1 //===- HexagonGenMux.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 // During instruction selection, MUX instructions are generated for
10 // conditional assignments. Since such assignments often present an
11 // opportunity to predicate instructions, HexagonExpandCondsets
12 // expands MUXes into pairs of conditional transfers, and then proceeds
13 // with predication of the producers/consumers of the registers involved.
14 // This happens after exiting from the SSA form, but before the machine
15 // instruction scheduler. After the scheduler and after the register
16 // allocation there can be cases of pairs of conditional transfers
17 // resulting from a MUX where neither of them was further predicated. If
18 // these transfers are now placed far enough from the instruction defining
19 // the predicate register, they cannot use the .new form. In such cases it
20 // is better to collapse them back to a single MUX instruction.
21
22 #include "HexagonInstrInfo.h"
23 #include "HexagonRegisterInfo.h"
24 #include "HexagonSubtarget.h"
25 #include "llvm/ADT/BitVector.h"
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/SmallVector.h"
28 #include "llvm/ADT/StringRef.h"
29 #include "llvm/CodeGen/LiveRegUnits.h"
30 #include "llvm/CodeGen/MachineBasicBlock.h"
31 #include "llvm/CodeGen/MachineFunction.h"
32 #include "llvm/CodeGen/MachineFunctionPass.h"
33 #include "llvm/CodeGen/MachineInstr.h"
34 #include "llvm/CodeGen/MachineInstrBuilder.h"
35 #include "llvm/CodeGen/MachineOperand.h"
36 #include "llvm/IR/DebugLoc.h"
37 #include "llvm/MC/MCInstrDesc.h"
38 #include "llvm/MC/MCRegisterInfo.h"
39 #include "llvm/Pass.h"
40 #include "llvm/Support/CommandLine.h"
41 #include "llvm/Support/MathExtras.h"
42 #include <algorithm>
43 #include <cassert>
44 #include <iterator>
45 #include <limits>
46 #include <utility>
47
48 #define DEBUG_TYPE "hexmux"
49
50 using namespace llvm;
51
52 namespace llvm {
53
54 FunctionPass *createHexagonGenMux();
55 void initializeHexagonGenMuxPass(PassRegistry& Registry);
56
57 } // end namespace llvm
58
59 // Initialize this to 0 to always prefer generating mux by default.
60 static cl::opt<unsigned> MinPredDist("hexagon-gen-mux-threshold", cl::Hidden,
61 cl::init(0), cl::desc("Minimum distance between predicate definition and "
62 "farther of the two predicated uses"));
63
64 namespace {
65
66 class HexagonGenMux : public MachineFunctionPass {
67 public:
68 static char ID;
69
HexagonGenMux()70 HexagonGenMux() : MachineFunctionPass(ID) {}
71
getPassName() const72 StringRef getPassName() const override {
73 return "Hexagon generate mux instructions";
74 }
75
getAnalysisUsage(AnalysisUsage & AU) const76 void getAnalysisUsage(AnalysisUsage &AU) const override {
77 MachineFunctionPass::getAnalysisUsage(AU);
78 }
79
80 bool runOnMachineFunction(MachineFunction &MF) override;
81
getRequiredProperties() const82 MachineFunctionProperties getRequiredProperties() const override {
83 return MachineFunctionProperties().set(
84 MachineFunctionProperties::Property::NoVRegs);
85 }
86
87 private:
88 const HexagonInstrInfo *HII = nullptr;
89 const HexagonRegisterInfo *HRI = nullptr;
90
91 struct CondsetInfo {
92 unsigned PredR = 0;
93 unsigned TrueX = std::numeric_limits<unsigned>::max();
94 unsigned FalseX = std::numeric_limits<unsigned>::max();
95
96 CondsetInfo() = default;
97 };
98
99 struct DefUseInfo {
100 BitVector Defs, Uses;
101
102 DefUseInfo() = default;
DefUseInfo__anoncf1483fd0111::HexagonGenMux::DefUseInfo103 DefUseInfo(const BitVector &D, const BitVector &U) : Defs(D), Uses(U) {}
104 };
105
106 struct MuxInfo {
107 MachineBasicBlock::iterator At;
108 unsigned DefR, PredR;
109 MachineOperand *SrcT, *SrcF;
