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/MathExtras.h"
26#include "llvm/Support/raw_ostream.h"
27#include <cassert>
28#include <tuple>
29
30using namespace llvm;
31
32#define DEBUG_TYPE "calcspillweights"
33
34void VirtRegAuxInfo::calculateSpillWeightsAndHints() {
35 LLVM_DEBUG(dbgs() << "********** Compute Spill Weights **********\n"
36 << "********** Function: " << MF.getName() << '\n');
37
38 MachineRegisterInfo &MRI = MF.getRegInfo();
39 for (unsigned I = 0, E = MRI.getNumVirtRegs(); I != E; ++I) {
40 Register Reg = Register::index2VirtReg(Index: I);
41 if (MRI.reg_nodbg_empty(RegNo: Reg))
42 continue;
43 calculateSpillWeightAndHint(LI&: LIS.getInterval(Reg));
44 }
45}
46
47// Return the preferred allocation register for reg, given a COPY instruction.
48Register VirtRegAuxInfo::copyHint(const MachineInstr *MI, unsigned Reg,
49 const TargetRegisterInfo &TRI,
50 const MachineRegisterInfo &MRI) {
51 unsigned Sub, HSub;
52 Register HReg;
53 if (MI->getOperand(i: 0).getReg() == Reg) {
54 Sub = MI->getOperand(i: 0).getSubReg();
55 HReg = MI->getOperand(i: 1).getReg();
56 HSub = MI->getOperand(i: 1).getSubReg();
57 } else {
58 Sub = MI->getOperand(i: 1).getSubReg();
59 HReg = MI->getOperand(i: 0).getReg();
60 HSub = MI->getOperand(i: 0).getSubReg();
61 }
62
63 if (!HReg)
64 return 0;
65
66 if (HReg.isVirtual())
67 return Sub == HSub ? HReg : Register();
68
69 const TargetRegisterClass *RC = MRI.getRegClass(Reg);
70 MCRegister CopiedPReg = HSub ? TRI.getSubReg(Reg: HReg, Idx: HSub) : HReg.asMCReg();
71 if (RC->contains(Reg: CopiedPReg))
72 return CopiedPReg;
73
74 // Check if reg:sub matches so that a super register could be hinted.
75 if (Sub)
76 return TRI.getMatchingSuperReg(Reg: CopiedPReg, SubIdx: Sub, RC);
77
78 return 0;
79}
80
81// Check if all values in LI are rematerializable
82bool VirtRegAuxInfo::isRematerializable(const LiveInterval &LI,
83 const LiveIntervals &LIS,
84 const VirtRegMap &VRM,
85 const TargetInstrInfo &TII) {
86 Register Reg = LI.reg();
87 Register Original = VRM.getOriginal(VirtReg: Reg);
88 for (LiveInterval::const_vni_iterator I = LI.vni_begin(), E = LI.vni_end();
89 I != E; ++I) {
90 const VNInfo *VNI = *I;
91 if (VNI->isUnused())
92 continue;
93 if (VNI->isPHIDef())
94 return false;
95
96 MachineInstr *MI = LIS.getInstructionFromIndex(index: VNI->def);
97 assert(MI && "Dead valno in interval");
98
99 // Trace copies introduced by live range splitting. The inline
100 // spiller can rematerialize through these copies, so the spill
101 // weight must reflect this.
102 while (TII.isFullCopyInstr(MI: *MI)) {
103 // The copy destination must match the interval register.
104 if (MI->getOperand(i: 0).getReg() != Reg)
105 return false;
106
107 // Get the source register.
108 Reg = MI->getOperand(i: 1).getReg();
109
110 // If the original (pre-splitting) registers match this
111 // copy came from a split.
112 if (!Reg.isVirtual() || VRM.getOriginal(VirtReg: Reg) != Original)
113 return false;
114
115 // Follow the copy live-in value.
116 const LiveInterval &SrcLI = LIS.getInterval(Reg);
117 LiveQueryResult SrcQ = SrcLI.Query(Idx: VNI->def);
118 VNI = SrcQ.valueIn();
119 assert(VNI && "Copy from non-existing value");
120 if (VNI->isPHIDef())
121 return false;
122 MI = LIS.getInstructionFromIndex(index: VNI->def);
123 assert(MI && "Dead valno in interval");
124 }
125
126 if (!TII.isTriviallyReMaterializable(MI: *MI))
127 return false;
128 }
129 return true;
130}
131
132bool VirtRegAuxInfo::isLiveAtStatepointVarArg(LiveInterval &LI) {
133 return any_of(Range: VRM.getRegInfo().reg_operands(Reg: LI.reg()),
134 P: [](MachineOperand &MO) {
135 MachineInstr *MI = MO.getParent();
136 if (MI->getOpcode() != TargetOpcode::STATEPOINT)
137 return false;
138 return StatepointOpers(MI).getVarIdx() <= MO.getOperandNo();
139 });
140}
141
142void VirtRegAuxInfo::calculateSpillWeightAndHint(LiveInterval &LI) {
143 float Weight = weightCalcHelper(LI);
144 // Check if unspillable.
