1 //===- CalcSpillWeights.cpp -----------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8
9 #include "llvm/CodeGen/CalcSpillWeights.h"
10 #include "llvm/ADT/SmallPtrSet.h"
11 #include "llvm/ADT/SmallSet.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/StackMaps.h"
20 #include "llvm/CodeGen/TargetInstrInfo.h"
21 #include "llvm/CodeGen/TargetRegisterInfo.h"
22 #include "llvm/CodeGen/TargetSubtargetInfo.h"
23 #include "llvm/CodeGen/VirtRegMap.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/raw_ostream.h"
26 #include <cassert>
27 #include <tuple>
28
29 using namespace llvm;
30
31 #define DEBUG_TYPE "calcspillweights"
32
calculateSpillWeightsAndHints()33 void VirtRegAuxInfo::calculateSpillWeightsAndHints() {
34 LLVM_DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
35 << "********** Function: " << MF.getName() << '\n');
36
37 MachineRegisterInfo &MRI = MF.getRegInfo();
38 for (unsigned I = 0, E = MRI.getNumVirtRegs(); I != E; ++I) {
39 Register Reg = Register::index2VirtReg(I);
40 if (MRI.reg_nodbg_empty(Reg))
41 continue;
42 calculateSpillWeightAndHint(LIS.getInterval(Reg));
43 }
44 }
45
46 // Return the preferred allocation register for reg, given a COPY instruction.
copyHint(const MachineInstr * MI,unsigned Reg,const TargetRegisterInfo & TRI,const MachineRegisterInfo & MRI)47 Register VirtRegAuxInfo::copyHint(const MachineInstr *MI, unsigned Reg,
48 const TargetRegisterInfo &TRI,
49 const MachineRegisterInfo &MRI) {
50 unsigned Sub, HSub;
51 Register HReg;
52 if (MI->getOperand(0).getReg() == Reg) {
53 Sub = MI->getOperand(0).getSubReg();
54 HReg = MI->getOperand(1).getReg();
55 HSub = MI->getOperand(1).getSubReg();
56 } else {
57 Sub = MI->getOperand(1).getSubReg();
58 HReg = MI->getOperand(0).getReg();
59 HSub = MI->getOperand(0).getSubReg();
60 }
61
62 if (!HReg)
63 return 0;
64
65 if (HReg.isVirtual())
66 return Sub == HSub ? HReg : Register();
67
68 const TargetRegisterClass *RC = MRI.getRegClass(Reg);
69 MCRegister CopiedPReg = HSub ? TRI.getSubReg(HReg, HSub) : HReg.asMCReg();
70 if (RC->contains(CopiedPReg))
71 return CopiedPReg;
72
73 // Check if reg:sub matches so that a super register could be hinted.
74 if (Sub)
75 return TRI.getMatchingSuperReg(CopiedPReg, Sub, RC);
76
77 return 0;
78 }
79
80 // Check if all values in LI are rematerializable
isRematerializable(const LiveInterval & LI,const LiveIntervals & LIS,const VirtRegMap & VRM,const TargetInstrInfo & TII)81 bool VirtRegAuxInfo::isRematerializable(const LiveInterval &LI,
82 const LiveIntervals &LIS,
83 const VirtRegMap &VRM,
84 const TargetInstrInfo &TII) {
85 Register Reg = LI.reg();
86 Register Original = VRM.getOriginal(Reg);
87 for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
88 I != E; ++I) {
89 const VNInfo *VNI = *I;
90 if (VNI->isUnused())
91 continue;
92 if (VNI->isPHIDef())
93 return false;
94
95 MachineInstr *MI = LIS.getInstructionFromIndex(VNI->def);
96 assert(MI && "Dead valno in interval");
97
98 // Trace copies introduced by live range splitting. The inline
99 // spiller can rematerialize through these copies, so the spill
100 // weight must reflect this.
101 while (TII.isFullCopyInstr(*MI)) {
102 // The copy destination must match the interval register.
103 if (MI->getOperand(0).getReg() != Reg)
104 return false;
105
106 // Get the source register.
107 Reg = MI->getOperand(1).getReg();
108
109 // If the original (pre-splitting) registers match this
110 // copy came from a split.
111 if (!Reg.isVirtual() || VRM.getOriginal(Reg) != Original)
112 return false;
113
114 // Follow the copy live-in value.