110 MachineInstr *Def1, *Def2;
111
MuxInfo__anoncf1483fd0111::HexagonGenMux::MuxInfo112 MuxInfo(MachineBasicBlock::iterator It, unsigned DR, unsigned PR,
113 MachineOperand *TOp, MachineOperand *FOp, MachineInstr &D1,
114 MachineInstr &D2)
115 : At(It), DefR(DR), PredR(PR), SrcT(TOp), SrcF(FOp), Def1(&D1),
116 Def2(&D2) {}
117 };
118
119 using InstrIndexMap = DenseMap<MachineInstr *, unsigned>;
120 using DefUseInfoMap = DenseMap<unsigned, DefUseInfo>;
121 using MuxInfoList = SmallVector<MuxInfo, 4>;
122
isRegPair(unsigned Reg) const123 bool isRegPair(unsigned Reg) const {
124 return Hexagon::DoubleRegsRegClass.contains(Reg);
125 }
126
127 void getSubRegs(unsigned Reg, BitVector &SRs) const;
128 void expandReg(unsigned Reg, BitVector &Set) const;
129 void getDefsUses(const MachineInstr *MI, BitVector &Defs,
130 BitVector &Uses) const;
131 void buildMaps(MachineBasicBlock &B, InstrIndexMap &I2X,
132 DefUseInfoMap &DUM);
133 bool isCondTransfer(unsigned Opc) const;
134 unsigned getMuxOpcode(const MachineOperand &Src1,
135 const MachineOperand &Src2) const;
136 bool genMuxInBlock(MachineBasicBlock &B);
137 };
138
139 } // end anonymous namespace
140
141 char HexagonGenMux::ID = 0;
142
143 INITIALIZE_PASS(HexagonGenMux, "hexagon-gen-mux",
144 "Hexagon generate mux instructions", false, false)
145
getSubRegs(unsigned Reg,BitVector & SRs) const146 void HexagonGenMux::getSubRegs(unsigned Reg, BitVector &SRs) const {
147 for (MCPhysReg I : HRI->subregs(Reg))
148 SRs[I] = true;
149 }
150
expandReg(unsigned Reg,BitVector & Set) const151 void HexagonGenMux::expandReg(unsigned Reg, BitVector &Set) const {
152 if (isRegPair(Reg))
153 getSubRegs(Reg, Set);
154 else
155 Set[Reg] = true;
156 }
157
getDefsUses(const MachineInstr * MI,BitVector & Defs,BitVector & Uses) const158 void HexagonGenMux::getDefsUses(const MachineInstr *MI, BitVector &Defs,
159 BitVector &Uses) const {
160 // First, get the implicit defs and uses for this instruction.
161 unsigned Opc = MI->getOpcode();
162 const MCInstrDesc &D = HII->get(Opc);
163 for (MCPhysReg R : D.implicit_defs())
164 expandReg(R, Defs);
165 for (MCPhysReg R : D.implicit_uses())
166 expandReg(R, Uses);
167
168 // Look over all operands, and collect explicit defs and uses.
169 for (const MachineOperand &MO : MI->operands()) {
170 if (!MO.isReg() || MO.isImplicit())
171 continue;
172 Register R = MO.getReg();
173 BitVector &Set = MO.isDef() ? Defs : Uses;
174 expandReg(R, Set);
175 }
176 }
177
buildMaps(MachineBasicBlock & B,InstrIndexMap & I2X,DefUseInfoMap & DUM)178 void HexagonGenMux::buildMaps(MachineBasicBlock &B, InstrIndexMap &I2X,
179 DefUseInfoMap &DUM) {
180 unsigned Index = 0;
181 unsigned NR = HRI->getNumRegs();
182 BitVector Defs(NR), Uses(NR);
183
184 for (MachineInstr &MI : B) {
185 I2X.insert(std::make_pair(&MI, Index));
186 Defs.reset();
187 Uses.reset();
188 getDefsUses(&MI, Defs, Uses);
189 DUM.insert(std::make_pair(Index, DefUseInfo(Defs, Uses)));
190 Index++;
191 }
192 }
193
isCondTransfer(unsigned Opc) const194 bool HexagonGenMux::isCondTransfer(unsigned Opc) const {
195 switch (Opc) {
196 case Hexagon::A2_tfrt:
197 case Hexagon::A2_tfrf:
198 case Hexagon::C2_cmoveit:
199 case Hexagon::C2_cmoveif:
200 return true;
201 }
202 return false;
203 }
204
getMuxOpcode(const MachineOperand & Src1,const MachineOperand & Src2) const205 unsigned HexagonGenMux::getMuxOpcode(const MachineOperand &Src1,
206 const MachineOperand &Src2) const {
207 bool IsReg1 = Src1.isReg(), IsReg2 = Src2.isReg();
208 if (IsReg1)
209 return IsReg2 ? Hexagon::C2_mux : Hexagon::C2_muxir;
210 if (IsReg2)
211 return Hexagon::C2_muxri;
212
213 // Neither is a register. The first source is extendable, but the second
214 // is not (s8).
