1 //===- ScopHelper.cpp - Some Helper Functions for Scop.  ------------------===//
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 // Small functions that help with Scop and LLVM-IR.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "polly/Support/ScopHelper.h"
15 #include "polly/Options.h"
16 #include "polly/ScopInfo.h"
17 #include "polly/Support/SCEVValidator.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/Analysis/RegionInfo.h"
20 #include "llvm/Analysis/ScalarEvolution.h"
21 #include "llvm/Analysis/ScalarEvolutionExpander.h"
22 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
23 #include "llvm/IR/CFG.h"
24 #include "llvm/IR/IntrinsicInst.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
27 
28 using namespace llvm;
29 using namespace polly;
30 
31 #define DEBUG_TYPE "polly-scop-helper"
32 
33 bool polly::hasInvokeEdge(const PHINode *PN) {
34   for (unsigned i = 0, e = PN->getNumIncomingValues(); i < e; ++i)
35     if (InvokeInst *II = dyn_cast<InvokeInst>(PN->getIncomingValue(i)))
36       if (II->getParent() == PN->getIncomingBlock(i))
37         return true;
38 
39   return false;
40 }
41 
42 // Ensures that there is just one predecessor to the entry node from outside the
43 // region.
44 // The identity of the region entry node is preserved.
45 static void simplifyRegionEntry(Region *R, DominatorTree *DT, LoopInfo *LI,
46                                 RegionInfo *RI) {
47   BasicBlock *EnteringBB = R->getEnteringBlock();
48   BasicBlock *Entry = R->getEntry();
49 
50   // Before (one of):
51   //
52   //                       \    /            //
53   //                      EnteringBB         //
54   //                        |    \------>    //
55   //   \   /                |                //
56   //   Entry <--\         Entry <--\         //
57   //   /   \    /         /   \    /         //
58   //        ....               ....          //
59 
60   // Create single entry edge if the region has multiple entry edges.
61   if (!EnteringBB) {
62     SmallVector<BasicBlock *, 4> Preds;
63     for (BasicBlock *P : predecessors(Entry))
64       if (!R->contains(P))
65         Preds.push_back(P);
66 
67     BasicBlock *NewEntering =
68         SplitBlockPredecessors(Entry, Preds, ".region_entering", DT, LI);
69 
70     if (RI) {
71       // The exit block of predecessing regions must be changed to NewEntering
72       for (BasicBlock *ExitPred : predecessors(NewEntering)) {
73         Region *RegionOfPred = RI->getRegionFor(ExitPred);
74         if (RegionOfPred->getExit() != Entry)
75           continue;
76 
77         while (!RegionOfPred->isTopLevelRegion() &&
78                RegionOfPred->getExit() == Entry) {
79           RegionOfPred->replaceExit(NewEntering);
80           RegionOfPred = RegionOfPred->getParent();
81         }
82       }
83 
84       // Make all ancestors use EnteringBB as entry; there might be edges to it
85       Region *AncestorR = R->getParent();
86       RI->setRegionFor(NewEntering, AncestorR);
87       while (!AncestorR->isTopLevelRegion() && AncestorR->getEntry() == Entry) {
88         AncestorR->replaceEntry(NewEntering);
89         AncestorR = AncestorR->getParent();
90       }
91     }
92 
93     EnteringBB = NewEntering;
94   }
95   assert(R->getEnteringBlock() == EnteringBB);
96 
97   // After:
98   //
99   //    \    /       //
100   //  EnteringBB     //
101   //      |          //
102   //      |          //
103   //    Entry <--\   //
104   //    /   \    /   //
105   //         ....    //
106 }
107 
108 // Ensure that the region has a single block that branches to the exit node.