115 const LiveInterval &SrcLI = LIS.getInterval(Reg);
116 LiveQueryResult SrcQ = SrcLI.Query(VNI->def);
117 VNI = SrcQ.valueIn();
118 assert(VNI && "Copy from non-existing value");
119 if (VNI->isPHIDef())
120 return false;
121 MI = LIS.getInstructionFromIndex(VNI->def);
122 assert(MI && "Dead valno in interval");
123 }
124
125 if (!TII.isTriviallyReMaterializable(*MI))
126 return false;
127 }
128 return true;
129 }
130
isLiveAtStatepointVarArg(LiveInterval & LI)131 bool VirtRegAuxInfo::isLiveAtStatepointVarArg(LiveInterval &LI) {
132 return any_of(VRM.getRegInfo().reg_operands(LI.reg()),
133 [](MachineOperand &MO) {
134 MachineInstr *MI = MO.getParent();
135 if (MI->getOpcode() != TargetOpcode::STATEPOINT)
136 return false;
137 return StatepointOpers(MI).getVarIdx() <= MO.getOperandNo();
138 });
139 }
140
calculateSpillWeightAndHint(LiveInterval & LI)141 void VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &LI) {
142 float Weight = weightCalcHelper(LI);
143 // Check if unspillable.
144 if (Weight < 0)
145 return;
146 LI.setWeight(Weight);
147 }
148
canMemFoldInlineAsm(LiveInterval & LI,const MachineRegisterInfo & MRI)149 static bool canMemFoldInlineAsm(LiveInterval &LI,
150 const MachineRegisterInfo &MRI) {
151 for (const MachineOperand &MO : MRI.reg_operands(LI.reg())) {
152 const MachineInstr *MI = MO.getParent();
153 if (MI->isInlineAsm() && MI->mayFoldInlineAsmRegOp(MI->getOperandNo(&MO)))
154 return true;
155 }
156
157 return false;
158 }
159
weightCalcHelper(LiveInterval & LI,SlotIndex * Start,SlotIndex * End)160 float VirtRegAuxInfo::weightCalcHelper(LiveInterval &LI, SlotIndex *Start,
161 SlotIndex *End) {
162 MachineRegisterInfo &MRI = MF.getRegInfo();
163 const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
164 const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
165 MachineBasicBlock *MBB = nullptr;
166 float TotalWeight = 0;
167 unsigned NumInstr = 0; // Number of instructions using LI
168 SmallPtrSet<MachineInstr *, 8> Visited;
169
170 std::pair<unsigned, Register> TargetHint = MRI.getRegAllocationHint(LI.reg());
171
172 if (LI.isSpillable()) {
173 Register Reg = LI.reg();
174 Register Original = VRM.getOriginal(Reg);
175 const LiveInterval &OrigInt = LIS.getInterval(Original);
176 // li comes from a split of OrigInt. If OrigInt was marked
177 // as not spillable, make sure the new interval is marked
178 // as not spillable as well.
179 if (!OrigInt.isSpillable())
180 LI.markNotSpillable();
181 }
182
183 // Don't recompute spill weight for an unspillable register.
184 bool IsSpillable = LI.isSpillable();
185
186 bool IsLocalSplitArtifact = Start && End;
187
188 // Do not update future local split artifacts.
189 bool ShouldUpdateLI = !IsLocalSplitArtifact;
190
191 if (IsLocalSplitArtifact) {
192 MachineBasicBlock *LocalMBB = LIS.getMBBFromIndex(*End);
193 assert(LocalMBB == LIS.getMBBFromIndex(*Start) &&
194 "start and end are expected to be in the same basic block");
195
196 // Local split artifact will have 2 additional copy instructions and they
197 // will be in the same BB.
198 // localLI = COPY other
199 // ...
200 // other = COPY localLI
201 TotalWeight += LiveIntervals::getSpillWeight(true, false, &MBFI, LocalMBB);
202 TotalWeight += LiveIntervals::getSpillWeight(false, true, &MBFI, LocalMBB);
203
204 NumInstr += 2;
205 }
206
207 // CopyHint is a sortable hint derived from a COPY instruction.
208 struct CopyHint {
209 const Register Reg;
210 const float Weight;
211 CopyHint(Register R, float W) : Reg(R), Weight(W) {}
212 bool operator<(const CopyHint &Rhs) const {
213 // Always prefer any physreg hint.
214 if (Reg.isPhysical() != Rhs.Reg.isPhysical())
215 return Reg.isPhysical();
216 if (Weight != Rhs.Weight)
217 return (Weight > Rhs.Weight);
218 return Reg.id() < Rhs.Reg.id(); // Tie-breaker.
219 }
220 };
221
222 bool IsExiting = false;
223 std::set<CopyHint> CopyHints;
224 DenseMap<unsigned, float> Hint;
225 for (MachineRegisterInfo::reg_instr_nodbg_iterator
226 I = MRI.reg_instr_nodbg_begin(LI.reg()),
227 E = MRI.reg_instr_nodbg_end();
228 I != E;) {
229 MachineInstr *MI = &*(I++);
230
231 // For local split artifacts, we are interested only in instructions between
232 // the expected start and end of the range.
