1 //===--------------- PPCVSXFMAMutate.cpp - VSX FMA Mutation ---------------===//
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 // This pass mutates the form of VSX FMA instructions to avoid unnecessary
10 // copies.
11 //
12 //===----------------------------------------------------------------------===//
13
14 #include "MCTargetDesc/PPCPredicates.h"
15 #include "PPC.h"
16 #include "PPCInstrBuilder.h"
17 #include "PPCInstrInfo.h"
18 #include "PPCMachineFunctionInfo.h"
19 #include "PPCTargetMachine.h"
20 #include "llvm/ADT/STLExtras.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/CodeGen/LiveIntervals.h"
23 #include "llvm/CodeGen/MachineDominators.h"
24 #include "llvm/CodeGen/MachineFrameInfo.h"
25 #include "llvm/CodeGen/MachineFunctionPass.h"
26 #include "llvm/CodeGen/MachineInstrBuilder.h"
27 #include "llvm/CodeGen/MachineMemOperand.h"
28 #include "llvm/CodeGen/MachineRegisterInfo.h"
29 #include "llvm/CodeGen/PseudoSourceValue.h"
30 #include "llvm/CodeGen/ScheduleDAG.h"
31 #include "llvm/CodeGen/SlotIndexes.h"
32 #include "llvm/InitializePasses.h"
33 #include "llvm/MC/MCAsmInfo.h"
34 #include "llvm/MC/TargetRegistry.h"
35 #include "llvm/Support/CommandLine.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/ErrorHandling.h"
38 #include "llvm/Support/raw_ostream.h"
39
40 using namespace llvm;
41
42 // Temporarily disable FMA mutation by default, since it doesn't handle
43 // cross-basic-block intervals well.
44 // See: http://lists.llvm.org/pipermail/llvm-dev/2016-February/095669.html
45 // http://reviews.llvm.org/D17087
46 static cl::opt<bool> DisableVSXFMAMutate(
47 "disable-ppc-vsx-fma-mutation",
48 cl::desc("Disable VSX FMA instruction mutation"), cl::init(true),
49 cl::Hidden);
50
51 #define DEBUG_TYPE "ppc-vsx-fma-mutate"
52
53 namespace llvm { namespace PPC {
54 int getAltVSXFMAOpcode(uint16_t Opcode);
55 } }
56
57 namespace {
58 // PPCVSXFMAMutate pass - For copies between VSX registers and non-VSX registers
59 // (Altivec and scalar floating-point registers), we need to transform the
60 // copies into subregister copies with other restrictions.
61 struct PPCVSXFMAMutate : public MachineFunctionPass {
62 static char ID;
PPCVSXFMAMutate__anon78ef7b5d0111::PPCVSXFMAMutate63 PPCVSXFMAMutate() : MachineFunctionPass(ID) {
64 initializePPCVSXFMAMutatePass(*PassRegistry::getPassRegistry());
65 }
66
67 LiveIntervals *LIS;
68 const PPCInstrInfo *TII;
69
70 protected:
processBlock__anon78ef7b5d0111::PPCVSXFMAMutate71 bool processBlock(MachineBasicBlock &MBB) {
72 bool Changed = false;
73
74 MachineRegisterInfo &MRI = MBB.getParent()->getRegInfo();
75 const TargetRegisterInfo *TRI = &TII->getRegisterInfo();
76 for (MachineBasicBlock::iterator I = MBB.begin(), IE = MBB.end();
77 I != IE; ++I) {
78 MachineInstr &MI = *I;
79
80 // The default (A-type) VSX FMA form kills the addend (it is taken from
81 // the target register, which is then updated to reflect the result of
82 // the FMA). If the instruction, however, kills one of the registers
83 // used for the product, then we can use the M-form instruction (which
84 // will take that value from the to-be-defined register).
85
86 int AltOpc = PPC::getAltVSXFMAOpcode(MI.getOpcode());
87 if (AltOpc == -1)
88 continue;
89
90 // This pass is run after register coalescing, and so we're looking for
91 // a situation like this:
92 // ...
93 // %5 = COPY %9; VSLRC:%5,%9
94 // %5<def,tied1> = XSMADDADP %5<tied0>, %17, %16,
95 // implicit %rm; VSLRC:%5,%17,%16
96 // ...
97 // %9<def,tied1> = XSMADDADP %9<tied0>, %17, %19,
98 // implicit %rm; VSLRC:%9,%17,%19
99 // ...
