1 //===- AVRShift.cpp - Shift Expansion Pass --------------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 /// \file 10 /// Expand non-8-bit and non-16-bit shift instructions (shl, lshr, ashr) to 11 /// inline loops, just like avr-gcc. This must be done in IR because otherwise 12 /// the type legalizer will turn 32-bit shifts into (non-existing) library calls 13 /// such as __ashlsi3. 14 // 15 //===----------------------------------------------------------------------===// 16 17 #include "AVR.h" 18 #include "llvm/IR/IRBuilder.h" 19 #include "llvm/IR/InstIterator.h" 20 21 using namespace llvm; 22 23 namespace { 24 25 class AVRShiftExpand : public FunctionPass { 26 public: 27 static char ID; 28 29 AVRShiftExpand() : FunctionPass(ID) {} 30 31 bool runOnFunction(Function &F) override; 32 33 StringRef getPassName() const override { return "AVR Shift Expansion"; } 34 35 private: 36 void expand(BinaryOperator *BI); 37 }; 38 39 } // end of anonymous namespace 40 41 char AVRShiftExpand::ID = 0; 42 43 INITIALIZE_PASS(AVRShiftExpand, "avr-shift-expand", "AVR Shift Expansion", 44 false, false) 45 46 Pass *llvm::createAVRShiftExpandPass() { return new AVRShiftExpand(); } 47 48 bool AVRShiftExpand::runOnFunction(Function &F) { 49 SmallVector<BinaryOperator *, 1> ShiftInsts; 50 auto &Ctx = F.getContext(); 51 for (Instruction &I : instructions(F)) { 52 if (!I.isShift()) 53 // Only expand shift instructions (shl, lshr, ashr). 54 continue; 55 if (I.getType() == Type::getInt8Ty(Ctx) || I.getType() == Type::getInt16Ty(Ctx)) 56 // Only expand non-8-bit and non-16-bit shifts, since those are expanded 57 // directly during isel. 58 continue; 59 if (isa<ConstantInt>(I.getOperand(1))) 60 // Only expand when the shift amount is not known. 61 // Known shift amounts are (currently) better expanded inline. 62 continue; 63 ShiftInsts.push_back(cast<BinaryOperator>(&I)); 64 } 65 66 // The expanding itself needs to be done separately as expand() will remove 67 // these instructions. Removing instructions while iterating over a basic 68 // block is not a great idea. 69 for (auto *I : ShiftInsts) { 70 expand(I); 71 } 72 73 // Return whether this function expanded any shift instructions. 74 return ShiftInsts.size() > 0; 75 } 76 77 void AVRShiftExpand::expand(BinaryOperator *BI) { 78 auto &Ctx = BI->getContext(); 79 IRBuilder<> Builder(BI); 80 Type *InputTy = cast<Instruction>(BI)->getType(); 81 Type *Int8Ty = Type::getInt8Ty(Ctx); 82 Value *Int8Zero = ConstantInt::get(Int8Ty, 0); 83 84 // Split the current basic block at the point of the existing shift 85 // instruction and insert a new basic block for the loop. 86 BasicBlock *BB = BI->getParent(); 87 Function *F = BB->getParent(); 88 BasicBlock *EndBB = BB->splitBasicBlock(BI, "shift.done"); 89 BasicBlock *LoopBB = BasicBlock::Create(Ctx, "shift.loop", F, EndBB); 90 91 // Truncate the shift amount to i8, which is trivially lowered to a single 92 // AVR register. 93 Builder.SetInsertPoint(&BB->back()); 94 Value *ShiftAmount = Builder.CreateTrunc(BI->getOperand(1), Int8Ty); 95 96 // Replace the unconditional branch that splitBasicBlock created with a 97 // conditional branch. 98 Value *Cmp1 = Builder.CreateICmpEQ(ShiftAmount, Int8Zero); 99 Builder.CreateCondBr(Cmp1, EndBB, LoopBB); 100 BB->back().eraseFromParent(); 101 102 // Create the loop body starting with PHI nodes. 103 Builder.SetInsertPoint(LoopBB); 104 PHINode *ShiftAmountPHI = Builder.CreatePHI(Int8Ty, 2); 105 ShiftAmountPHI->addIncoming(ShiftAmount, BB); 106 PHINode *ValuePHI = Builder.CreatePHI(InputTy, 2); 107 ValuePHI->addIncoming(BI->getOperand(0), BB); 108 109 // Subtract the shift amount by one, as we're shifting one this loop 110 // iteration. 111 Value *ShiftAmountSub = 112 Builder.CreateSub(ShiftAmountPHI, ConstantInt::get(Int8Ty, 1)); 113 ShiftAmountPHI->addIncoming(ShiftAmountSub, LoopBB); 114 115 // Emit the actual shift instruction. The difference is that this shift 116 // instruction has a constant shift amount, which can be emitted inline 117 // without a library call. 118 Value *ValueShifted; 119 switch (BI->getOpcode()) { 120 case Instruction::Shl: 121 ValueShifted = Builder.CreateShl(ValuePHI, ConstantInt::get(InputTy, 1)); 122 break; 123 case Instruction::LShr: 124 ValueShifted = Builder.CreateLShr(ValuePHI, ConstantInt::get(InputTy, 1)); 125 break; 126 case Instruction::AShr: 127 ValueShifted = Builder.CreateAShr(ValuePHI, ConstantInt::get(InputTy, 1)); 128 break; 129 default: 130 llvm_unreachable("asked to expand an instruction that is not a shift"); 131 } 132 ValuePHI->addIncoming(ValueShifted, LoopBB); 133 134 // Branch to either the loop again (if there is more to shift) or to the 135 // basic block after the loop (if all bits are shifted). 136 Value *Cmp2 = Builder.CreateICmpEQ(ShiftAmountSub, Int8Zero); 137 Builder.CreateCondBr(Cmp2, EndBB, LoopBB); 138 139 // Collect the resulting value. This is necessary in the IR but won't produce 140 // any actual instructions. 141 Builder.SetInsertPoint(BI); 142 PHINode *Result = Builder.CreatePHI(InputTy, 2); 143 Result->addIncoming(BI->getOperand(0), BB); 144 Result->addIncoming(ValueShifted, LoopBB); 145 146 // Replace the original shift instruction. 147 BI->replaceAllUsesWith(Result); 148 BI->eraseFromParent(); 149 } 150