1 //===-- LiveRangeEdit.cpp - Basic tools for editing a register live range -===//
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 // The LiveRangeEdit class represents changes done to a virtual register when it
10 // is spilled or split.
11 //===----------------------------------------------------------------------===//
12 
13 #include "llvm/CodeGen/LiveRangeEdit.h"
14 #include "llvm/ADT/Statistic.h"
15 #include "llvm/CodeGen/CalcSpillWeights.h"
16 #include "llvm/CodeGen/LiveIntervals.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18 #include "llvm/CodeGen/TargetInstrInfo.h"
19 #include "llvm/CodeGen/VirtRegMap.h"
20 #include "llvm/Support/Debug.h"
21 #include "llvm/Support/raw_ostream.h"
22 
23 using namespace llvm;
24 
25 #define DEBUG_TYPE "regalloc"
26 
27 STATISTIC(NumDCEDeleted,     "Number of instructions deleted by DCE");
28 STATISTIC(NumDCEFoldedLoads, "Number of single use loads folded after DCE");
29 STATISTIC(NumFracRanges,     "Number of live ranges fractured by DCE");
30 
31 void LiveRangeEdit::Delegate::anchor() { }
32 
33 LiveInterval &LiveRangeEdit::createEmptyIntervalFrom(Register OldReg,
34                                                      bool createSubRanges) {
35   Register VReg = MRI.createVirtualRegister(MRI.getRegClass(OldReg));
36   if (VRM)
37     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
38 
39   LiveInterval &LI = LIS.createEmptyInterval(VReg);
40   if (Parent && !Parent->isSpillable())
41     LI.markNotSpillable();
42   if (createSubRanges) {
43     // Create empty subranges if the OldReg's interval has them. Do not create
44     // the main range here---it will be constructed later after the subranges
45     // have been finalized.
46     LiveInterval &OldLI = LIS.getInterval(OldReg);
47     VNInfo::Allocator &Alloc = LIS.getVNInfoAllocator();
48     for (LiveInterval::SubRange &S : OldLI.subranges())
49       LI.createSubRange(Alloc, S.LaneMask);
50   }
51   return LI;
52 }
53 
54 Register LiveRangeEdit::createFrom(Register OldReg) {
55   Register VReg = MRI.createVirtualRegister(MRI.getRegClass(OldReg));
56   if (VRM) {
57     VRM->setIsSplitFromReg(VReg, VRM->getOriginal(OldReg));
58   }
59   // FIXME: Getting the interval here actually computes it.
60   // In theory, this may not be what we want, but in practice
61   // the createEmptyIntervalFrom API is used when this is not
62   // the case. Generally speaking we just want to annotate the
63   // LiveInterval when it gets created but we cannot do that at
64   // the moment.
65   if (Parent && !Parent->isSpillable())
66     LIS.getInterval(VReg).markNotSpillable();
67   return VReg;
68 }
69 
70 bool LiveRangeEdit::checkRematerializable(VNInfo *VNI,
71                                           const MachineInstr *DefMI,
72                                           AAResults *aa) {
73   assert(DefMI && "Missing instruction");
74   ScannedRemattable = true;
75   if (!TII.isTriviallyReMaterializable(*DefMI, aa))
76     return false;
77   Remattable.insert(VNI);
78   return true;
79 }
80 
81 void LiveRangeEdit::scanRemattable(AAResults *aa) {
82   for (VNInfo *VNI : getParent().valnos) {
83     if (VNI->isUnused())
84       continue;
85     unsigned Original = VRM->getOriginal(getReg());
86     LiveInterval &OrigLI = LIS.getInterval(Original);
87     VNInfo *OrigVNI = OrigLI.getVNInfoAt(VNI->def);
88     if (!OrigVNI)
89       continue;
90     MachineInstr *DefMI = LIS.getInstructionFromIndex(OrigVNI->def);
91     if (!DefMI)
92       continue;
93     checkRematerializable(OrigVNI, DefMI, aa);
94   }
95   ScannedRemattable = true;
96 }
97 
98 bool LiveRangeEdit::anyRematerializable(AAResults *aa) {
99   if (!ScannedRemattable)
100     scanRemattable(aa);
101   return !Remattable.empty();
102 }
103 
104 /// allUsesAvailableAt - Return true if all registers used by OrigMI at
105 /// OrigIdx are also available with the same value at UseIdx.