145 if (Weight < 0)
146 return;
147 LI.setWeight(Weight);
148}
149
150static bool canMemFoldInlineAsm(LiveInterval &LI,
151 const MachineRegisterInfo &MRI) {
152 for (const MachineOperand &MO : MRI.reg_operands(Reg: LI.reg())) {
153 const MachineInstr *MI = MO.getParent();
154 if (MI->isInlineAsm() && MI->mayFoldInlineAsmRegOp(OpId: MI->getOperandNo(I: &MO)))
155 return true;
156 }
157
158 return false;
159}
160
161float VirtRegAuxInfo::weightCalcHelper(LiveInterval &LI, SlotIndex *Start,
162 SlotIndex *End) {
163 MachineRegisterInfo &MRI = MF.getRegInfo();
164 const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
165 const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
166 MachineBasicBlock *MBB = nullptr;
167 float TotalWeight = 0;
168 unsigned NumInstr = 0; // Number of instructions using LI
169 SmallPtrSet<MachineInstr *, 8> Visited;
170
171 std::pair<unsigned, Register> TargetHint = MRI.getRegAllocationHint(VReg: LI.reg());
172
173 if (LI.isSpillable()) {
174 Register Reg = LI.reg();
175 Register Original = VRM.getOriginal(VirtReg: Reg);
176 const LiveInterval &OrigInt = LIS.getInterval(Reg: Original);
177 // li comes from a split of OrigInt. If OrigInt was marked
178 // as not spillable, make sure the new interval is marked
179 // as not spillable as well.
180 if (!OrigInt.isSpillable())
181 LI.markNotSpillable();
182 }
183
184 // Don't recompute spill weight for an unspillable register.
185 bool IsSpillable = LI.isSpillable();
186
187 bool IsLocalSplitArtifact = Start && End;
188
189 // Do not update future local split artifacts.
190 bool ShouldUpdateLI = !IsLocalSplitArtifact;
191
192 if (IsLocalSplitArtifact) {
193 MachineBasicBlock *LocalMBB = LIS.getMBBFromIndex(index: *End);
194 assert(LocalMBB == LIS.getMBBFromIndex(*Start) &&
195 "start and end are expected to be in the same basic block");
196
197 // Local split artifact will have 2 additional copy instructions and they
198 // will be in the same BB.
199 // localLI = COPY other
200 // ...
201 // other = COPY localLI
202 TotalWeight += LiveIntervals::getSpillWeight(isDef: true, isUse: false, MBFI: &MBFI, MBB: LocalMBB);
203 TotalWeight += LiveIntervals::getSpillWeight(isDef: false, isUse: true, MBFI: &MBFI, MBB: LocalMBB);
204
205 NumInstr += 2;
206 }
207
208 // CopyHint is a sortable hint derived from a COPY instruction.
209 struct CopyHint {
210 const Register Reg;
211 const float Weight;
212 CopyHint(Register R, float W) : Reg(R), Weight(W) {}
213 bool operator<(const CopyHint &Rhs) const {
214 // Always prefer any physreg hint.
215 if (Reg.isPhysical() != Rhs.Reg.isPhysical())
216 return Reg.isPhysical();
217 if (Weight != Rhs.Weight)
218 return (Weight > Rhs.Weight);
219 return Reg.id() < Rhs.Reg.id(); // Tie-breaker.
220 }
221 };
222
223 bool IsExiting = false;
224 std::set<CopyHint> CopyHints;
225 DenseMap<unsigned, float> Hint;
226 for (MachineRegisterInfo::reg_instr_nodbg_iterator
227 I = MRI.reg_instr_nodbg_begin(RegNo: LI.reg()),
228 E = MRI.reg_instr_nodbg_end();
229 I != E;) {
230 MachineInstr *MI = &*(I++);
231
232 // For local split artifacts, we are interested only in instructions between
233 // the expected start and end of the range.
234 SlotIndex SI = LIS.getInstructionIndex(Instr: *MI);
235 if (IsLocalSplitArtifact && ((SI < *Start) || (SI > *End)))
236 continue;
237
238 NumInstr++;
239 bool identityCopy = false;
240 auto DestSrc = TII.isCopyInstr(MI: *MI);
241 if (DestSrc) {
242 const MachineOperand *DestRegOp = DestSrc->Destination;
243 const MachineOperand *SrcRegOp = DestSrc->Source;
244 identityCopy = DestRegOp->getReg() == SrcRegOp->getReg() &&
245 DestRegOp->getSubReg() == SrcRegOp->getSubReg();
246 }
247
248 if (identityCopy || MI->isImplicitDef())
249 continue;
250 if (!Visited.insert(Ptr: MI).second)
251 continue;
252
253 // For terminators that produce values, ask the backend if the register is
254 // not spillable.
