1 //===-- MachineLICM.cpp - Machine Loop Invariant Code Motion Pass ---------===//
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 //===----------------------------------------------------------------------===//
10 // This pass performs loop invariant code motion on machine instructions. We
11 // attempt to remove as much code from the body of a loop as possible.
13 //===----------------------------------------------------------------------===//
15 #define DEBUG_TYPE "machine-licm"
16 #include "llvm/ADT/IndexedMap.h"
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/ADT/Statistic.h"
19 #include "llvm/CodeGen/MachineBasicBlock.h"
20 #include "llvm/CodeGen/MachineDominators.h"
21 #include "llvm/CodeGen/MachineInstr.h"
22 #include "llvm/CodeGen/MachineLoopInfo.h"
23 #include "llvm/CodeGen/MachineRegisterInfo.h"
24 #include "llvm/CodeGen/Passes.h"
25 #include "llvm/Support/CFG.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Compiler.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Target/MRegisterInfo.h"
30 #include "llvm/Target/TargetInstrInfo.h"
31 #include "llvm/Target/TargetMachine.h"
36 // Hidden options to help debugging
38 PerformLICM("machine-licm",
39 cl::init(false), cl::Hidden,
40 cl::desc("Perform loop-invariant code motion on machine code"));
43 STATISTIC(NumHoisted, "Number of machine instructions hoisted out of loops");
46 class VISIBILITY_HIDDEN MachineLICM : public MachineFunctionPass {
47 const TargetMachine *TM;
48 const TargetInstrInfo *TII;
49 MachineFunction *CurMF; // Current MachineFunction
51 // Various analyses that we use...
52 MachineLoopInfo *LI; // Current MachineLoopInfo
53 MachineDominatorTree *DT; // Machine dominator tree for the current Loop
54 MachineRegisterInfo *RegInfo; // Machine register information
56 // State that is updated as we process loops
57 bool Changed; // True if a loop is changed.
58 MachineLoop *CurLoop; // The current loop we are working on.
60 static char ID; // Pass identification, replacement for typeid
61 MachineLICM() : MachineFunctionPass((intptr_t)&ID) {}
63 virtual bool runOnMachineFunction(MachineFunction &MF);
65 /// FIXME: Loop preheaders?
67 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
69 AU.addRequired<MachineLoopInfo>();
70 AU.addRequired<MachineDominatorTree>();
73 /// VisitAllLoops - Visit all of the loops in depth first order and try to
74 /// hoist invariant instructions from them.
76 void VisitAllLoops(MachineLoop *L) {
77 const std::vector<MachineLoop*> &SubLoops = L->getSubLoops();
79 for (MachineLoop::iterator
80 I = SubLoops.begin(), E = SubLoops.end(); I != E; ++I) {
83 // Traverse the body of the loop in depth first order on the dominator
84 // tree so that we are guaranteed to see definitions before we see uses.
86 HoistRegion(DT->getNode(ML->getHeader()));
89 HoistRegion(DT->getNode(L->getHeader()));
92 /// IsInSubLoop - A little predicate that returns true if the specified
93 /// basic block is in a subloop of the current one, not the current one
96 bool IsInSubLoop(MachineBasicBlock *BB) {
97 assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
98 return LI->getLoopFor(BB) != CurLoop;
101 /// IsLoopInvariantInst - Returns true if the instruction is loop
102 /// invariant. I.e., all virtual register operands are defined outside of
103 /// the loop, physical registers aren't accessed (explicitly or implicitly),
104 /// and the instruction is hoistable.
106 bool IsLoopInvariantInst(MachineInstr &I);
108 /// FindPredecessors - Get all of the predecessors of the loop that are not
111 void FindPredecessors(std::vector<MachineBasicBlock*> &Preds) {
112 const MachineBasicBlock *Header = CurLoop->getHeader();
114 for (MachineBasicBlock::const_pred_iterator
115 I = Header->pred_begin(), E = Header->pred_end(); I != E; ++I)
116 if (!CurLoop->contains(*I))
120 /// MoveInstToEndOfBlock - Moves the machine instruction to the bottom of
121 /// the predecessor basic block (but before the terminator instructions).
123 void MoveInstToEndOfBlock(MachineBasicBlock *ToMBB,
124 MachineBasicBlock *FromMBB,
127 DOUT << "Hoisting " << *MI;
128 if (ToMBB->getBasicBlock())
129 DOUT << " to MachineBasicBlock "
130 << ToMBB->getBasicBlock()->getName();
134 MachineBasicBlock::iterator WhereIter = ToMBB->getFirstTerminator();
135 MachineBasicBlock::iterator To, From = FromMBB->begin();
140 assert(From != FromMBB->end() && "Didn't find instr in BB!");
143 ToMBB->splice(WhereIter, FromMBB, From, ++To);
147 /// HoistRegion - Walk the specified region of the CFG (defined by all
148 /// blocks dominated by the specified block, and that are in the current
149 /// loop) in depth first order w.r.t the DominatorTree. This allows us to
150 /// visit definitions before uses, allowing us to hoist a loop body in one
151 /// pass without iteration.
153 void HoistRegion(MachineDomTreeNode *N);
155 /// Hoist - When an instruction is found to only use loop invariant operands
156 /// that is safe to hoist, this instruction is called to do the dirty work.
158 void Hoist(MachineInstr &MI);
161 char MachineLICM::ID = 0;
162 RegisterPass<MachineLICM> X("machine-licm",
163 "Machine Loop Invariant Code Motion");
164 } // end anonymous namespace
166 FunctionPass *llvm::createMachineLICMPass() { return new MachineLICM(); }
168 /// Hoist expressions out of the specified loop. Note, alias info for inner loop
169 /// is not preserved so it is not a good idea to run LICM multiple times on one
172 bool MachineLICM::runOnMachineFunction(MachineFunction &MF) {
173 if (!PerformLICM) return false; // For debugging.