106 bool LiveRangeEdit::allUsesAvailableAt(const MachineInstr *OrigMI,
107                                        SlotIndex OrigIdx,
108                                        SlotIndex UseIdx) const {
109   OrigIdx = OrigIdx.getRegSlot(true);
110   UseIdx = UseIdx.getRegSlot(true);
111   for (unsigned i = 0, e = OrigMI->getNumOperands(); i != e; ++i) {
112     const MachineOperand &MO = OrigMI->getOperand(i);
113     if (!MO.isReg() || !MO.getReg() || !MO.readsReg())
114       continue;
115 
116     // We can't remat physreg uses, unless it is a constant or target wants
117     // to ignore this use.
118     if (Register::isPhysicalRegister(MO.getReg())) {
119       if (MRI.isConstantPhysReg(MO.getReg()) || TII.isIgnorableUse(MO))
120         continue;
121       return false;
122     }
123 
124     LiveInterval &li = LIS.getInterval(MO.getReg());
125     const VNInfo *OVNI = li.getVNInfoAt(OrigIdx);
126     if (!OVNI)
127       continue;
128 
129     // Don't allow rematerialization immediately after the original def.
130     // It would be incorrect if OrigMI redefines the register.
131     // See PR14098.
132     if (SlotIndex::isSameInstr(OrigIdx, UseIdx))
133       return false;
134 
135     if (OVNI != li.getVNInfoAt(UseIdx))
136       return false;
137   }
138   return true;
139 }
140 
141 bool LiveRangeEdit::canRematerializeAt(Remat &RM, VNInfo *OrigVNI,
142                                        SlotIndex UseIdx, bool cheapAsAMove) {
143   assert(ScannedRemattable && "Call anyRematerializable first");
144 
145   // Use scanRemattable info.
146   if (!Remattable.count(OrigVNI))
147     return false;
148 
149   // No defining instruction provided.
150   SlotIndex DefIdx;
151   assert(RM.OrigMI && "No defining instruction for remattable value");
152   DefIdx = LIS.getInstructionIndex(*RM.OrigMI);
153 
154   // If only cheap remats were requested, bail out early.
155   if (cheapAsAMove && !TII.isAsCheapAsAMove(*RM.OrigMI))
156     return false;
157 
158   // Verify that all used registers are available with the same values.
159   if (!allUsesAvailableAt(RM.OrigMI, DefIdx, UseIdx))
160     return false;
161 
162   return true;
163 }
164 
165 SlotIndex LiveRangeEdit::rematerializeAt(MachineBasicBlock &MBB,
166                                          MachineBasicBlock::iterator MI,
167                                          unsigned DestReg,
168                                          const Remat &RM,
169                                          const TargetRegisterInfo &tri,
170                                          bool Late) {
171   assert(RM.OrigMI && "Invalid remat");
172   TII.reMaterialize(MBB, MI, DestReg, 0, *RM.OrigMI, tri);
173   // DestReg of the cloned instruction cannot be Dead. Set isDead of DestReg
174   // to false anyway in case the isDead flag of RM.OrigMI's dest register
175   // is true.
176   (*--MI).getOperand(0).setIsDead(false);
177   Rematted.insert(RM.ParentVNI);
178   return LIS.getSlotIndexes()->insertMachineInstrInMaps(*MI, Late).getRegSlot();
179 }
180 
181 void LiveRangeEdit::eraseVirtReg(Register Reg) {
182   if (TheDelegate && TheDelegate->LRE_CanEraseVirtReg(Reg))
183     LIS.removeInterval(Reg);
184 }
185 
186 bool LiveRangeEdit::foldAsLoad(LiveInterval *LI,
187                                SmallVectorImpl<MachineInstr*> &Dead) {
188   MachineInstr *DefMI = nullptr, *UseMI = nullptr;
189 
190   // Check that there is a single def and a single use.
191   for (MachineOperand &MO : MRI.reg_nodbg_operands(LI->reg())) {
192     MachineInstr *MI = MO.getParent();
193     if (MO.isDef()) {
194       if (DefMI && DefMI != MI)
195         return false;
196       if (!MI->canFoldAsLoad())
197         return false;
198       DefMI = MI;
199     } else if (!MO.isUndef()) {
200       if (UseMI && UseMI != MI)
201         return false;
202       // FIXME: Targets don't know how to fold subreg uses.