109 static void simplifyRegionExit(Region *R, DominatorTree *DT, LoopInfo *LI,
110                                RegionInfo *RI) {
111   BasicBlock *ExitBB = R->getExit();
112   BasicBlock *ExitingBB = R->getExitingBlock();
113 
114   // Before:
115   //
116   //   (Region)   ______/  //
117   //      \  |   /         //
118   //       ExitBB          //
119   //       /    \          //
120 
121   if (!ExitingBB) {
122     SmallVector<BasicBlock *, 4> Preds;
123     for (BasicBlock *P : predecessors(ExitBB))
124       if (R->contains(P))
125         Preds.push_back(P);
126 
127     //  Preds[0] Preds[1]      otherBB //
128     //         \  |  ________/         //
129     //          \ | /                  //
130     //           BB                    //
131     ExitingBB =
132         SplitBlockPredecessors(ExitBB, Preds, ".region_exiting", DT, LI);
133     // Preds[0] Preds[1]      otherBB  //
134     //        \  /           /         //
135     // BB.region_exiting    /          //
136     //                  \  /           //
137     //                   BB            //
138 
139     if (RI)
140       RI->setRegionFor(ExitingBB, R);
141 
142     // Change the exit of nested regions, but not the region itself,
143     R->replaceExitRecursive(ExitingBB);
144     R->replaceExit(ExitBB);
145   }
146   assert(ExitingBB == R->getExitingBlock());
147 
148   // After:
149   //
150   //     \   /                //
151   //    ExitingBB     _____/  //
152   //          \      /        //
153   //           ExitBB         //
154   //           /    \         //
155 }
156 
157 void polly::simplifyRegion(Region *R, DominatorTree *DT, LoopInfo *LI,
158                            RegionInfo *RI) {
159   assert(R && !R->isTopLevelRegion());
160   assert(!RI || RI == R->getRegionInfo());
161   assert((!RI || DT) &&
162          "RegionInfo requires DominatorTree to be updated as well");
163 
164   simplifyRegionEntry(R, DT, LI, RI);
165   simplifyRegionExit(R, DT, LI, RI);
166   assert(R->isSimple());
167 }
168 
169 // Split the block into two successive blocks.
170 //
171 // Like llvm::SplitBlock, but also preserves RegionInfo
172 static BasicBlock *splitBlock(BasicBlock *Old, Instruction *SplitPt,
173                               DominatorTree *DT, llvm::LoopInfo *LI,
174                               RegionInfo *RI) {
175   assert(Old && SplitPt);
176 
177   // Before:
178   //
179   //  \   /  //
180   //   Old   //
181   //  /   \  //
182 
183   BasicBlock *NewBlock = llvm::SplitBlock(Old, SplitPt, DT, LI);
184 
185   if (RI) {
186     Region *R = RI->getRegionFor(Old);
187     RI->setRegionFor(NewBlock, R);
188   }
189 
190   // After:
191   //
192   //   \   /    //
193   //    Old     //
194   //     |      //
195   //  NewBlock  //
196   //   /   \    //
197 
198   return NewBlock;
199 }
200 
201 void polly::splitEntryBlockForAlloca(BasicBlock *EntryBlock, Pass *P) {
202   // Find first non-alloca instruction. Every basic block has a non-alloc
203   // instruction, as every well formed basic block has a terminator.
204   BasicBlock::iterator I = EntryBlock->begin();
205   while (isa<AllocaInst>(I))
206     ++I;
207 
208   auto *DTWP = P->getAnalysisIfAvailable<DominatorTreeWrapperPass>();
209   auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
210   auto *LIWP = P->getAnalysisIfAvailable<LoopInfoWrapperPass>();
211   auto *LI = LIWP ? &LIWP->getLoopInfo() : nullptr;
212   RegionInfoPass *RIP = P->getAnalysisIfAvailable<RegionInfoPass>();
213   RegionInfo *RI = RIP ? &RIP->getRegionInfo() : nullptr;
214 
215   // splitBlock updates DT, LI and RI.
216   splitBlock(EntryBlock, &*I, DT, LI, RI);
217 }
218 
219 /// The SCEVExpander will __not__ generate any code for an existing SDiv/SRem
220 /// instruction but just use it, if it is referenced as a SCEVUnknown. We want
221 /// however to generate new code if the instruction is in the analyzed region
222 /// and we generate code outside/in front of that region. Hence, we generate the
223 /// code for the SDiv/SRem operands in front of the analyzed region and then
224 /// create a new SDiv/SRem operation there too.
225 struct ScopExpander : SCEVVisitor<ScopExpander, const SCEV *> {
226   friend struct SCEVVisitor<ScopExpander, const SCEV *>;
227 
228   explicit ScopExpander(const Region &R, ScalarEvolution &SE,
229                         const DataLayout &DL, const char *Name, ValueMapT *VMap)
230       : Expander(SCEVExpander(SE, DL, Name)), SE(SE), Name(Name), R(R),
231         VMap(VMap) {}
232 
233   Value *expandCodeFor(const SCEV *E, Type *Ty, Instruction *I) {
234     // If we generate code in the region we will immediately fall back to the
235     // SCEVExpander, otherwise we will stop at all unknowns in the SCEV and if
236     // needed replace them by copies computed in the entering block.
