1 //===- CalcSpillWeights.cpp -----------------------------------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 
10 #include "llvm/CodeGen/CalcSpillWeights.h"
11 #include "llvm/ADT/SmallPtrSet.h"
12 #include "llvm/CodeGen/LiveInterval.h"
13 #include "llvm/CodeGen/LiveIntervals.h"
14 #include "llvm/CodeGen/MachineFunction.h"
15 #include "llvm/CodeGen/MachineInstr.h"
16 #include "llvm/CodeGen/MachineLoopInfo.h"
17 #include "llvm/CodeGen/MachineOperand.h"
18 #include "llvm/CodeGen/MachineRegisterInfo.h"
19 #include "llvm/CodeGen/TargetInstrInfo.h"
20 #include "llvm/CodeGen/TargetRegisterInfo.h"
21 #include "llvm/CodeGen/TargetSubtargetInfo.h"
22 #include "llvm/CodeGen/VirtRegMap.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 #include <cassert>
26 #include <tuple>
27 
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "calcspillweights"
31 
32 void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
33                            MachineFunction &MF,
34                            VirtRegMap *VRM,
35                            const MachineLoopInfo &MLI,
36                            const MachineBlockFrequencyInfo &MBFI,
37                            VirtRegAuxInfo::NormalizingFn norm) {
38   DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
39                << "********** Function: " << MF.getName() << '\n');
40 
41   MachineRegisterInfo &MRI = MF.getRegInfo();
42   VirtRegAuxInfo VRAI(MF, LIS, VRM, MLI, MBFI, norm);
43   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
44     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
45     if (MRI.reg_nodbg_empty(Reg))
46       continue;
47     VRAI.calculateSpillWeightAndHint(LIS.getInterval(Reg));
48   }
49 }
50 
51 // Return the preferred allocation register for reg, given a COPY instruction.
52 static unsigned copyHint(const MachineInstr *mi, unsigned reg,
53                          const TargetRegisterInfo &tri,
54                          const MachineRegisterInfo &mri) {
55   unsigned sub, hreg, hsub;
56   if (mi->getOperand(0).getReg() == reg) {
57     sub = mi->getOperand(0).getSubReg();
58     hreg = mi->getOperand(1).getReg();
59     hsub = mi->getOperand(1).getSubReg();
60   } else {
61     sub = mi->getOperand(1).getSubReg();
62     hreg = mi->getOperand(0).getReg();
63     hsub = mi->getOperand(0).getSubReg();
64   }
65 
66   if (!hreg)
67     return 0;
68 
69   if (TargetRegisterInfo::isVirtualRegister(hreg))
70     return sub == hsub ? hreg : 0;
71 
72   const TargetRegisterClass *rc = mri.getRegClass(reg);
73   if (!tri.enableMultipleCopyHints()) {
74     // Only allow physreg hints in rc.
75     if (sub == 0)
76       return rc->contains(hreg) ? hreg : 0;
77 
78     // reg:sub should match the physreg hreg.
79     return tri.getMatchingSuperReg(hreg, sub, rc);
80   }
81 
82   unsigned CopiedPReg = (hsub ? tri.getSubReg(hreg, hsub) : hreg);
83   if (rc->contains(CopiedPReg))
84     return CopiedPReg;
85 
86   // Check if reg:sub matches so that a super register could be hinted.
87   if (sub)
88     return tri.getMatchingSuperReg(CopiedPReg, sub, rc);
89 
90   return 0;
91 }
92 
93 // Check if all values in LI are rematerializable
94 static bool isRematerializable(const LiveInterval &LI,
95                                const LiveIntervals &LIS,
96                                VirtRegMap *VRM,
97                                const TargetInstrInfo &TII) {
98   unsigned Reg = LI.reg;
99   unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
100   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
101        I != E; ++I) {
102     const VNInfo *VNI = *I;
103     if (VNI->isUnused())
104       continue;
105     if (VNI->isPHIDef())
106       return false;
107 
108     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
109     assert(MI && "Dead valno in interval");
110 
111     // Trace copies introduced by live range splitting.  The inline
112     // spiller can rematerialize through these copies, so the spill
113     // weight must reflect this.