215 if (Src2.isImm() && isInt<8>(Src2.getImm()))
216 return Hexagon::C2_muxii;
217
218 return 0;
219 }
220
genMuxInBlock(MachineBasicBlock & B)221 bool HexagonGenMux::genMuxInBlock(MachineBasicBlock &B) {
222 bool Changed = false;
223 InstrIndexMap I2X;
224 DefUseInfoMap DUM;
225 buildMaps(B, I2X, DUM);
226
227 using CondsetMap = DenseMap<unsigned, CondsetInfo>;
228
229 CondsetMap CM;
230 MuxInfoList ML;
231
232 for (MachineInstr &MI : llvm::make_early_inc_range(B)) {
233 unsigned Opc = MI.getOpcode();
234 if (!isCondTransfer(Opc))
235 continue;
236 Register DR = MI.getOperand(0).getReg();
237 if (isRegPair(DR))
238 continue;
239 MachineOperand &PredOp = MI.getOperand(1);
240 if (PredOp.isUndef())
241 continue;
242
243 Register PR = PredOp.getReg();
244 unsigned Idx = I2X.lookup(&MI);
245 CondsetMap::iterator F = CM.find(DR);
246 bool IfTrue = HII->isPredicatedTrue(Opc);
247
248 // If there is no record of a conditional transfer for this register,
249 // or the predicate register differs, create a new record for it.
250 if (F != CM.end() && F->second.PredR != PR) {
251 CM.erase(F);
252 F = CM.end();
253 }
254 if (F == CM.end()) {
255 auto It = CM.insert(std::make_pair(DR, CondsetInfo()));
256 F = It.first;
257 F->second.PredR = PR;
258 }
259 CondsetInfo &CI = F->second;
260 if (IfTrue)
261 CI.TrueX = Idx;
262 else
263 CI.FalseX = Idx;
264 if (CI.TrueX == std::numeric_limits<unsigned>::max() ||
265 CI.FalseX == std::numeric_limits<unsigned>::max())
266 continue;
267
268 // There is now a complete definition of DR, i.e. we have the predicate
269 // register, the definition if-true, and definition if-false.
270
271 // First, check if the definitions are far enough from the definition
272 // of the predicate register.
273 unsigned MinX = std::min(CI.TrueX, CI.FalseX);
274 unsigned MaxX = std::max(CI.TrueX, CI.FalseX);
275 // Specifically, check if the predicate definition is within a prescribed
276 // distance from the farther of the two predicated instructions.
277 unsigned SearchX = (MaxX >= MinPredDist) ? MaxX-MinPredDist : 0;
278 bool NearDef = false;
279 for (unsigned X = SearchX; X < MaxX; ++X) {
280 const DefUseInfo &DU = DUM.lookup(X);
281 if (!DU.Defs[PR])
282 continue;
283 NearDef = true;
284 break;
285 }
286 if (NearDef)
287 continue;
288
289 // The predicate register is not defined in the last few instructions.
290 // Check if the conversion to MUX is possible (either "up", i.e. at the
291 // place of the earlier partial definition, or "down", where the later
292 // definition is located). Examine all defs and uses between these two
293 // definitions.
