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 //===----------------------------------------------------------------------===//
10 // This pass moves instructions into successor blocks when possible, so that
11 // they aren't executed on paths where their results aren't needed.
13 // This pass is not intended to be a replacement or a complete alternative
14 // for an LLVM-IR-level sinking pass. It is only designed to sink simple
15 // constructs that are not exposed before lowering and instruction selection.
17 //===----------------------------------------------------------------------===//
19 #define DEBUG_TYPE "machine-sink"
20 #include "llvm/CodeGen/Passes.h"
21 #include "llvm/CodeGen/MachineRegisterInfo.h"
22 #include "llvm/CodeGen/MachineDominators.h"
23 #include "llvm/CodeGen/MachineLoopInfo.h"
24 #include "llvm/Analysis/AliasAnalysis.h"
25 #include "llvm/Target/TargetRegisterInfo.h"
26 #include "llvm/Target/TargetInstrInfo.h"
27 #include "llvm/Target/TargetMachine.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
35 SplitEdges("machine-sink-split",
36 cl::desc("Split critical edges during machine sinking"),
37 cl::init(false), cl::Hidden);
38 static cl::opt<unsigned>
39 SplitLimit("split-limit",
40 cl::init(~0u), cl::Hidden);
42 STATISTIC(NumSunk, "Number of machine instructions sunk");
43 STATISTIC(NumSplit, "Number of critical edges split");
46 class MachineSinking : public MachineFunctionPass {
47 const TargetInstrInfo *TII;
48 const TargetRegisterInfo *TRI;
49 MachineRegisterInfo *RegInfo; // Machine register information
50 MachineDominatorTree *DT; // Machine dominator tree
53 BitVector AllocatableSet; // Which physregs are allocatable?
56 static char ID; // Pass identification
57 MachineSinking() : MachineFunctionPass(ID) {}
59 virtual bool runOnMachineFunction(MachineFunction &MF);
61 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
63 MachineFunctionPass::getAnalysisUsage(AU);
64 AU.addRequired<AliasAnalysis>();
65 AU.addRequired<MachineDominatorTree>();
66 AU.addRequired<MachineLoopInfo>();
67 AU.addPreserved<MachineDominatorTree>();
68 AU.addPreserved<MachineLoopInfo>();
71 bool ProcessBlock(MachineBasicBlock &MBB);
72 MachineBasicBlock *SplitCriticalEdge(MachineBasicBlock *From,
73 MachineBasicBlock *To);
74 bool SinkInstruction(MachineInstr *MI, bool &SawStore);
75 bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB,
76 MachineBasicBlock *DefMBB, bool &LocalUse) const;
78 } // end anonymous namespace
80 char MachineSinking::ID = 0;
81 INITIALIZE_PASS(MachineSinking, "machine-sink",
82 "Machine code sinking", false, false);
84 FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
86 /// AllUsesDominatedByBlock - Return true if all uses of the specified register
87 /// occur in blocks dominated by the specified block. If any use is in the
88 /// definition block, then return false since it is never legal to move def
90 bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
91 MachineBasicBlock *MBB,
92 MachineBasicBlock *DefMBB,
93 bool &LocalUse) const {
94 assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
95 "Only makes sense for vregs");
96 // Ignoring debug uses is necessary so debug info doesn't affect the code.
97 // This may leave a referencing dbg_value in the original block, before
98 // the definition of the vreg. Dwarf generator handles this although the
99 // user might not get the right info at runtime.
100 for (MachineRegisterInfo::use_nodbg_iterator
101 I = RegInfo->use_nodbg_begin(Reg), E = RegInfo->use_nodbg_end();
103 // Determine the block of the use.
104 MachineInstr *UseInst = &*I;
105 MachineBasicBlock *UseBlock = UseInst->getParent();
106 if (UseBlock == DefMBB) {
111 if (UseInst->isPHI()) {
112 // PHI nodes use the operand in the predecessor block, not the block with
114 UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
117 // Check that it dominates.
118 if (!DT->dominates(MBB, UseBlock))
125 bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
126 DEBUG(dbgs() << "******** Machine Sinking ********\n");
128 const TargetMachine &TM = MF.getTarget();
129 TII = TM.getInstrInfo();
130 TRI = TM.getRegisterInfo();
131 RegInfo = &MF.getRegInfo();
132 DT = &getAnalysis<MachineDominatorTree>();
133 LI = &getAnalysis<MachineLoopInfo>();
134 AA = &getAnalysis<AliasAnalysis>();
135 AllocatableSet = TRI->getAllocatableSet(MF);
137 bool EverMadeChange = false;
140 bool MadeChange = false;
142 // Process all basic blocks.