203       if (MO.getSubReg())
204         return false;
205       UseMI = MI;
206     }
207   }
208   if (!DefMI || !UseMI)
209     return false;
210 
211   // Since we're moving the DefMI load, make sure we're not extending any live
212   // ranges.
213   if (!allUsesAvailableAt(DefMI, LIS.getInstructionIndex(*DefMI),
214                           LIS.getInstructionIndex(*UseMI)))
215     return false;
216 
217   // We also need to make sure it is safe to move the load.
218   // Assume there are stores between DefMI and UseMI.
219   bool SawStore = true;
220   if (!DefMI->isSafeToMove(nullptr, SawStore))
221     return false;
222 
223   LLVM_DEBUG(dbgs() << "Try to fold single def: " << *DefMI
224                     << "       into single use: " << *UseMI);
225 
226   SmallVector<unsigned, 8> Ops;
227   if (UseMI->readsWritesVirtualRegister(LI->reg(), &Ops).second)
228     return false;
229 
230   MachineInstr *FoldMI = TII.foldMemoryOperand(*UseMI, Ops, *DefMI, &LIS);
231   if (!FoldMI)
232     return false;
233   LLVM_DEBUG(dbgs() << "                folded: " << *FoldMI);
234   LIS.ReplaceMachineInstrInMaps(*UseMI, *FoldMI);
235   // Update the call site info.
236   if (UseMI->shouldUpdateCallSiteInfo())
237     UseMI->getMF()->moveCallSiteInfo(UseMI, FoldMI);
238   UseMI->eraseFromParent();
239   DefMI->addRegisterDead(LI->reg(), nullptr);
240   Dead.push_back(DefMI);
241   ++NumDCEFoldedLoads;
242   return true;
243 }
244 
245 bool LiveRangeEdit::useIsKill(const LiveInterval &LI,
246                               const MachineOperand &MO) const {
247   const MachineInstr &MI = *MO.getParent();
248   SlotIndex Idx = LIS.getInstructionIndex(MI).getRegSlot();
249   if (LI.Query(Idx).isKill())
250     return true;
251   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
252   unsigned SubReg = MO.getSubReg();
253   LaneBitmask LaneMask = TRI.getSubRegIndexLaneMask(SubReg);
254   for (const LiveInterval::SubRange &S : LI.subranges()) {
255     if ((S.LaneMask & LaneMask).any() && S.Query(Idx).isKill())
256       return true;
257   }
258   return false;
259 }
260 
261 /// Find all live intervals that need to shrink, then remove the instruction.
262 void LiveRangeEdit::eliminateDeadDef(MachineInstr *MI, ToShrinkSet &ToShrink,
263                                      AAResults *AA) {
264   assert(MI->allDefsAreDead() && "Def isn't really dead");
265   SlotIndex Idx = LIS.getInstructionIndex(*MI).getRegSlot();
266 
267   // Never delete a bundled instruction.
268   if (MI->isBundled()) {
269     return;
270   }
271   // Never delete inline asm.
272   if (MI->isInlineAsm()) {
273     LLVM_DEBUG(dbgs() << "Won't delete: " << Idx << '\t' << *MI);
274     return;
275   }
276 
277   // Use the same criteria as DeadMachineInstructionElim.
278   bool SawStore = false;
279   if (!MI->isSafeToMove(nullptr, SawStore)) {
280     LLVM_DEBUG(dbgs() << "Can't delete: " << Idx << '\t' << *MI);
281     return;
282   }
283 
284   LLVM_DEBUG(dbgs() << "Deleting dead def " << Idx << '\t' << *MI);
285 
286   // Collect virtual registers to be erased after MI is gone.
287   SmallVector<unsigned, 8> RegsToErase;
288   bool ReadsPhysRegs = false;
289   bool isOrigDef = false;
290   unsigned Dest;
291   // Only optimize rematerialize case when the instruction has one def, since
292   // otherwise we could leave some dead defs in the code.  This case is
293   // extremely rare.