237     if (!R.contains(I))
238       E = visit(E);
239     return Expander.expandCodeFor(E, Ty, I);
240   }
241 
242 private:
243   SCEVExpander Expander;
244   ScalarEvolution &SE;
245   const char *Name;
246   const Region &R;
247   ValueMapT *VMap;
248 
249   const SCEV *visitUnknown(const SCEVUnknown *E) {
250 
251     // If a value mapping was given try if the underlying value is remapped.
252     if (VMap)
253       if (Value *NewVal = VMap->lookup(E->getValue()))
254         if (NewVal != E->getValue())
255           return visit(SE.getSCEV(NewVal));
256 
257     Instruction *Inst = dyn_cast<Instruction>(E->getValue());
258     if (!Inst || (Inst->getOpcode() != Instruction::SRem &&
259                   Inst->getOpcode() != Instruction::SDiv))
260       return E;
261 
262     if (!R.contains(Inst))
263       return E;
264 
265     Instruction *StartIP = R.getEnteringBlock()->getTerminator();
266 
267     const SCEV *LHSScev = visit(SE.getSCEV(Inst->getOperand(0)));
268     const SCEV *RHSScev = visit(SE.getSCEV(Inst->getOperand(1)));
269 
270     Value *LHS = Expander.expandCodeFor(LHSScev, E->getType(), StartIP);
271     Value *RHS = Expander.expandCodeFor(RHSScev, E->getType(), StartIP);
272 
273     Inst = BinaryOperator::Create((Instruction::BinaryOps)Inst->getOpcode(),
274                                   LHS, RHS, Inst->getName() + Name, StartIP);
275     return SE.getSCEV(Inst);
276   }
277 
278   /// The following functions will just traverse the SCEV and rebuild it with
279   /// the new operands returned by the traversal.
280   ///
281   ///{
282   const SCEV *visitConstant(const SCEVConstant *E) { return E; }
283   const SCEV *visitTruncateExpr(const SCEVTruncateExpr *E) {
284     return SE.getTruncateExpr(visit(E->getOperand()), E->getType());
285   }
286   const SCEV *visitZeroExtendExpr(const SCEVZeroExtendExpr *E) {
287     return SE.getZeroExtendExpr(visit(E->getOperand()), E->getType());
288   }
289   const SCEV *visitSignExtendExpr(const SCEVSignExtendExpr *E) {
290     return SE.getSignExtendExpr(visit(E->getOperand()), E->getType());
291   }
292   const SCEV *visitUDivExpr(const SCEVUDivExpr *E) {
293     return SE.getUDivExpr(visit(E->getLHS()), visit(E->getRHS()));
294   }
295   const SCEV *visitAddExpr(const SCEVAddExpr *E) {
296     SmallVector<const SCEV *, 4> NewOps;
297     for (const SCEV *Op : E->operands())
298       NewOps.push_back(visit(Op));
299     return SE.getAddExpr(NewOps);
300   }
301   const SCEV *visitMulExpr(const SCEVMulExpr *E) {
302     SmallVector<const SCEV *, 4> NewOps;
303     for (const SCEV *Op : E->operands())
304       NewOps.push_back(visit(Op));
305     return SE.getMulExpr(NewOps);
306   }
307   const SCEV *visitUMaxExpr(const SCEVUMaxExpr *E) {
308     SmallVector<const SCEV *, 4> NewOps;
309     for (const SCEV *Op : E->operands())
310       NewOps.push_back(visit(Op));
311     return SE.getUMaxExpr(NewOps);
312   }
313   const SCEV *visitSMaxExpr(const SCEVSMaxExpr *E) {
314     SmallVector<const SCEV *, 4> NewOps;
315     for (const SCEV *Op : E->operands())
316       NewOps.push_back(visit(Op));
317     return SE.getSMaxExpr(NewOps);
318   }
319   const SCEV *visitAddRecExpr(const SCEVAddRecExpr *E) {
320     SmallVector<const SCEV *, 4> NewOps;
321     for (const SCEV *Op : E->operands())
322       NewOps.push_back(visit(Op));
323     return SE.getAddRecExpr(NewOps, E->getLoop(), E->getNoWrapFlags());
324   }
325   ///}
326 };
327 
328 Value *polly::expandCodeFor(Scop &S, ScalarEvolution &SE, const DataLayout &DL,
329                             const char *Name, const SCEV *E, Type *Ty,
330                             Instruction *IP, ValueMapT *VMap) {
331   ScopExpander Expander(S.getRegion(), SE, DL, Name, VMap);
332   return Expander.expandCodeFor(E, Ty, IP);
333 }
334 
335 bool polly::isErrorBlock(BasicBlock &BB, const Region &R, LoopInfo &LI,
336                          const DominatorTree &DT) {
337 
338   if (isa<UnreachableInst>(BB.getTerminator()))
339     return true;
340 
341   if (LI.isLoopHeader(&BB))
342     return false;
343 
344   // Basic blocks that are always executed are not considered error blocks,
345   // as their execution can not be a rare event.