114     if (VRM) {
115       while (MI->isFullCopy()) {
116         // The copy destination must match the interval register.
117         if (MI->getOperand(0).getReg() != Reg)
118           return false;
119 
120         // Get the source register.
121         Reg = MI->getOperand(1).getReg();
122 
123         // If the original (pre-splitting) registers match this
124         // copy came from a split.
125         if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
126             VRM->getOriginal(Reg) != Original)
127           return false;
128 
129         // Follow the copy live-in value.
130         const LiveInterval &SrcLI = LIS.getInterval(Reg);
131         LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
132         VNI = SrcQ.valueIn();
133         assert(VNI && "Copy from non-existing value");
134         if (VNI->isPHIDef())
135           return false;
136         MI = LIS.getInstructionFromIndex(VNI->def);
137         assert(MI && "Dead valno in interval");
138       }
139     }
140 
141     if (!TII.isTriviallyReMaterializable(*MI, LIS.getAliasAnalysis()))
142       return false;
143   }
144   return true;
145 }
146 
147 void VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
148   float weight = weightCalcHelper(li);
149   // Check if unspillable.
150   if (weight < 0)
151     return;
152   li.weight = weight;
153 }
154 
155 float VirtRegAuxInfo::futureWeight(LiveInterval &li, SlotIndex start,
156                                    SlotIndex end) {
157   return weightCalcHelper(li, &start, &end);
158 }
159 
160 float VirtRegAuxInfo::weightCalcHelper(LiveInterval &li, SlotIndex *start,
161                                        SlotIndex *end) {
162   MachineRegisterInfo &mri = MF.getRegInfo();
163   const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
164   MachineBasicBlock *mbb = nullptr;
165   MachineLoop *loop = nullptr;
166   bool isExiting = false;
167   float totalWeight = 0;
168   unsigned numInstr = 0; // Number of instructions using li
169   SmallPtrSet<MachineInstr*, 8> visited;
170 
171   std::pair<unsigned, unsigned> TargetHint = mri.getRegAllocationHint(li.reg);
172 
173   // Don't recompute spill weight for an unspillable register.
174   bool Spillable = li.isSpillable();
175 
176   bool localSplitArtifact = start && end;
177 
178   // Do not update future local split artifacts.
179   bool updateLI = !localSplitArtifact;
180 
181   if (localSplitArtifact) {
182     MachineBasicBlock *localMBB = LIS.getMBBFromIndex(*end);
183     assert(localMBB == LIS.getMBBFromIndex(*start) &&
184            "start and end are expected to be in the same basic block");
185 
186     // Local split artifact will have 2 additional copy instructions and they
187     // will be in the same BB.
188     // localLI = COPY other
189     // ...
190     // other   = COPY localLI
191     totalWeight += LiveIntervals::getSpillWeight(true, false, &MBFI, localMBB);
192     totalWeight += LiveIntervals::getSpillWeight(false, true, &MBFI, localMBB);
193 
194     numInstr += 2;
195   }
196 
197   // CopyHint is a sortable hint derived from a COPY instruction.
198   struct CopyHint {
199     unsigned Reg;
200     float Weight;
201     bool IsPhys;
202     unsigned HintOrder;
203     CopyHint(unsigned R, float W, bool P, unsigned HR) :
204       Reg(R), Weight(W), IsPhys(P), HintOrder(HR) {}
205     bool operator<(const CopyHint &rhs) const {
206       // Always prefer any physreg hint.
207       if (IsPhys != rhs.IsPhys)
208         return (IsPhys && !rhs.IsPhys);
209       if (Weight != rhs.Weight)
210         return (Weight > rhs.Weight);
211 
212       // This is just a temporary way to achive NFC for targets that don't
213       // enable multiple copy hints. HintOrder should be removed when all
214       // targets return true in enableMultipleCopyHints().
215       return (HintOrder < rhs.HintOrder);
216 
217 #if 0 // Should replace the HintOrder check, see above.