175 DOUT << "******** Machine LICM ********\n";
179 TM = &CurMF->getTarget();
180 TII = TM->getInstrInfo();
181 RegInfo = &CurMF->getRegInfo();
183 // Get our Loop information...
184 LI = &getAnalysis<MachineLoopInfo>();
185 DT = &getAnalysis<MachineDominatorTree>();
187 for (MachineLoopInfo::iterator
188 I = LI->begin(), E = LI->end(); I != E; ++I) {
191 // Visit all of the instructions of the loop. We want to visit the subloops
192 // first, though, so that we can hoist their invariants first into their
193 // containing loop before we process that loop.
194 VisitAllLoops(CurLoop);
200 /// HoistRegion - Walk the specified region of the CFG (defined by all blocks
201 /// dominated by the specified block, and that are in the current loop) in depth
202 /// first order w.r.t the DominatorTree. This allows us to visit definitions
203 /// before uses, allowing us to hoist a loop body in one pass without iteration.
205 void MachineLICM::HoistRegion(MachineDomTreeNode *N) {
206 assert(N != 0 && "Null dominator tree node?");
207 MachineBasicBlock *BB = N->getBlock();
209 // If this subregion is not in the top level loop at all, exit.
210 if (!CurLoop->contains(BB)) return;
212 // Only need to process the contents of this block if it is not part of a
213 // subloop (which would already have been processed).
214 if (!IsInSubLoop(BB))
215 for (MachineBasicBlock::iterator
216 I = BB->begin(), E = BB->end(); I != E; ) {
217 MachineInstr &MI = *I++;
219 // Try hoisting the instruction out of the loop. We can only do this if
220 // all of the operands of the instruction are loop invariant and if it is
221 // safe to hoist the instruction.
225 const std::vector<MachineDomTreeNode*> &Children = N->getChildren();
227 for (unsigned I = 0, E = Children.size(); I != E; ++I)
228 HoistRegion(Children[I]);
231 /// IsLoopInvariantInst - Returns true if the instruction is loop
232 /// invariant. I.e., all virtual register operands are defined outside of the
233 /// loop, physical registers aren't accessed explicitly, and there are no side
234 /// effects that aren't captured by the operands or other flags.
236 bool MachineLICM::IsLoopInvariantInst(MachineInstr &I) {
238 DOUT << "--- Checking if we can hoist " << I;
239 if (I.getInstrDescriptor()->ImplicitUses) {
240 DOUT << " * Instruction has implicit uses:\n";
242 const MRegisterInfo *MRI = TM->getRegisterInfo();
243 const unsigned *ImpUses = I.getInstrDescriptor()->ImplicitUses;
245 for (; *ImpUses; ++ImpUses)
246 DOUT << " -> " << MRI->getName(*ImpUses) << "\n";
249 if (I.getInstrDescriptor()->ImplicitDefs) {
250 DOUT << " * Instruction has implicit defines:\n";
252 const MRegisterInfo *MRI = TM->getRegisterInfo();
253 const unsigned *ImpDefs = I.getInstrDescriptor()->ImplicitDefs;
255 for (; *ImpDefs; ++ImpDefs)
256 DOUT << " -> " << MRI->getName(*ImpDefs) << "\n";
259 if (TII->hasUnmodelledSideEffects(&I))
260 DOUT << " * Instruction has side effects.\n";
263 // The instruction is loop invariant if all of its operands are loop-invariant
264 for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
265 const MachineOperand &MO = I.getOperand(i);
267 if (!(MO.isRegister() && MO.getReg() && MO.isUse()))
270 unsigned Reg = MO.getReg();
272 // Don't hoist instructions that access physical registers.
273 if (!MRegisterInfo::isVirtualRegister(Reg))
276 assert(RegInfo->getVRegDef(Reg)&&"Machine instr not mapped for this vreg?");
278 // If the loop contains the definition of an operand, then the instruction
279 // isn't loop invariant.
280 if (CurLoop->contains(RegInfo->getVRegDef(Reg)->getParent()))
284 // Don't hoist something that has unmodelled side effects.
285 if (TII->hasUnmodelledSideEffects(&I)) return false;
287 // If we got this far, the instruction is loop invariant!
291 /// Hoist - When an instruction is found to only use loop invariant operands
292 /// that is safe to hoist, this instruction is called to do the dirty work.
294 void MachineLICM::Hoist(MachineInstr &MI) {
295 if (!IsLoopInvariantInst(MI)) return;
297 std::vector<MachineBasicBlock*> Preds;
299 // Non-back-edge predecessors.
300 FindPredecessors(Preds);
302 // Either we don't have any predecessors(?!) or we have more than one, which
304 if (Preds.empty() || Preds.size() != 1) return;
306 // Check that the predecessor is qualified to take the hoisted
307 // instruction. I.e., there is only one edge from the predecessor, and it's to
309 MachineBasicBlock *MBB = Preds.front();
311 // FIXME: We are assuming at first that the basic block coming into this loop
312 // has only one successor. This isn't the case in general because we haven't
313 // broken critical edges or added preheaders.
314 if (MBB->succ_size() != 1) return;
315 assert(*MBB->succ_begin() == CurLoop->getHeader() &&
316 "The predecessor doesn't feed directly into the loop header!");
318 // Now move the instructions to the predecessor.
319 MoveInstToEndOfBlock(MBB, MI.getParent(), &MI);