294   if (VRM && MI->getOperand(0).isReg() && MI->getOperand(0).isDef() &&
295       MI->getDesc().getNumDefs() == 1) {
296     Dest = MI->getOperand(0).getReg();
297     unsigned Original = VRM->getOriginal(Dest);
298     LiveInterval &OrigLI = LIS.getInterval(Original);
299     VNInfo *OrigVNI = OrigLI.getVNInfoAt(Idx);
300     // The original live-range may have been shrunk to
301     // an empty live-range. It happens when it is dead, but
302     // we still keep it around to be able to rematerialize
303     // other values that depend on it.
304     if (OrigVNI)
305       isOrigDef = SlotIndex::isSameInstr(OrigVNI->def, Idx);
306   }
307 
308   // Check for live intervals that may shrink
309   for (MachineInstr::mop_iterator MOI = MI->operands_begin(),
310          MOE = MI->operands_end(); MOI != MOE; ++MOI) {
311     if (!MOI->isReg())
312       continue;
313     Register Reg = MOI->getReg();
314     if (!Register::isVirtualRegister(Reg)) {
315       // Check if MI reads any unreserved physregs.
316       if (Reg && MOI->readsReg() && !MRI.isReserved(Reg))
317         ReadsPhysRegs = true;
318       else if (MOI->isDef())
319         LIS.removePhysRegDefAt(Reg.asMCReg(), Idx);
320       continue;
321     }
322     LiveInterval &LI = LIS.getInterval(Reg);
323 
324     // Shrink read registers, unless it is likely to be expensive and
325     // unlikely to change anything. We typically don't want to shrink the
326     // PIC base register that has lots of uses everywhere.
327     // Always shrink COPY uses that probably come from live range splitting.
328     if ((MI->readsVirtualRegister(Reg) && (MI->isCopy() || MOI->isDef())) ||
329         (MOI->readsReg() && (MRI.hasOneNonDBGUse(Reg) || useIsKill(LI, *MOI))))
330       ToShrink.insert(&LI);
331 
332     // Remove defined value.
333     if (MOI->isDef()) {
334       if (TheDelegate && LI.getVNInfoAt(Idx) != nullptr)
335         TheDelegate->LRE_WillShrinkVirtReg(LI.reg());
336       LIS.removeVRegDefAt(LI, Idx);
337       if (LI.empty())
338         RegsToErase.push_back(Reg);
339     }
340   }
341 
342   // Currently, we don't support DCE of physreg live ranges. If MI reads
343   // any unreserved physregs, don't erase the instruction, but turn it into
344   // a KILL instead. This way, the physreg live ranges don't end up
345   // dangling.
346   // FIXME: It would be better to have something like shrinkToUses() for
347   // physregs. That could potentially enable more DCE and it would free up
348   // the physreg. It would not happen often, though.
349   if (ReadsPhysRegs) {
350     MI->setDesc(TII.get(TargetOpcode::KILL));
351     // Remove all operands that aren't physregs.
352     for (unsigned i = MI->getNumOperands(); i; --i) {
353       const MachineOperand &MO = MI->getOperand(i-1);
354       if (MO.isReg() && Register::isPhysicalRegister(MO.getReg()))
355         continue;
356       MI->RemoveOperand(i-1);
357     }
358     LLVM_DEBUG(dbgs() << "Converted physregs to:\t" << *MI);
359   } else {
360     // If the dest of MI is an original reg and MI is reMaterializable,
361     // don't delete the inst. Replace the dest with a new reg, and keep
362     // the inst for remat of other siblings. The inst is saved in
363     // LiveRangeEdit::DeadRemats and will be deleted after all the
364     // allocations of the func are done.
365     if (isOrigDef && DeadRemats && TII.isTriviallyReMaterializable(*MI, AA)) {
366       LiveInterval &NewLI = createEmptyIntervalFrom(Dest, false);
367       VNInfo *VNI = NewLI.getNextValue(Idx, LIS.getVNInfoAllocator());
368       NewLI.addSegment(LiveInterval::Segment(Idx, Idx.getDeadSlot(), VNI));
369       pop_back();
370       DeadRemats->insert(MI);
371       const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
372       MI->substituteRegister(Dest, NewLI.reg(), 0, TRI);
373       MI->getOperand(0).setIsDead(true);
374     } else {
375       if (TheDelegate)
376         TheDelegate->LRE_WillEraseInstruction(MI);
377       LIS.RemoveMachineInstrFromMaps(*MI);
378       MI->eraseFromParent();
379       ++NumDCEDeleted;
380     }
381   }
382 
383   // Erase any virtregs that are now empty and unused. There may be <undef>
384   // uses around. Keep the empty live range in that case.