233 SlotIndex SI = LIS.getInstructionIndex(*MI);
234 if (IsLocalSplitArtifact && ((SI < *Start) || (SI > *End)))
235 continue;
236
237 NumInstr++;
238 bool identityCopy = false;
239 auto DestSrc = TII.isCopyInstr(*MI);
240 if (DestSrc) {
241 const MachineOperand *DestRegOp = DestSrc->Destination;
242 const MachineOperand *SrcRegOp = DestSrc->Source;
243 identityCopy = DestRegOp->getReg() == SrcRegOp->getReg() &&
244 DestRegOp->getSubReg() == SrcRegOp->getSubReg();
245 }
246
247 if (identityCopy || MI->isImplicitDef())
248 continue;
249 if (!Visited.insert(MI).second)
250 continue;
251
252 // For terminators that produce values, ask the backend if the register is
253 // not spillable.
254 if (TII.isUnspillableTerminator(MI) && MI->definesRegister(LI.reg())) {
255 LI.markNotSpillable();
256 return -1.0f;
257 }
258
259 float Weight = 1.0f;
260 if (IsSpillable) {
261 // Get loop info for mi.
262 if (MI->getParent() != MBB) {
263 MBB = MI->getParent();
264 const MachineLoop *Loop = Loops.getLoopFor(MBB);
265 IsExiting = Loop ? Loop->isLoopExiting(MBB) : false;
266 }
267
268 // Calculate instr weight.
269 bool Reads, Writes;
270 std::tie(Reads, Writes) = MI->readsWritesVirtualRegister(LI.reg());
271 Weight = LiveIntervals::getSpillWeight(Writes, Reads, &MBFI, *MI);
272
273 // Give extra weight to what looks like a loop induction variable update.
274 if (Writes && IsExiting && LIS.isLiveOutOfMBB(LI, MBB))
275 Weight *= 3;
276
277 TotalWeight += Weight;
278 }
279
280 // Get allocation hints from copies.
281 if (!TII.isCopyInstr(*MI))
282 continue;
283 Register HintReg = copyHint(MI, LI.reg(), TRI, MRI);
284 if (!HintReg)
285 continue;
286 // Force hweight onto the stack so that x86 doesn't add hidden precision,
287 // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
288 //
289 // FIXME: we probably shouldn't use floats at all.
290 volatile float HWeight = Hint[HintReg] += Weight;
291 if (HintReg.isVirtual() || MRI.isAllocatable(HintReg))
292 CopyHints.insert(CopyHint(HintReg, HWeight));
293 }
294
295 // Pass all the sorted copy hints to mri.
296 if (ShouldUpdateLI && CopyHints.size()) {
297 // Remove a generic hint if previously added by target.
298 if (TargetHint.first == 0 && TargetHint.second)
299 MRI.clearSimpleHint(LI.reg());
300
301 SmallSet<Register, 4> HintedRegs;
302 for (const auto &Hint : CopyHints) {
303 if (!HintedRegs.insert(Hint.Reg).second ||
304 (TargetHint.first != 0 && Hint.Reg == TargetHint.second))
305 // Don't add the same reg twice or the target-type hint again.
306 continue;
307 MRI.addRegAllocationHint(LI.reg(), Hint.Reg);
308 }
309
310 // Weakly boost the spill weight of hinted registers.
311 TotalWeight *= 1.01F;
312 }
313
314 // If the live interval was already unspillable, leave it that way.
315 if (!IsSpillable)
316 return -1.0;
317
318 // Mark li as unspillable if all live ranges are tiny and the interval
319 // is not live at any reg mask. If the interval is live at a reg mask
320 // spilling may be required. If li is live as use in statepoint instruction
321 // spilling may be required due to if we mark interval with use in statepoint
322 // as not spillable we are risky to end up with no register to allocate.
323 // At the same time STATEPOINT instruction is perfectly fine to have this
324 // operand on stack, so spilling such interval and folding its load from stack
325 // into instruction itself makes perfect sense.
326 if (ShouldUpdateLI && LI.isZeroLength(LIS.getSlotIndexes()) &&
327 !LI.isLiveAtIndexes(LIS.getRegMaskSlots()) &&
328 !isLiveAtStatepointVarArg(LI) && !canMemFoldInlineAsm(LI, MRI)) {
329 LI.markNotSpillable();
330 return -1.0;
331 }
332
333 // If all of the definitions of the interval are re-materializable,
334 // it is a preferred candidate for spilling.
335 // FIXME: this gets much more complicated once we support non-trivial
336 // re-materialization.
337 if (isRematerializable(LI, LIS, VRM, *MF.getSubtarget().getInstrInfo()))
338 TotalWeight *= 0.5F;
339
340 if (IsLocalSplitArtifact)
341 return normalize(TotalWeight, Start->distance(*End), NumInstr);
342 return normalize(TotalWeight, LI.getSize(), NumInstr);
343 }
344