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 #define DEBUG_TYPE "calcspillweights"
11 
12 #include "llvm/Function.h"
13 #include "llvm/ADT/SmallSet.h"
14 #include "llvm/CodeGen/CalcSpillWeights.h"
15 #include "llvm/CodeGen/LiveIntervalAnalysis.h"
16 #include "llvm/CodeGen/MachineFunction.h"
17 #include "llvm/CodeGen/MachineLoopInfo.h"
18 #include "llvm/CodeGen/MachineRegisterInfo.h"
19 #include "llvm/CodeGen/SlotIndexes.h"
20 #include "llvm/Support/Debug.h"
21 #include "llvm/Support/raw_ostream.h"
22 #include "llvm/Target/TargetInstrInfo.h"
23 #include "llvm/Target/TargetMachine.h"
24 #include "llvm/Target/TargetRegisterInfo.h"
25 using namespace llvm;
26 
27 char CalculateSpillWeights::ID = 0;
28 INITIALIZE_PASS_BEGIN(CalculateSpillWeights, "calcspillweights",
29                 "Calculate spill weights", false, false)
30 INITIALIZE_PASS_DEPENDENCY(LiveIntervals)
31 INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
32 INITIALIZE_PASS_END(CalculateSpillWeights, "calcspillweights",
33                 "Calculate spill weights", false, false)
34 
35 void CalculateSpillWeights::getAnalysisUsage(AnalysisUsage &au) const {
36   au.addRequired<LiveIntervals>();
37   au.addRequired<MachineLoopInfo>();
38   au.setPreservesAll();
39   MachineFunctionPass::getAnalysisUsage(au);
40 }
41 
42 bool CalculateSpillWeights::runOnMachineFunction(MachineFunction &MF) {
43 
44   DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
45                << "********** Function: "
46                << MF.getFunction()->getName() << '\n');
47 
48   LiveIntervals &LIS = getAnalysis<LiveIntervals>();
49   MachineRegisterInfo &MRI = MF.getRegInfo();
50   VirtRegAuxInfo VRAI(MF, LIS, getAnalysis<MachineLoopInfo>());
51   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
52     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
53     if (MRI.reg_nodbg_empty(Reg))
54       continue;
55     VRAI.CalculateWeightAndHint(LIS.getInterval(Reg));
56   }
57   return false;
58 }
59 
60 // Return the preferred allocation register for reg, given a COPY instruction.
61 static unsigned copyHint(const MachineInstr *mi, unsigned reg,
62                          const TargetRegisterInfo &tri,
63                          const MachineRegisterInfo &mri) {
64   unsigned sub, hreg, hsub;
65   if (mi->getOperand(0).getReg() == reg) {
66     sub = mi->getOperand(0).getSubReg();
67     hreg = mi->getOperand(1).getReg();
68     hsub = mi->getOperand(1).getSubReg();
69   } else {
70     sub = mi->getOperand(1).getSubReg();
71     hreg = mi->getOperand(0).getReg();
72     hsub = mi->getOperand(0).getSubReg();
73   }
74 
75   if (!hreg)
76     return 0;
77 
78   if (TargetRegisterInfo::isVirtualRegister(hreg))
79     return sub == hsub ? hreg : 0;
80 
81   const TargetRegisterClass *rc = mri.getRegClass(reg);
82 
83   // Only allow physreg hints in rc.
84   if (sub == 0)
85     return rc->contains(hreg) ? hreg : 0;
86 
87   // reg:sub should match the physreg hreg.
88   return tri.getMatchingSuperReg(hreg, sub, rc);
89 }
90 
91 // Check if all values in LI are rematerializable
92 static bool isRematerializable(const LiveInterval &LI,
93                                const LiveIntervals &LIS,
94                                const TargetInstrInfo &TII) {
95   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
96        I != E; ++I) {
97     const VNInfo *VNI = *I;
98     if (VNI->isUnused())
99       continue;
100     if (VNI->isPHIDef())
101       return false;
102 
103     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
104     assert(MI && "Dead valno in interval");
105 
106     if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
107       return false;
108   }
109   return true;
110 }
111 
112 void VirtRegAuxInfo::CalculateWeightAndHint(LiveInterval &li) {
113   MachineRegisterInfo &mri = MF.getRegInfo();
114   const TargetRegisterInfo &tri = *MF.getTarget().getRegisterInfo();
115   MachineBasicBlock *mbb = 0;
116   MachineLoop *loop = 0;
117   unsigned loopDepth = 0;
118   bool isExiting = false;
119   float totalWeight = 0;
120   SmallPtrSet<MachineInstr*, 8> visited;
121 
122   // Find the best physreg hist and the best virtreg hint.
123   float bestPhys = 0, bestVirt = 0;
124   unsigned hintPhys = 0, hintVirt = 0;
125 
126   // Don't recompute a target specific hint.
127   bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
128 
129   // Don't recompute spill weight for an unspillable register.
130   bool Spillable = li.isSpillable();
131 
132   for (MachineRegisterInfo::reg_iterator I = mri.reg_begin(li.reg);
133        MachineInstr *mi = I.skipInstruction();) {
134     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
135       continue;
136     if (!visited.insert(mi))
137       continue;
138 
139     float weight = 1.0f;
140     if (Spillable) {
141       // Get loop info for mi.
142       if (mi->getParent() != mbb) {
143         mbb = mi->getParent();
144         loop = Loops.getLoopFor(mbb);
145         loopDepth = loop ? loop->getLoopDepth() : 0;
146         isExiting = loop ? loop->isLoopExiting(mbb) : false;
147       }
148 
149       // Calculate instr weight.
150       bool reads, writes;
151       tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
152       weight = LiveIntervals::getSpillWeight(writes, reads, loopDepth);
153 
154       // Give extra weight to what looks like a loop induction variable update.
155       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
156         weight *= 3;
157 
158       totalWeight += weight;
159     }
160 
161     // Get allocation hints from copies.
162     if (noHint || !mi->isCopy())
163       continue;
164     unsigned hint = copyHint(mi, li.reg, tri, mri);
165     if (!hint)
166       continue;
167     float hweight = Hint[hint] += weight;
168     if (TargetRegisterInfo::isPhysicalRegister(hint)) {
169       if (hweight > bestPhys && LIS.isAllocatable(hint))
170         bestPhys = hweight, hintPhys = hint;
171     } else {
172       if (hweight > bestVirt)
173         bestVirt = hweight, hintVirt = hint;
174     }
175   }
176 
177   Hint.clear();
178 
179   // Always prefer the physreg hint.
180   if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
181     mri.setRegAllocationHint(li.reg, 0, hint);
182     // Weakly boost the spill weight of hinted registers.
183     totalWeight *= 1.01F;
184   }
185 
186   // If the live interval was already unspillable, leave it that way.
187   if (!Spillable)
188     return;
189 
190   // Mark li as unspillable if all live ranges are tiny.
191   if (li.isZeroLength(LIS.getSlotIndexes())) {
192     li.markNotSpillable();
193     return;
194   }
195 
196   // If all of the definitions of the interval are re-materializable,
197   // it is a preferred candidate for spilling.
198   // FIXME: this gets much more complicated once we support non-trivial
199   // re-materialization.
200   if (isRematerializable(li, LIS, *MF.getTarget().getInstrInfo()))
201     totalWeight *= 0.5F;
202 
203   li.weight = normalizeSpillWeight(totalWeight, li.getSize());
204 }
205