1 //===-- MachineSink.cpp - Sinking for machine instructions ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "machine-sink"
15 #include "llvm/CodeGen/Passes.h"
16 #include "llvm/CodeGen/MachineRegisterInfo.h"
17 #include "llvm/CodeGen/MachineDominators.h"
18 #include "llvm/Target/TargetRegisterInfo.h"
19 #include "llvm/Target/TargetInstrInfo.h"
20 #include "llvm/Target/TargetMachine.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/Support/Compiler.h"
23 #include "llvm/Support/Debug.h"
26 STATISTIC(NumSunk, "Number of machine instructions sunk");
29 class VISIBILITY_HIDDEN MachineSinking : public MachineFunctionPass {
30 const TargetMachine *TM;
31 const TargetInstrInfo *TII;
32 MachineFunction *CurMF; // Current MachineFunction
33 MachineRegisterInfo *RegInfo; // Machine register information
34 MachineDominatorTree *DT; // Machine dominator tree for the current Loop
37 static char ID; // Pass identification
38 MachineSinking() : MachineFunctionPass(&ID) {}
40 virtual bool runOnMachineFunction(MachineFunction &MF);
42 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43 MachineFunctionPass::getAnalysisUsage(AU);
44 AU.addRequired<MachineDominatorTree>();
45 AU.addPreserved<MachineDominatorTree>();
48 bool ProcessBlock(MachineBasicBlock &MBB);
49 bool SinkInstruction(MachineInstr *MI, bool &SawStore);
50 bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB) const;
52 } // end anonymous namespace
54 char MachineSinking::ID = 0;
55 static RegisterPass<MachineSinking>
56 X("machine-sink", "Machine code sinking");
58 FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
60 /// AllUsesDominatedByBlock - Return true if all uses of the specified register
61 /// occur in blocks dominated by the specified block.
62 bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
63 MachineBasicBlock *MBB) const {
64 assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
65 "Only makes sense for vregs");
66 for (MachineRegisterInfo::reg_iterator I = RegInfo->reg_begin(Reg),
67 E = RegInfo->reg_end(); I != E; ++I) {
68 if (I.getOperand().isDef()) continue; // ignore def.
70 // Determine the block of the use.
71 MachineInstr *UseInst = &*I;
72 MachineBasicBlock *UseBlock = UseInst->getParent();
73 if (UseInst->getOpcode() == TargetInstrInfo::PHI) {
74 // PHI nodes use the operand in the predecessor block, not the block with
76 UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
78 // Check that it dominates.
79 if (!DT->dominates(MBB, UseBlock))
87 bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
88 DOUT << "******** Machine Sinking ********\n";
91 TM = &CurMF->getTarget();
92 TII = TM->getInstrInfo();
93 RegInfo = &CurMF->getRegInfo();
94 DT = &getAnalysis<MachineDominatorTree>();
96 bool EverMadeChange = false;
99 bool MadeChange = false;
101 // Process all basic blocks.
102 for (MachineFunction::iterator I = CurMF->begin(), E = CurMF->end();
104 MadeChange |= ProcessBlock(*I);
106 // If this iteration over the code changed anything, keep iterating.
107 if (!MadeChange) break;
108 EverMadeChange = true;
110 return EverMadeChange;
113 bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
114 bool MadeChange = false;
116 // Can't sink anything out of a block that has less than two successors.
117 if (MBB.succ_size() <= 1) return false;
119 // Walk the basic block bottom-up. Remember if we saw a store.
120 bool SawStore = false;
121 for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
122 MachineBasicBlock::iterator LastIt = I;
123 if (SinkInstruction(--I, SawStore)) {
132 /// SinkInstruction - Determine whether it is safe to sink the specified machine
133 /// instruction out of its current block into a successor.
134 bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
135 // Check if it's safe to move the instruction.
136 if (!MI->isSafeToMove(TII, SawStore))
139 // FIXME: This should include support for sinking instructions within the
140 // block they are currently in to shorten the live ranges. We often get
141 // instructions sunk into the top of a large block, but it would be better to
142 // also sink them down before their first use in the block. This xform has to
143 // be careful not to *increase* register pressure though, e.g. sinking
144 // "x = y + z" down if it kills y and z would increase the live ranges of y
145 // and z only the shrink the live range of x.
147 // Loop over all the operands of the specified instruction. If there is
148 // anything we can't handle, bail out.
149 MachineBasicBlock *ParentBlock = MI->getParent();
151 // SuccToSinkTo - This is the successor to sink this instruction to, once we
153 MachineBasicBlock *SuccToSinkTo = 0;
155 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
156 const MachineOperand &MO = MI->getOperand(i);
157 if (!MO.isReg()) continue; // Ignore non-register operands.
159 unsigned Reg = MO.getReg();
160 if (Reg == 0) continue;
162 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
163 // If this is a physical register use, we can't move it. If it is a def,
164 // we can move it, but only if the def is dead.
165 if (MO.isUse() || !MO.isDead())
168 // Virtual register uses are always safe to sink.
169 if (MO.isUse()) continue;
171 // FIXME: This picks a successor to sink into based on having one
172 // successor that dominates all the uses. However, there are cases where
173 // sinking can happen but where the sink point isn't a successor. For
178 // the instruction could be sunk over the whole diamond for the
179 // if/then/else (or loop, etc), allowing it to be sunk into other blocks
182 // Virtual register defs can only be sunk if all their uses are in blocks
183 // dominated by one of the successors.
185 // If a previous operand picked a block to sink to, then this operand
186 // must be sinkable to the same block.
187 if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo))
192 // Otherwise, we should look at all the successors and decide which one
193 // we should sink to.
194 for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
195 E = ParentBlock->succ_end(); SI != E; ++SI) {
196 if (AllUsesDominatedByBlock(Reg, *SI)) {
202 // If we couldn't find a block to sink to, ignore this instruction.
203 if (SuccToSinkTo == 0)
208 // If there are no outputs, it must have side-effects.
209 if (SuccToSinkTo == 0)
212 DEBUG(cerr << "Sink instr " << *MI);
213 DEBUG(cerr << "to block " << *SuccToSinkTo);
215 // If the block has multiple predecessors, this would introduce computation on
216 // a path that it doesn't already exist. We could split the critical edge,
217 // but for now we just punt.
218 // FIXME: Split critical edges if not backedges.
219 if (SuccToSinkTo->pred_size() > 1) {
220 DEBUG(cerr << " *** PUNTING: Critical edge found\n");
224 // Determine where to insert into. Skip phi nodes.
225 MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
226 while (InsertPos != SuccToSinkTo->end() &&
227 InsertPos->getOpcode() == TargetInstrInfo::PHI)
230 // Move the instruction.
231 SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
232 ++MachineBasicBlock::iterator(MI));