385   for (unsigned i = 0, e = RegsToErase.size(); i != e; ++i) {
386     Register Reg = RegsToErase[i];
387     if (LIS.hasInterval(Reg) && MRI.reg_nodbg_empty(Reg)) {
388       ToShrink.remove(&LIS.getInterval(Reg));
389       eraseVirtReg(Reg);
390     }
391   }
392 }
393 
394 void LiveRangeEdit::eliminateDeadDefs(SmallVectorImpl<MachineInstr *> &Dead,
395                                       ArrayRef<Register> RegsBeingSpilled,
396                                       AAResults *AA) {
397   ToShrinkSet ToShrink;
398 
399   for (;;) {
400     // Erase all dead defs.
401     while (!Dead.empty())
402       eliminateDeadDef(Dead.pop_back_val(), ToShrink, AA);
403 
404     if (ToShrink.empty())
405       break;
406 
407     // Shrink just one live interval. Then delete new dead defs.
408     LiveInterval *LI = ToShrink.back();
409     ToShrink.pop_back();
410     if (foldAsLoad(LI, Dead))
411       continue;
412     unsigned VReg = LI->reg();
413     if (TheDelegate)
414       TheDelegate->LRE_WillShrinkVirtReg(VReg);
415     if (!LIS.shrinkToUses(LI, &Dead))
416       continue;
417 
418     // Don't create new intervals for a register being spilled.
419     // The new intervals would have to be spilled anyway so its not worth it.
420     // Also they currently aren't spilled so creating them and not spilling
421     // them results in incorrect code.
422     bool BeingSpilled = false;
423     for (unsigned i = 0, e = RegsBeingSpilled.size(); i != e; ++i) {
424       if (VReg == RegsBeingSpilled[i]) {
425         BeingSpilled = true;
426         break;
427       }
428     }
429 
430     if (BeingSpilled) continue;
431 
432     // LI may have been separated, create new intervals.
433     LI->RenumberValues();
434     SmallVector<LiveInterval*, 8> SplitLIs;
435     LIS.splitSeparateComponents(*LI, SplitLIs);
436     if (!SplitLIs.empty())
437       ++NumFracRanges;
438 
439     Register Original = VRM ? VRM->getOriginal(VReg) : Register();
440     for (const LiveInterval *SplitLI : SplitLIs) {
441       // If LI is an original interval that hasn't been split yet, make the new
442       // intervals their own originals instead of referring to LI. The original
443       // interval must contain all the split products, and LI doesn't.
444       if (Original != VReg && Original != 0)
445         VRM->setIsSplitFromReg(SplitLI->reg(), Original);
446       if (TheDelegate)
447         TheDelegate->LRE_DidCloneVirtReg(SplitLI->reg(), VReg);
448     }
449   }
450 }
451 
452 // Keep track of new virtual registers created via
453 // MachineRegisterInfo::createVirtualRegister.
454 void
455 LiveRangeEdit::MRI_NoteNewVirtualRegister(Register VReg) {
456   if (VRM)
457     VRM->grow();
458 
459   NewRegs.push_back(VReg);
460 }
461 
462 void LiveRangeEdit::calculateRegClassAndHint(MachineFunction &MF,
463                                              VirtRegAuxInfo &VRAI) {
464   for (unsigned I = 0, Size = size(); I < Size; ++I) {
465     LiveInterval &LI = LIS.getInterval(get(I));
466     if (MRI.recomputeRegClass(LI.reg()))
467       LLVM_DEBUG({
468         const TargetRegisterInfo *TRI = MF.getSubtarget().getRegisterInfo();
469         dbgs() << "Inflated " << printReg(LI.reg()) << " to "
470                << TRI->getRegClassName(MRI.getRegClass(LI.reg())) << '\n';
471       });
472     VRAI.calculateSpillWeightAndHint(LI);
473   }
474 }
475