100 // Where we can eliminate the copy by changing from the A-type to the
101 // M-type instruction. Specifically, for this example, this means:
102 // %5<def,tied1> = XSMADDADP %5<tied0>, %17, %16,
103 // implicit %rm; VSLRC:%5,%17,%16
104 // is replaced by:
105 // %16<def,tied1> = XSMADDMDP %16<tied0>, %18, %9,
106 // implicit %rm; VSLRC:%16,%18,%9
107 // and we remove: %5 = COPY %9; VSLRC:%5,%9
108
109 SlotIndex FMAIdx = LIS->getInstructionIndex(MI);
110
111 VNInfo *AddendValNo =
112 LIS->getInterval(MI.getOperand(1).getReg()).Query(FMAIdx).valueIn();
113
114 // This can be null if the register is undef.
115 if (!AddendValNo)
116 continue;
117
118 MachineInstr *AddendMI = LIS->getInstructionFromIndex(AddendValNo->def);
119
120 // The addend and this instruction must be in the same block.
121
122 if (!AddendMI || AddendMI->getParent() != MI.getParent())
123 continue;
124
125 // The addend must be a full copy within the same register class.
126
127 if (!AddendMI->isFullCopy())
128 continue;
129
130 Register AddendSrcReg = AddendMI->getOperand(1).getReg();
131 if (AddendSrcReg.isVirtual()) {
132 if (MRI.getRegClass(AddendMI->getOperand(0).getReg()) !=
133 MRI.getRegClass(AddendSrcReg))
134 continue;
135 } else {
136 // If AddendSrcReg is a physical register, make sure the destination
137 // register class contains it.
138 if (!MRI.getRegClass(AddendMI->getOperand(0).getReg())
139 ->contains(AddendSrcReg))
140 continue;
141 }
142
143 // In theory, there could be other uses of the addend copy before this
144 // fma. We could deal with this, but that would require additional
145 // logic below and I suspect it will not occur in any relevant
146 // situations. Additionally, check whether the copy source is killed
147 // prior to the fma. In order to replace the addend here with the
148 // source of the copy, it must still be live here. We can't use
149 // interval testing for a physical register, so as long as we're
150 // walking the MIs we may as well test liveness here.
151 //
152 // FIXME: There is a case that occurs in practice, like this:
153 // %9 = COPY %f1; VSSRC:%9
154 // ...
155 // %6 = COPY %9; VSSRC:%6,%9
156 // %7 = COPY %9; VSSRC:%7,%9
157 // %9<def,tied1> = XSMADDASP %9<tied0>, %1, %4; VSSRC:
158 // %6<def,tied1> = XSMADDASP %6<tied0>, %1, %2; VSSRC:
159 // %7<def,tied1> = XSMADDASP %7<tied0>, %1, %3; VSSRC:
160 // which prevents an otherwise-profitable transformation.
161 bool OtherUsers = false, KillsAddendSrc = false;
162 for (auto J = std::prev(I), JE = MachineBasicBlock::iterator(AddendMI);
163 J != JE; --J) {
164 if (J->readsVirtualRegister(AddendMI->getOperand(0).getReg())) {
165 OtherUsers = true;
166 break;
167 }
168 if (J->modifiesRegister(AddendSrcReg, TRI) ||
169 J->killsRegister(AddendSrcReg, TRI)) {
170 KillsAddendSrc = true;
171 break;
172 }
173 }
174
175 if (OtherUsers || KillsAddendSrc)
176 continue;
177
178
179 // The transformation doesn't work well with things like:
180 // %5 = A-form-op %5, %11, %5;
181 // unless %11 is also a kill, so skip when it is not,
182 // and check operand 3 to see it is also a kill to handle the case:
183 // %5 = A-form-op %5, %5, %11;
184 // where %5 and %11 are both kills. This case would be skipped
185 // otherwise.
186 Register OldFMAReg = MI.getOperand(0).getReg();
187
188 // Find one of the product operands that is killed by this instruction.
189 unsigned KilledProdOp = 0, OtherProdOp = 0;
190 Register Reg2 = MI.getOperand(2).getReg();
191 Register Reg3 = MI.getOperand(3).getReg();
192 if (LIS->getInterval(Reg2).Query(FMAIdx).isKill()
193 && Reg2 != OldFMAReg) {
194 KilledProdOp = 2;
195 OtherProdOp = 3;
196 } else if (LIS->getInterval(Reg3).Query(FMAIdx).isKill()
197 && Reg3 != OldFMAReg) {
198 KilledProdOp = 3;
199 OtherProdOp = 2;
200 }
201
202 // If there are no usable killed product operands, then this
203 // transformation is likely not profitable.
