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
AVRShiftExpand()29 AVRShiftExpand() : FunctionPass(ID) {}
30
31 bool runOnFunction(Function &F) override;
32
getPassName() const33 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
createAVRShiftExpandPass()46 Pass *llvm::createAVRShiftExpandPass() { return new AVRShiftExpand(); }
47
runOnFunction(Function & F)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) ||
56 I.getType() == Type::getInt16Ty(Ctx))
57 // Only expand non-8-bit and non-16-bit shifts, since those are expanded
58 // directly during isel.
59 continue;
60 if (isa<ConstantInt>(I.getOperand(1)))
61 // Only expand when the shift amount is not known.
62 // Known shift amounts are (currently) better expanded inline.
63 continue;
64 ShiftInsts.push_back(cast<BinaryOperator>(&I));
65 }
66
67 // The expanding itself needs to be done separately as expand() will remove
68 // these instructions. Removing instructions while iterating over a basic
69 // block is not a great idea.
70 for (auto *I : ShiftInsts) {
71 expand(I);
72 }
73
74 // Return whether this function expanded any shift instructions.
75 return ShiftInsts.size() > 0;
76 }
77
expand(BinaryOperator * BI)78 void AVRShiftExpand::expand(BinaryOperator *BI) {
79 auto &Ctx = BI->getContext();
80 IRBuilder<> Builder(BI);
81 Type *InputTy = cast<Instruction>(BI)->getType();
82 Type *Int8Ty = Type::getInt8Ty(Ctx);
83 Value *Int8Zero = ConstantInt::get(Int8Ty, 0);
84
85 // Split the current basic block at the point of the existing shift
86 // instruction and insert a new basic block for the loop.
87 BasicBlock *BB = BI->getParent();
88 Function *F = BB->getParent();
89 BasicBlock *EndBB = BB->splitBasicBlock(BI, "shift.done");
90 BasicBlock *LoopBB = BasicBlock::Create(Ctx, "shift.loop", F, EndBB);
91
92 // Truncate the shift amount to i8, which is trivially lowered to a single
93 // AVR register.
94 Builder.SetInsertPoint(&BB->back());
95 Value *ShiftAmount = Builder.CreateTrunc(BI->getOperand(1), Int8Ty);
96
97 // Replace the unconditional branch that splitBasicBlock created with a
98 // conditional branch.
99 Value *Cmp1 = Builder.CreateICmpEQ(ShiftAmount, Int8Zero);
100 Builder.CreateCondBr(Cmp1, EndBB, LoopBB);
101 BB->back().eraseFromParent();
102
103 // Create the loop body starting with PHI nodes.
104 Builder.SetInsertPoint(LoopBB);
105 PHINode *ShiftAmountPHI = Builder.CreatePHI(Int8Ty, 2);
106 ShiftAmountPHI->addIncoming(ShiftAmount, BB);
107 PHINode *ValuePHI = Builder.CreatePHI(InputTy, 2);
108 ValuePHI->addIncoming(BI->getOperand(0), BB);
109
110 // Subtract the shift amount by one, as we're shifting one this loop
111 // iteration.
112 Value *ShiftAmountSub =
113 Builder.CreateSub(ShiftAmountPHI, ConstantInt::get(Int8Ty, 1));
114 ShiftAmountPHI->addIncoming(ShiftAmountSub, LoopBB);
115
116 // Emit the actual shift instruction. The difference is that this shift
117 // instruction has a constant shift amount, which can be emitted inline
118 // without a library call.
119 Value *ValueShifted;
120 switch (BI->getOpcode()) {
121 case Instruction::Shl:
122 ValueShifted = Builder.CreateShl(ValuePHI, ConstantInt::get(InputTy, 1));
123 break;
124 case Instruction::LShr:
125 ValueShifted = Builder.CreateLShr(ValuePHI, ConstantInt::get(InputTy, 1));
126 break;
127 case Instruction::AShr:
128 ValueShifted = Builder.CreateAShr(ValuePHI, ConstantInt::get(InputTy, 1));
129 break;
130 default:
131 llvm_unreachable("asked to expand an instruction that is not a shift");
132 }
133 ValuePHI->addIncoming(ValueShifted, LoopBB);
134
135 // Branch to either the loop again (if there is more to shift) or to the
136 // basic block after the loop (if all bits are shifted).
137 Value *Cmp2 = Builder.CreateICmpEQ(ShiftAmountSub, Int8Zero);
138 Builder.CreateCondBr(Cmp2, EndBB, LoopBB);
139
140 // Collect the resulting value. This is necessary in the IR but won't produce
141 // any actual instructions.
142 Builder.SetInsertPoint(BI);
143 PHINode *Result = Builder.CreatePHI(InputTy, 2);
144 Result->addIncoming(BI->getOperand(0), BB);
145 Result->addIncoming(ValueShifted, LoopBB);
146
147 // Replace the original shift instruction.
148 BI->replaceAllUsesWith(Result);
149 BI->eraseFromParent();
150 }
151