346   bool DominatesAllPredecessors = true;
347   for (auto Pred : predecessors(R.getExit()))
348     if (R.contains(Pred) && !DT.dominates(&BB, Pred))
349       DominatesAllPredecessors = false;
350 
351   if (DominatesAllPredecessors)
352     return false;
353 
354   // FIXME: This is a simple heuristic to determine if the load is executed
355   //        in a conditional. However, we actually would need the control
356   //        condition, i.e., the post dominance frontier. Alternatively we
357   //        could walk up the dominance tree until we find a block that is
358   //        not post dominated by the load and check if it is a conditional
359   //        or a loop header.
360   auto *DTNode = DT.getNode(&BB);
361   auto *IDomBB = DTNode->getIDom()->getBlock();
362   if (LI.isLoopHeader(IDomBB))
363     return false;
364 
365   for (Instruction &Inst : BB)
366     if (CallInst *CI = dyn_cast<CallInst>(&Inst)) {
367       if (isIgnoredIntrinsic(CI))
368         return false;
369 
370       if (!CI->doesNotAccessMemory())
371         return true;
372       if (CI->doesNotReturn())
373         return true;
374     }
375 
376   return false;
377 }
378 
379 Value *polly::getConditionFromTerminator(TerminatorInst *TI) {
380   if (BranchInst *BR = dyn_cast<BranchInst>(TI)) {
381     if (BR->isUnconditional())
382       return ConstantInt::getTrue(Type::getInt1Ty(TI->getContext()));
383 
384     return BR->getCondition();
385   }
386 
387   if (SwitchInst *SI = dyn_cast<SwitchInst>(TI))
388     return SI->getCondition();
389 
390   return nullptr;
391 }
392 
393 bool polly::isHoistableLoad(LoadInst *LInst, Region &R, LoopInfo &LI,
394                             ScalarEvolution &SE) {
395   Loop *L = LI.getLoopFor(LInst->getParent());
396   const SCEV *PtrSCEV = SE.getSCEVAtScope(LInst->getPointerOperand(), L);
397   while (L && R.contains(L)) {
398     if (!SE.isLoopInvariant(PtrSCEV, L))
399       return false;
400     L = L->getParentLoop();
401   }
402 
403   return true;
404 }
405 
406 bool polly::isIgnoredIntrinsic(const Value *V) {
407   if (auto *IT = dyn_cast<IntrinsicInst>(V)) {
408     switch (IT->getIntrinsicID()) {
409     // Lifetime markers are supported/ignored.
410     case llvm::Intrinsic::lifetime_start:
411     case llvm::Intrinsic::lifetime_end:
412     // Invariant markers are supported/ignored.
413     case llvm::Intrinsic::invariant_start:
414     case llvm::Intrinsic::invariant_end:
415     // Some misc annotations are supported/ignored.
416     case llvm::Intrinsic::var_annotation:
417     case llvm::Intrinsic::ptr_annotation:
418     case llvm::Intrinsic::annotation:
419     case llvm::Intrinsic::donothing:
420     case llvm::Intrinsic::assume:
421     case llvm::Intrinsic::expect:
422     // Some debug info intrisics are supported/ignored.
423     case llvm::Intrinsic::dbg_value:
424     case llvm::Intrinsic::dbg_declare:
425       return true;
426     default:
427       break;
428     }
429   }
430   return false;
431 }
432 
433 bool polly::canSynthesize(const Value *V, const llvm::LoopInfo *LI,
434                           ScalarEvolution *SE, const Region *R) {
435   if (!V || !SE->isSCEVable(V->getType()))
436     return false;
437 
438   if (const SCEV *Scev = SE->getSCEV(const_cast<Value *>(V)))
439     if (!isa<SCEVCouldNotCompute>(Scev))
440       if (!hasScalarDepsInsideRegion(Scev, R))
441         return true;
442 
443   return false;
444 }
445