294 // SR1, SR2 - source registers from the first and the second definition.
295 MachineBasicBlock::iterator It1 = B.begin(), It2 = B.begin();
296 std::advance(It1, MinX);
297 std::advance(It2, MaxX);
298 MachineInstr &Def1 = *It1, &Def2 = *It2;
299 MachineOperand *Src1 = &Def1.getOperand(2), *Src2 = &Def2.getOperand(2);
300 Register SR1 = Src1->isReg() ? Src1->getReg() : Register();
301 Register SR2 = Src2->isReg() ? Src2->getReg() : Register();
302 bool Failure = false, CanUp = true, CanDown = true;
303 for (unsigned X = MinX+1; X < MaxX; X++) {
304 const DefUseInfo &DU = DUM.lookup(X);
305 if (DU.Defs[PR] || DU.Defs[DR] || DU.Uses[DR]) {
306 Failure = true;
307 break;
308 }
309 if (CanDown && DU.Defs[SR1])
310 CanDown = false;
311 if (CanUp && DU.Defs[SR2])
312 CanUp = false;
313 }
314 if (Failure || (!CanUp && !CanDown))
315 continue;
316
317 MachineOperand *SrcT = (MinX == CI.TrueX) ? Src1 : Src2;
318 MachineOperand *SrcF = (MinX == CI.FalseX) ? Src1 : Src2;
319 // Prefer "down", since this will move the MUX farther away from the
320 // predicate definition.
321 MachineBasicBlock::iterator At = CanDown ? Def2 : Def1;
322 ML.push_back(MuxInfo(At, DR, PR, SrcT, SrcF, Def1, Def2));
323 }
324
325 for (MuxInfo &MX : ML) {
326 unsigned MxOpc = getMuxOpcode(*MX.SrcT, *MX.SrcF);
327 if (!MxOpc)
328 continue;
329 // Basic correctness check: since we are deleting instructions, validate the
330 // iterators. There is a possibility that one of Def1 or Def2 is translated
331 // to "mux" and being considered for other "mux" instructions.
332 if (!MX.At->getParent() || !MX.Def1->getParent() || !MX.Def2->getParent())
333 continue;
334
335 MachineBasicBlock &B = *MX.At->getParent();
336 const DebugLoc &DL = B.findDebugLoc(MX.At);
337 auto NewMux = BuildMI(B, MX.At, DL, HII->get(MxOpc), MX.DefR)
338 .addReg(MX.PredR)
339 .add(*MX.SrcT)
340 .add(*MX.SrcF);
341 NewMux->clearKillInfo();
342 B.remove(MX.Def1);
343 B.remove(MX.Def2);
344 Changed = true;
345 }
346
347 // Fix up kill flags.
348
349 LiveRegUnits LPR(*HRI);
350 LPR.addLiveOuts(B);
351 for (MachineInstr &I : llvm::reverse(B)) {
352 if (I.isDebugInstr())
353 continue;
354 // This isn't 100% accurate, but it's safe.
355 // It won't detect (as a kill) a case like this
356 // r0 = add r0, 1 <-- r0 should be "killed"
357 // ... = r0
358 for (MachineOperand &Op : I.operands()) {
359 if (!Op.isReg() || !Op.isUse())
360 continue;
361 assert(Op.getSubReg() == 0 && "Should have physical registers only");
362 bool Live = !LPR.available(Op.getReg());
363 Op.setIsKill(!Live);
364 }
365 LPR.stepBackward(I);
366 }
367
368 return Changed;
369 }
370
runOnMachineFunction(MachineFunction & MF)371 bool HexagonGenMux::runOnMachineFunction(MachineFunction &MF) {
372 if (skipFunction(MF.getFunction()))
373 return false;
374 HII = MF.getSubtarget<HexagonSubtarget>().getInstrInfo();
375 HRI = MF.getSubtarget<HexagonSubtarget>().getRegisterInfo();
376 bool Changed = false;
377 for (auto &I : MF)
378 Changed |= genMuxInBlock(I);
379 return Changed;
380 }
381
createHexagonGenMux()382 FunctionPass *llvm::createHexagonGenMux() {
383 return new HexagonGenMux();
384 }
385