255 if (TII.isUnspillableTerminator(MI) &&
256 MI->definesRegister(Reg: LI.reg(), /*TRI=*/nullptr)) {
257 LI.markNotSpillable();
258 return -1.0f;
259 }
260
261 // Force Weight onto the stack so that x86 doesn't add hidden precision,
262 // similar to HWeight below.
263 stack_float_t Weight = 1.0f;
264 if (IsSpillable) {
265 // Get loop info for mi.
266 if (MI->getParent() != MBB) {
267 MBB = MI->getParent();
268 const MachineLoop *Loop = Loops.getLoopFor(BB: MBB);
269 IsExiting = Loop ? Loop->isLoopExiting(BB: MBB) : false;
270 }
271
272 // Calculate instr weight.
273 bool Reads, Writes;
274 std::tie(args&: Reads, args&: Writes) = MI->readsWritesVirtualRegister(Reg: LI.reg());
275 Weight = LiveIntervals::getSpillWeight(isDef: Writes, isUse: Reads, MBFI: &MBFI, MI: *MI);
276
277 // Give extra weight to what looks like a loop induction variable update.
278 if (Writes && IsExiting && LIS.isLiveOutOfMBB(LR: LI, mbb: MBB))
279 Weight *= 3;
280
281 TotalWeight += Weight;
282 }
283
284 // Get allocation hints from copies.
285 if (!TII.isCopyInstr(MI: *MI))
286 continue;
287 Register HintReg = copyHint(MI, Reg: LI.reg(), TRI, MRI);
288 if (!HintReg)
289 continue;
290 // Force HWeight onto the stack so that x86 doesn't add hidden precision,
291 // making the comparison incorrectly pass (i.e., 1 > 1 == true??).
292 stack_float_t HWeight = Hint[HintReg] += Weight;
293 if (HintReg.isVirtual() || MRI.isAllocatable(PhysReg: HintReg))
294 CopyHints.insert(x: CopyHint(HintReg, HWeight));
295 }
296
297 // Pass all the sorted copy hints to mri.
298 if (ShouldUpdateLI && CopyHints.size()) {
299 // Remove a generic hint if previously added by target.
300 if (TargetHint.first == 0 && TargetHint.second)
301 MRI.clearSimpleHint(VReg: LI.reg());
302
303 SmallSet<Register, 4> HintedRegs;
304 for (const auto &Hint : CopyHints) {
305 if (!HintedRegs.insert(V: Hint.Reg).second ||
306 (TargetHint.first != 0 && Hint.Reg == TargetHint.second))
307 // Don't add the same reg twice or the target-type hint again.
308 continue;
309 MRI.addRegAllocationHint(VReg: LI.reg(), PrefReg: Hint.Reg);
310 }
311
312 // Weakly boost the spill weight of hinted registers.
313 TotalWeight *= 1.01F;
314 }
315
316 // If the live interval was already unspillable, leave it that way.
317 if (!IsSpillable)
318 return -1.0;
319
320 // Mark li as unspillable if all live ranges are tiny and the interval
321 // is not live at any reg mask. If the interval is live at a reg mask
322 // spilling may be required. If li is live as use in statepoint instruction
323 // spilling may be required due to if we mark interval with use in statepoint
324 // as not spillable we are risky to end up with no register to allocate.
325 // At the same time STATEPOINT instruction is perfectly fine to have this
326 // operand on stack, so spilling such interval and folding its load from stack
327 // into instruction itself makes perfect sense.
328 if (ShouldUpdateLI && LI.isZeroLength(Indexes: LIS.getSlotIndexes()) &&
329 !LI.isLiveAtIndexes(Slots: LIS.getRegMaskSlots()) &&
330 !isLiveAtStatepointVarArg(LI) && !canMemFoldInlineAsm(LI, MRI)) {
331 LI.markNotSpillable();
332 return -1.0;
333 }
334
335 // If all of the definitions of the interval are re-materializable,
336 // it is a preferred candidate for spilling.
337 // FIXME: this gets much more complicated once we support non-trivial
338 // re-materialization.
339 if (isRematerializable(LI, LIS, VRM, TII: *MF.getSubtarget().getInstrInfo()))
340 TotalWeight *= 0.5F;
341
342 if (IsLocalSplitArtifact)
343 return normalize(UseDefFreq: TotalWeight, Size: Start->distance(other: *End), NumInstr);
344 return normalize(UseDefFreq: TotalWeight, Size: LI.getSize(), NumInstr);
345}
346