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/LiveIntervalAnalysis.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/VirtRegMap.h"
20 #include "llvm/Support/Debug.h"
21 #include "llvm/Support/raw_ostream.h"
22 #include "llvm/Target/TargetInstrInfo.h"
23 #include "llvm/Target/TargetRegisterInfo.h"
24 #include "llvm/Target/TargetSubtargetInfo.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 
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 // Check if all values in LI are rematerializable
83 static bool isRematerializable(const LiveInterval &LI,
84                                const LiveIntervals &LIS,
85                                VirtRegMap *VRM,
86                                const TargetInstrInfo &TII) {
87   unsigned Reg = LI.reg;
88   unsigned Original = VRM ? VRM->getOriginal(Reg) : 0;
89   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
90        I != E; ++I) {
91     const VNInfo *VNI = *I;
92     if (VNI->isUnused())
93       continue;
94     if (VNI->isPHIDef())
95       return false;
96 
97     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
98     assert(MI && "Dead valno in interval");
99 
100     // Trace copies introduced by live range splitting.  The inline
101     // spiller can rematerialize through these copies, so the spill
102     // weight must reflect this.
103     if (VRM) {
104       while (MI->isFullCopy()) {
105         // The copy destination must match the interval register.
106         if (MI->getOperand(0).getReg() != Reg)
107           return false;
108 
109         // Get the source register.
110         Reg = MI->getOperand(1).getReg();
111 
112         // If the original (pre-splitting) registers match this
113         // copy came from a split.
114         if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
115             VRM->getOriginal(Reg) != Original)
116           return false;
117 
118         // Follow the copy live-in value.
119         const LiveInterval &SrcLI = LIS.getInterval(Reg);
120         LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
121         VNI = SrcQ.valueIn();
122         assert(VNI && "Copy from non-existing value");
123         if (VNI->isPHIDef())
124           return false;
125         MI = LIS.getInstructionFromIndex(VNI->def);
126         assert(MI && "Dead valno in interval");
127       }
128     }
129 
130     if (!TII.isTriviallyReMaterializable(*MI, LIS.getAliasAnalysis()))
131       return false;
132   }
133   return true;
134 }
135 
136 void VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
137   float weight = weightCalcHelper(li);
138   // Check if unspillable.
139   if (weight < 0)
140     return;
141   li.weight = weight;
142 }
143 
144 float VirtRegAuxInfo::futureWeight(LiveInterval &li, SlotIndex start,
145                                    SlotIndex end) {
146   return weightCalcHelper(li, &start, &end);
147 }
148 
149 float VirtRegAuxInfo::weightCalcHelper(LiveInterval &li, SlotIndex *start,
150                                        SlotIndex *end) {
151   MachineRegisterInfo &mri = MF.getRegInfo();
152   const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
153   MachineBasicBlock *mbb = nullptr;
154   MachineLoop *loop = nullptr;
155   bool isExiting = false;
156   float totalWeight = 0;
157   unsigned numInstr = 0; // Number of instructions using li
158   SmallPtrSet<MachineInstr*, 8> visited;
159 
160   // Find the best physreg hint and the best virtreg hint.
161   float bestPhys = 0, bestVirt = 0;
162   unsigned hintPhys = 0, hintVirt = 0;
163 
164   // Don't recompute a target specific hint.
165   bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
166 
167   // Don't recompute spill weight for an unspillable register.
168   bool Spillable = li.isSpillable();
169 
170   bool localSplitArtifact = start && end;
171 
172   // Do not update future local split artifacts.
173   bool updateLI = !localSplitArtifact;
174 
175   if (localSplitArtifact) {
176     MachineBasicBlock *localMBB = LIS.getMBBFromIndex(*end);
177     assert(localMBB == LIS.getMBBFromIndex(*start) &&
178            "start and end are expected to be in the same basic block");
179 
180     // Local split artifact will have 2 additional copy instructions and they
181     // will be in the same BB.
182     // localLI = COPY other
183     // ...
184     // other   = COPY localLI
185     totalWeight += LiveIntervals::getSpillWeight(true, false, &MBFI, localMBB);
186     totalWeight += LiveIntervals::getSpillWeight(false, true, &MBFI, localMBB);
187 
188     numInstr += 2;
189   }
190 
191   for (MachineRegisterInfo::reg_instr_iterator
192        I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
193        I != E; ) {
194     MachineInstr *mi = &*(I++);
195 
196     // For local split artifacts, we are interested only in instructions between
197     // the expected start and end of the range.
198     SlotIndex si = LIS.getInstructionIndex(*mi);
199     if (localSplitArtifact && ((si < *start) || (si > *end)))
200       continue;
201 
202     numInstr++;
203     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
204       continue;
205     if (!visited.insert(mi).second)
206       continue;
207 
208     float weight = 1.0f;
209     if (Spillable) {
210       // Get loop info for mi.
211       if (mi->getParent() != mbb) {
212         mbb = mi->getParent();
213         loop = Loops.getLoopFor(mbb);
214         isExiting = loop ? loop->isLoopExiting(mbb) : false;
215       }
216 
217       // Calculate instr weight.
218       bool reads, writes;
219       std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
220       weight = LiveIntervals::getSpillWeight(writes, reads, &MBFI, *mi);
221 
222       // Give extra weight to what looks like a loop induction variable update.
223       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
224         weight *= 3;
225 
226       totalWeight += weight;
227     }
228 
229     // Get allocation hints from copies.
230     if (noHint || !mi->isCopy())
231       continue;
232     unsigned hint = copyHint(mi, li.reg, tri, mri);
233     if (!hint)
234       continue;
235     // Force hweight onto the stack so that x86 doesn't add hidden precision,
236     // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
237     //
238     // FIXME: we probably shouldn't use floats at all.
239     volatile float hweight = Hint[hint] += weight;
240     if (TargetRegisterInfo::isPhysicalRegister(hint)) {
241       if (hweight > bestPhys && mri.isAllocatable(hint)) {
242         bestPhys = hweight;
243         hintPhys = hint;
244       }
245     } else {
246       if (hweight > bestVirt) {
247         bestVirt = hweight;
248         hintVirt = hint;
249       }
250     }
251   }
252 
253   Hint.clear();
254 
255   // Always prefer the physreg hint.
256   if (updateLI) {
257     if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
258       mri.setRegAllocationHint(li.reg, 0, hint);
259       // Weakly boost the spill weight of hinted registers.
260       totalWeight *= 1.01F;
261     }
262   }
263 
264   // If the live interval was already unspillable, leave it that way.
265   if (!Spillable)
266     return -1.0;
267 
268   // Mark li as unspillable if all live ranges are tiny and the interval
269   // is not live at any reg mask.  If the interval is live at a reg mask
270   // spilling may be required.
271   if (updateLI && li.isZeroLength(LIS.getSlotIndexes()) &&
272       !li.isLiveAtIndexes(LIS.getRegMaskSlots())) {
273     li.markNotSpillable();
274     return -1.0;
275   }
276 
277   // If all of the definitions of the interval are re-materializable,
278   // it is a preferred candidate for spilling.
279   // FIXME: this gets much more complicated once we support non-trivial
280   // re-materialization.
281   if (isRematerializable(li, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
282     totalWeight *= 0.5F;
283 
284   if (localSplitArtifact)
285     return normalize(totalWeight, start->distance(*end), numInstr);
286   return normalize(totalWeight, li.getSize(), numInstr);
287 }
288