218       // (just for the purpose of maintaining the set)
219       return Reg < rhs.Reg;
220 #endif
221     }
222   };
223   std::set<CopyHint> CopyHints;
224 
225   // Temporary: see comment for HintOrder above.
226   unsigned CopyHintOrder = 0;
227   for (MachineRegisterInfo::reg_instr_iterator
228        I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
229        I != E; ) {
230     MachineInstr *mi = &*(I++);
231 
232     // For local split artifacts, we are interested only in instructions between
233     // the expected start and end of the range.
234     SlotIndex si = LIS.getInstructionIndex(*mi);
235     if (localSplitArtifact && ((si < *start) || (si > *end)))
236       continue;
237 
238     numInstr++;
239     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
240       continue;
241     if (!visited.insert(mi).second)
242       continue;
243 
244     float weight = 1.0f;
245     if (Spillable) {
246       // Get loop info for mi.
247       if (mi->getParent() != mbb) {
248         mbb = mi->getParent();
249         loop = Loops.getLoopFor(mbb);
250         isExiting = loop ? loop->isLoopExiting(mbb) : false;
251       }
252 
253       // Calculate instr weight.
254       bool reads, writes;
255       std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
256       weight = LiveIntervals::getSpillWeight(writes, reads, &MBFI, *mi);
257 
258       // Give extra weight to what looks like a loop induction variable update.
259       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
260         weight *= 3;
261 
262       totalWeight += weight;
263     }
264 
265     // Get allocation hints from copies.
266     if (!mi->isCopy() ||
267         (TargetHint.first != 0 && !tri.enableMultipleCopyHints()))
268       continue;
269     unsigned hint = copyHint(mi, li.reg, tri, mri);
270     if (!hint)
271       continue;
272     // Force hweight onto the stack so that x86 doesn't add hidden precision,
273     // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
274     //
275     // FIXME: we probably shouldn't use floats at all.
276     volatile float hweight = Hint[hint] += weight;
277     if (TargetRegisterInfo::isVirtualRegister(hint) || mri.isAllocatable(hint))
278       CopyHints.insert(CopyHint(hint, hweight, tri.isPhysicalRegister(hint),
279                      (tri.enableMultipleCopyHints() ? hint : CopyHintOrder++)));
280   }
281 
282   Hint.clear();
283 
284   // Pass all the sorted copy hints to mri.
285   if (updateLI && CopyHints.size()) {
286     // Remove a generic hint if previously added by target.
287     if (TargetHint.first == 0 && TargetHint.second)
288       mri.clearSimpleHint(li.reg);
289 
290     for (auto &Hint : CopyHints) {
291       if (TargetHint.first != 0 && Hint.Reg == TargetHint.second)
292         // Don't add again the target-type hint.
293         continue;
294       mri.addRegAllocationHint(li.reg, Hint.Reg);
295       if (!tri.enableMultipleCopyHints())
296         break;
297     }
298 
299     // Weakly boost the spill weight of hinted registers.
300     totalWeight *= 1.01F;
301   }
302 
303   // If the live interval was already unspillable, leave it that way.
304   if (!Spillable)
305     return -1.0;
306 
307   // Mark li as unspillable if all live ranges are tiny and the interval
308   // is not live at any reg mask.  If the interval is live at a reg mask
309   // spilling may be required.
310   if (updateLI && li.isZeroLength(LIS.getSlotIndexes()) &&
311       !li.isLiveAtIndexes(LIS.getRegMaskSlots())) {
312     li.markNotSpillable();
313     return -1.0;
314   }
315 
316   // If all of the definitions of the interval are re-materializable,
317   // it is a preferred candidate for spilling.
318   // FIXME: this gets much more complicated once we support non-trivial
319   // re-materialization.
320   if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
321     totalWeight *= 0.5F;
322 
323   if (localSplitArtifact)
324     return normalize(totalWeight, start->distance(*end), numInstr);
325   return normalize(totalWeight, li.getSize(), numInstr);
326 }
327