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
calculateSpillWeightsAndHints(LiveIntervals & LIS,MachineFunction & MF,const MachineLoopInfo & MLI,const MachineBlockFrequencyInfo & MBFI,VirtRegAuxInfo::NormalizingFn norm)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.
copyHint(const MachineInstr * mi,unsigned reg,const TargetRegisterInfo & tri,const MachineRegisterInfo & mri)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
isRematerializable(const LiveInterval & LI,const LiveIntervals & LIS,const TargetInstrInfo & TII)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
calculateSpillWeightAndHint(LiveInterval & li)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