to 0 in at most bitwidth(K) iterations. + StableValue = ConstantInt::get(cast<IntegerType>(RHS->getType()), 0); + break; + } + + auto *Result = + ConstantFoldCompareInstOperands(Pred, StableValue, RHS, DL, &TLI); + assert(Result->getType()->isIntegerTy(1) && + "Otherwise cannot be an operand to a branch instruction"); + + if (Result->isZeroValue()) { + unsigned BitWidth = getTypeSizeInBits(RHS->getType()); + const SCEV *UpperBound = + getConstant(getEffectiveSCEVType(RHS->getType()), BitWidth); + return ExitLimit(getCouldNotCompute(), UpperBound); + } + + return getCouldNotCompute(); +} /// CanConstantFold - Return true if we can constant fold an instruction of the /// specified type, assuming that all operands were constants. |