143 for (MachineFunction::iterator I = MF.begin(), E = MF.end();
145 MadeChange |= ProcessBlock(*I);
147 // If this iteration over the code changed anything, keep iterating.
148 if (!MadeChange) break;
149 EverMadeChange = true;
151 return EverMadeChange;
154 bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
155 // Can't sink anything out of a block that has less than two successors.
156 if (MBB.succ_size() <= 1 || MBB.empty()) return false;
158 // Don't bother sinking code out of unreachable blocks. In addition to being
159 // unprofitable, it can also lead to infinite looping, because in an
160 // unreachable loop there may be nowhere to stop.
161 if (!DT->isReachableFromEntry(&MBB)) return false;
163 bool MadeChange = false;
165 // Walk the basic block bottom-up. Remember if we saw a store.
166 MachineBasicBlock::iterator I = MBB.end();
168 bool ProcessedBegin, SawStore = false;
170 MachineInstr *MI = I; // The instruction to sink.
172 // Predecrement I (if it's not begin) so that it isn't invalidated by
174 ProcessedBegin = I == MBB.begin();
178 if (MI->isDebugValue())
181 if (SinkInstruction(MI, SawStore))
182 ++NumSunk, MadeChange = true;
184 // If we just processed the first instruction in the block, we're done.
185 } while (!ProcessedBegin);
190 MachineBasicBlock *MachineSinking::SplitCriticalEdge(MachineBasicBlock *FromBB,
191 MachineBasicBlock *ToBB) {
192 // Avoid breaking back edge. From == To means backedge for single BB loop.
193 if (!SplitEdges || NumSplit == SplitLimit || FromBB == ToBB)
196 // Check for more "complex" loops.
197 if (LI->getLoopFor(FromBB) != LI->getLoopFor(ToBB) ||
198 !LI->isLoopHeader(ToBB)) {
199 // It's not always legal to break critical edges and sink the computation
207 // ... no uses of v1024
213 // If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted:
222 // ... no uses of v1024
228 // This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3
229 // flow. We need to ensure the new basic block where the computation is
230 // sunk to dominates all the uses.
231 // It's only legal to break critical edge and sink the computation to the
232 // new block if all the predecessors of "To", except for "From", are
233 // not dominated by "From". Given SSA property, this means these
234 // predecessors are dominated by "To".
235 for (MachineBasicBlock::pred_iterator PI = ToBB->pred_begin(),
236 E = ToBB->pred_end(); PI != E; ++PI) {
239 if (!DT->dominates(ToBB, *PI))
243 // FIXME: Determine if it's cost effective to break this edge.
244 return FromBB->SplitCriticalEdge(ToBB, this);
250 /// SinkInstruction - Determine whether it is safe to sink the specified machine
251 /// instruction out of its current block into a successor.
252 bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
253 // Check if it's safe to move the instruction.
254 if (!MI->isSafeToMove(TII, AA, SawStore))
257 // FIXME: This should include support for sinking instructions within the
258 // block they are currently in to shorten the live ranges. We often get
259 // instructions sunk into the top of a large block, but it would be better to
260 // also sink them down before their first use in the block. This xform has to
261 // be careful not to *increase* register pressure though, e.g. sinking
262 // "x = y + z" down if it kills y and z would increase the live ranges of y
263 // and z and only shrink the live range of x.
265 // Loop over all the operands of the specified instruction. If there is
266 // anything we can't handle, bail out.
267 MachineBasicBlock *ParentBlock = MI->getParent();
269 // SuccToSinkTo - This is the successor to sink this instruction to, once we
271 MachineBasicBlock *SuccToSinkTo = 0;
273 bool LocalUse = false;
274 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
275 const MachineOperand &MO = MI->getOperand(i);
276 if (!MO.isReg()) continue; // Ignore non-register operands.
278 unsigned Reg = MO.getReg();
279 if (Reg == 0) continue;
281 if (TargetRegisterInfo::isPhysicalRegister(Reg)) {
283 // If the physreg has no defs anywhere, it's just an ambient register
284 // and we can freely move its uses. Alternatively, if it's allocatable,
285 // it could get allocated to something with a def during allocation.
286 if (!RegInfo->def_empty(Reg))
289 if (AllocatableSet.test(Reg))
292 // Check for a def among the register's aliases too.