204 if (!KilledProdOp)
205 continue;
206
207 // If the addend copy is used only by this MI, then the addend source
208 // register is likely not live here. This could be fixed (based on the
209 // legality checks above, the live range for the addend source register
210 // could be extended), but it seems likely that such a trivial copy can
211 // be coalesced away later, and thus is not worth the effort.
212 if (AddendSrcReg.isVirtual() &&
213 !LIS->getInterval(AddendSrcReg).liveAt(FMAIdx))
214 continue;
215
216 // Transform: (O2 * O3) + O1 -> (O2 * O1) + O3.
217
218 Register KilledProdReg = MI.getOperand(KilledProdOp).getReg();
219 Register OtherProdReg = MI.getOperand(OtherProdOp).getReg();
220
221 unsigned AddSubReg = AddendMI->getOperand(1).getSubReg();
222 unsigned KilledProdSubReg = MI.getOperand(KilledProdOp).getSubReg();
223 unsigned OtherProdSubReg = MI.getOperand(OtherProdOp).getSubReg();
224
225 bool AddRegKill = AddendMI->getOperand(1).isKill();
226 bool KilledProdRegKill = MI.getOperand(KilledProdOp).isKill();
227 bool OtherProdRegKill = MI.getOperand(OtherProdOp).isKill();
228
229 bool AddRegUndef = AddendMI->getOperand(1).isUndef();
230 bool KilledProdRegUndef = MI.getOperand(KilledProdOp).isUndef();
231 bool OtherProdRegUndef = MI.getOperand(OtherProdOp).isUndef();
232
233 // If there isn't a class that fits, we can't perform the transform.
234 // This is needed for correctness with a mixture of VSX and Altivec
235 // instructions to make sure that a low VSX register is not assigned to
236 // the Altivec instruction.
237 if (!MRI.constrainRegClass(KilledProdReg,
238 MRI.getRegClass(OldFMAReg)))
239 continue;
240
241 assert(OldFMAReg == AddendMI->getOperand(0).getReg() &&
242 "Addend copy not tied to old FMA output!");
243
244 LLVM_DEBUG(dbgs() << "VSX FMA Mutation:\n " << MI);
245
246 MI.getOperand(0).setReg(KilledProdReg);
247 MI.getOperand(1).setReg(KilledProdReg);
248 MI.getOperand(3).setReg(AddendSrcReg);
249
250 MI.getOperand(0).setSubReg(KilledProdSubReg);
251 MI.getOperand(1).setSubReg(KilledProdSubReg);
252 MI.getOperand(3).setSubReg(AddSubReg);
253
254 MI.getOperand(1).setIsKill(KilledProdRegKill);
255 MI.getOperand(3).setIsKill(AddRegKill);
256
257 MI.getOperand(1).setIsUndef(KilledProdRegUndef);
258 MI.getOperand(3).setIsUndef(AddRegUndef);
259
260 MI.setDesc(TII->get(AltOpc));
261
262 // If the addend is also a multiplicand, replace it with the addend
263 // source in both places.
264 if (OtherProdReg == AddendMI->getOperand(0).getReg()) {
265 MI.getOperand(2).setReg(AddendSrcReg);
266 MI.getOperand(2).setSubReg(AddSubReg);
267 MI.getOperand(2).setIsKill(AddRegKill);
268 MI.getOperand(2).setIsUndef(AddRegUndef);
269 } else {
270 MI.getOperand(2).setReg(OtherProdReg);
271 MI.getOperand(2).setSubReg(OtherProdSubReg);
272 MI.getOperand(2).setIsKill(OtherProdRegKill);
273 MI.getOperand(2).setIsUndef(OtherProdRegUndef);
274 }
275
276 LLVM_DEBUG(dbgs() << " -> " << MI);
277
278 // The killed product operand was killed here, so we can reuse it now
279 // for the result of the fma.
280
281 LiveInterval &FMAInt = LIS->getInterval(OldFMAReg);
282 VNInfo *FMAValNo = FMAInt.getVNInfoAt(FMAIdx.getRegSlot());
283 for (auto UI = MRI.reg_nodbg_begin(OldFMAReg), UE = MRI.reg_nodbg_end();
284 UI != UE;) {
285 MachineOperand &UseMO = *UI;
286 MachineInstr *UseMI = UseMO.getParent();
287 ++UI;
288
289 // Don't replace the result register of the copy we're about to erase.
