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/CodeGen/LiveIntervalAnalysis.h"
12 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
13 #include "llvm/CodeGen/MachineFunction.h"
14 #include "llvm/CodeGen/MachineLoopInfo.h"
15 #include "llvm/CodeGen/MachineRegisterInfo.h"
16 #include "llvm/Support/Debug.h"
17 #include "llvm/Support/raw_ostream.h"
18 #include "llvm/Target/TargetInstrInfo.h"
19 #include "llvm/Target/TargetRegisterInfo.h"
20 #include "llvm/Target/TargetSubtargetInfo.h"
21 using namespace llvm;
22 
23 #define DEBUG_TYPE "calcspillweights"
24 
25 void llvm::calculateSpillWeightsAndHints(LiveIntervals &LIS,
26                            MachineFunction &MF,
27                            const MachineLoopInfo &MLI,
28                            const MachineBlockFrequencyInfo &MBFI,
29                            VirtRegAuxInfo::NormalizingFn norm) {
30   DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
31                << "********** Function: " << MF.getName() << '\n');
32 
33   MachineRegisterInfo &MRI = MF.getRegInfo();
34   VirtRegAuxInfo VRAI(MF, LIS, MLI, MBFI, norm);
35   for (unsigned i = 0, e = MRI.getNumVirtRegs(); i != e; ++i) {
36     unsigned Reg = TargetRegisterInfo::index2VirtReg(i);
37     if (MRI.reg_nodbg_empty(Reg))
38       continue;
39     VRAI.calculateSpillWeightAndHint(LIS.getInterval(Reg));
40   }
41 }
42 
43 // Return the preferred allocation register for reg, given a COPY instruction.
44 static unsigned copyHint(const MachineInstr *mi, unsigned reg,
45                          const TargetRegisterInfo &tri,
46                          const MachineRegisterInfo &mri) {
47   unsigned sub, hreg, hsub;
48   if (mi->getOperand(0).getReg() == reg) {
49     sub = mi->getOperand(0).getSubReg();
50     hreg = mi->getOperand(1).getReg();
51     hsub = mi->getOperand(1).getSubReg();
52   } else {
53     sub = mi->getOperand(1).getSubReg();
54     hreg = mi->getOperand(0).getReg();
55     hsub = mi->getOperand(0).getSubReg();
56   }
57 
58   if (!hreg)
59     return 0;
60 
61   if (TargetRegisterInfo::isVirtualRegister(hreg))
62     return sub == hsub ? hreg : 0;
63 
64   const TargetRegisterClass *rc = mri.getRegClass(reg);
65 
66   // Only allow physreg hints in rc.
67   if (sub == 0)
68     return rc->contains(hreg) ? hreg : 0;
69 
70   // reg:sub should match the physreg hreg.
71   return tri.getMatchingSuperReg(hreg, sub, rc);
72 }
73 
74 // Check if all values in LI are rematerializable
75 static bool isRematerializable(const LiveInterval &LI,
76                                const LiveIntervals &LIS,
77                                const TargetInstrInfo &TII) {
78   for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
79        I != E; ++I) {
80     const VNInfo *VNI = *I;
81     if (VNI->isUnused())
82       continue;
83     if (VNI->isPHIDef())
84       return false;
85 
86     MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
87     assert(MI && "Dead valno in interval");
88 
89     if (!TII.isTriviallyReMaterializable(MI, LIS.getAliasAnalysis()))
90       return false;
91   }
92   return true;
93 }
94 
95 void
96 VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &li) {
97   MachineRegisterInfo &mri = MF.getRegInfo();
98   const TargetRegisterInfo &tri = *MF.getSubtarget().getRegisterInfo();
99   MachineBasicBlock *mbb = nullptr;
100   MachineLoop *loop = nullptr;
101   bool isExiting = false;
102   float totalWeight = 0;
103   unsigned numInstr = 0; // Number of instructions using li
104   SmallPtrSet<MachineInstr*, 8> visited;
105 
106   // Find the best physreg hint and the best virtreg hint.
107   float bestPhys = 0, bestVirt = 0;
108   unsigned hintPhys = 0, hintVirt = 0;
109 
110   // Don't recompute a target specific hint.
111   bool noHint = mri.getRegAllocationHint(li.reg).first != 0;
112 
113   // Don't recompute spill weight for an unspillable register.
114   bool Spillable = li.isSpillable();
115 
116   for (MachineRegisterInfo::reg_instr_iterator
117        I = mri.reg_instr_begin(li.reg), E = mri.reg_instr_end();
118        I != E; ) {
119     MachineInstr *mi = &*(I++);
120     numInstr++;
121     if (mi->isIdentityCopy() || mi->isImplicitDef() || mi->isDebugValue())
122       continue;
123     if (!visited.insert(mi).second)
124       continue;
125 
126     float weight = 1.0f;
127     if (Spillable) {
128       // Get loop info for mi.
129       if (mi->getParent() != mbb) {
130         mbb = mi->getParent();
131         loop = Loops.getLoopFor(mbb);
132         isExiting = loop ? loop->isLoopExiting(mbb) : false;
133       }
134 
135       // Calculate instr weight.
136       bool reads, writes;
137       std::tie(reads, writes) = mi->readsWritesVirtualRegister(li.reg);
138       weight = LiveIntervals::getSpillWeight(
139         writes, reads, &MBFI, mi);
140 
141       // Give extra weight to what looks like a loop induction variable update.
142       if (writes && isExiting && LIS.isLiveOutOfMBB(li, mbb))
143         weight *= 3;
144 
145       totalWeight += weight;
146     }
147 
148     // Get allocation hints from copies.
149     if (noHint || !mi->isCopy())
150       continue;
151     unsigned hint = copyHint(mi, li.reg, tri, mri);
152     if (!hint)
153       continue;
154     // Force hweight onto the stack so that x86 doesn't add hidden precision,
155     // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
156     //
157     // FIXME: we probably shouldn't use floats at all.
158     volatile float hweight = Hint[hint] += weight;
159     if (TargetRegisterInfo::isPhysicalRegister(hint)) {
160       if (hweight > bestPhys && mri.isAllocatable(hint))
161         bestPhys = hweight, hintPhys = hint;
162     } else {
163       if (hweight > bestVirt)
164         bestVirt = hweight, hintVirt = hint;
165     }
166   }
167 
168   Hint.clear();
169 
170   // Always prefer the physreg hint.
171   if (unsigned hint = hintPhys ? hintPhys : hintVirt) {
172     mri.setRegAllocationHint(li.reg, 0, hint);
173     // Weakly boost the spill weight of hinted registers.
174     totalWeight *= 1.01F;
175   }
176 
177   // If the live interval was already unspillable, leave it that way.
178   if (!Spillable)
179     return;
180 
181   // Mark li as unspillable if all live ranges are tiny.
182   if (li.isZeroLength(LIS.getSlotIndexes())) {
183     li.markNotSpillable();
184     return;
185   }
186 
187   // If all of the definitions of the interval are re-materializable,
188   // it is a preferred candidate for spilling.
189   // FIXME: this gets much more complicated once we support non-trivial
190   // re-materialization.
191   if (isRematerializable(li, LIS, *MF.getSubtarget().getInstrInfo()))
192     totalWeight *= 0.5F;
193 
194   li.weight = normalize(totalWeight, li.getSize(), numInstr);
195 }
196