293 for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
294 unsigned AliasReg = *Alias;
295 if (!RegInfo->def_empty(AliasReg))
298 if (AllocatableSet.test(AliasReg))
301 } else if (!MO.isDead()) {
302 // A def that isn't dead. We can't move it.
306 // Virtual register uses are always safe to sink.
307 if (MO.isUse()) continue;
309 // If it's not safe to move defs of the register class, then abort.
310 if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg)))
313 // FIXME: This picks a successor to sink into based on having one
314 // successor that dominates all the uses. However, there are cases where
315 // sinking can happen but where the sink point isn't a successor. For
322 // the instruction could be sunk over the whole diamond for the
323 // if/then/else (or loop, etc), allowing it to be sunk into other blocks
326 // Virtual register defs can only be sunk if all their uses are in blocks
327 // dominated by one of the successors.
329 // If a previous operand picked a block to sink to, then this operand
330 // must be sinkable to the same block.
331 if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, ParentBlock, LocalUse))
337 // Otherwise, we should look at all the successors and decide which one
338 // we should sink to.
339 for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
340 E = ParentBlock->succ_end(); SI != E; ++SI) {
341 if (AllUsesDominatedByBlock(Reg, *SI, ParentBlock, LocalUse)) {
346 // Def is used locally, it's never safe to move this def.
350 // If we couldn't find a block to sink to, ignore this instruction.
351 if (SuccToSinkTo == 0)
356 // If there are no outputs, it must have side-effects.
357 if (SuccToSinkTo == 0)
360 // It's not safe to sink instructions to EH landing pad. Control flow into
361 // landing pad is implicitly defined.
362 if (SuccToSinkTo->isLandingPad())
365 // It is not possible to sink an instruction into its own block. This can
366 // happen with loops.
367 if (MI->getParent() == SuccToSinkTo)
370 // If the instruction to move defines a dead physical register which is live
371 // when leaving the basic block, don't move it because it could turn into a
372 // "zombie" define of that preg. E.g., EFLAGS. (<rdar://problem/8030636>)
373 for (unsigned I = 0, E = MI->getNumOperands(); I != E; ++I) {
374 const MachineOperand &MO = MI->getOperand(I);
375 if (!MO.isReg()) continue;
376 unsigned Reg = MO.getReg();
377 if (Reg == 0 || !TargetRegisterInfo::isPhysicalRegister(Reg)) continue;
378 if (SuccToSinkTo->isLiveIn(Reg))
382 DEBUG(dbgs() << "Sink instr " << *MI << "\tinto block " << *SuccToSinkTo);
384 // If the block has multiple predecessors, this would introduce computation on
385 // a path that it doesn't already exist. We could split the critical edge,
386 // but for now we just punt.
387 // FIXME: Split critical edges if not backedges.
388 if (SuccToSinkTo->pred_size() > 1) {
389 // We cannot sink a load across a critical edge - there may be stores in
391 bool TryBreak = false;
393 if (!MI->isSafeToMove(TII, AA, store)) {
394 DEBUG(dbgs() << " *** PUNTING: Won't sink load along critical edge.\n");
398 // We don't want to sink across a critical edge if we don't dominate the
399 // successor. We could be introducing calculations to new code paths.
400 if (!TryBreak && !DT->dominates(ParentBlock, SuccToSinkTo)) {
401 DEBUG(dbgs() << " *** PUNTING: Critical edge found\n");
405 // Don't sink instructions into a loop.
406 if (!TryBreak && LI->isLoopHeader(SuccToSinkTo)) {
407 DEBUG(dbgs() << " *** PUNTING: Loop header found\n");
411 // Otherwise we are OK with sinking along a critical edge.
413 DEBUG(dbgs() << "Sinking along critical edge.\n");
415 MachineBasicBlock *NewSucc = SplitCriticalEdge(ParentBlock, SuccToSinkTo);
418 " *** PUNTING: Not legal or profitable to break critical edge\n");
421 DEBUG(dbgs() << "*** Splitting critical edge:"
422 " BB#" << ParentBlock->getNumber()
423 << " -- BB#" << NewSucc->getNumber()
424 << " -- BB#" << SuccToSinkTo->getNumber() << '\n');
425 SuccToSinkTo = NewSucc;
431 // Determine where to insert into. Skip phi nodes.
432 MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
433 while (InsertPos != SuccToSinkTo->end() && InsertPos->isPHI())
436 // Move the instruction.
437 SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
438 ++MachineBasicBlock::iterator(MI));
440 // Conservatively, clear any kill flags, since it's possible that they are no