290 if (UseMI == AddendMI)
291 continue;
292
293 UseMO.substVirtReg(KilledProdReg, KilledProdSubReg, *TRI);
294 }
295
296 // Extend the live intervals of the killed product operand to hold the
297 // fma result.
298
299 LiveInterval &NewFMAInt = LIS->getInterval(KilledProdReg);
300 for (auto &AI : FMAInt) {
301 // Don't add the segment that corresponds to the original copy.
302 if (AI.valno == AddendValNo)
303 continue;
304
305 VNInfo *NewFMAValNo =
306 NewFMAInt.getNextValue(AI.start, LIS->getVNInfoAllocator());
307
308 NewFMAInt.addSegment(
309 LiveInterval::Segment(AI.start, AI.end, NewFMAValNo));
310 }
311 LLVM_DEBUG(dbgs() << " extended: " << NewFMAInt << '\n');
312
313 // Extend the live interval of the addend source (it might end at the
314 // copy to be removed, or somewhere in between there and here). This
315 // is necessary only if it is a physical register.
316 if (!AddendSrcReg.isVirtual())
317 for (MCRegUnit Unit : TRI->regunits(AddendSrcReg.asMCReg())) {
318 LiveRange &AddendSrcRange = LIS->getRegUnit(Unit);
319 AddendSrcRange.extendInBlock(LIS->getMBBStartIdx(&MBB),
320 FMAIdx.getRegSlot());
321 LLVM_DEBUG(dbgs() << " extended: " << AddendSrcRange << '\n');
322 }
323
324 FMAInt.removeValNo(FMAValNo);
325 LLVM_DEBUG(dbgs() << " trimmed: " << FMAInt << '\n');
326
327 // Remove the (now unused) copy.
328
329 LLVM_DEBUG(dbgs() << " removing: " << *AddendMI << '\n');
330 LIS->RemoveMachineInstrFromMaps(*AddendMI);
331 AddendMI->eraseFromParent();
332
333 Changed = true;
334 }
335
336 return Changed;
337 }
338
339 public:
runOnMachineFunction__anon78ef7b5d0111::PPCVSXFMAMutate340 bool runOnMachineFunction(MachineFunction &MF) override {
341 if (skipFunction(MF.getFunction()))
342 return false;
343
344 // If we don't have VSX then go ahead and return without doing
345 // anything.
346 const PPCSubtarget &STI = MF.getSubtarget<PPCSubtarget>();
347 if (!STI.hasVSX())
348 return false;
349
350 LIS = &getAnalysis<LiveIntervalsWrapperPass>().getLIS();
351
352 TII = STI.getInstrInfo();
353
354 bool Changed = false;
355
356 if (DisableVSXFMAMutate)
357 return Changed;
358
359 for (MachineBasicBlock &B : llvm::make_early_inc_range(MF))
360 if (processBlock(B))
361 Changed = true;
362
363 return Changed;
364 }
365
getAnalysisUsage__anon78ef7b5d0111::PPCVSXFMAMutate366 void getAnalysisUsage(AnalysisUsage &AU) const override {
367 AU.addRequired<LiveIntervalsWrapperPass>();
368 AU.addPreserved<LiveIntervalsWrapperPass>();
369 AU.addRequired<SlotIndexesWrapperPass>();
370 AU.addPreserved<SlotIndexesWrapperPass>();
371 AU.addRequired<MachineDominatorTreeWrapperPass>();
372 AU.addPreserved<MachineDominatorTreeWrapperPass>();
373 MachineFunctionPass::getAnalysisUsage(AU);
374 }
375 };
376 }
377
378 INITIALIZE_PASS_BEGIN(PPCVSXFMAMutate, DEBUG_TYPE,
379 "PowerPC VSX FMA Mutation", false, false)
380 INITIALIZE_PASS_DEPENDENCY(LiveIntervalsWrapperPass)
381 INITIALIZE_PASS_DEPENDENCY(SlotIndexesWrapperPass)
382 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
383 INITIALIZE_PASS_END(PPCVSXFMAMutate, DEBUG_TYPE,
384 "PowerPC VSX FMA Mutation", false, false)
385
386 char &llvm::PPCVSXFMAMutateID = PPCVSXFMAMutate::ID;
387
388 char PPCVSXFMAMutate::ID = 0;
createPPCVSXFMAMutatePass()389 FunctionPass *llvm::createPPCVSXFMAMutatePass() {
390 return new PPCVSXFMAMutate();
391 }
392