1 //===-- SIAnnotateControlFlow.cpp -  ------------------===//
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 /// \file
11 /// Annotates the control flow with hardware specific intrinsics.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "AMDGPU.h"
16 #include "llvm/ADT/DepthFirstIterator.h"
17 #include "llvm/Analysis/DivergenceAnalysis.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/IR/Constants.h"
20 #include "llvm/IR/Dominators.h"
21 #include "llvm/IR/Instructions.h"
22 #include "llvm/IR/Module.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
25 #include "llvm/Transforms/Utils/SSAUpdater.h"
26 
27 using namespace llvm;
28 
29 #define DEBUG_TYPE "si-annotate-control-flow"
30 
31 namespace {
32 
33 // Complex types used in this pass
34 typedef std::pair<BasicBlock *, Value *> StackEntry;
35 typedef SmallVector<StackEntry, 16> StackVector;
36 
37 // Intrinsic names the control flow is annotated with
38 static const char *const IfIntrinsic = "llvm.amdgcn.if";
39 static const char *const ElseIntrinsic = "llvm.amdgcn.else";
40 static const char *const BreakIntrinsic = "llvm.amdgcn.break";
41 static const char *const IfBreakIntrinsic = "llvm.amdgcn.if.break";
42 static const char *const ElseBreakIntrinsic = "llvm.amdgcn.else.break";
43 static const char *const LoopIntrinsic = "llvm.amdgcn.loop";
44 static const char *const EndCfIntrinsic = "llvm.amdgcn.end.cf";
45 
46 class SIAnnotateControlFlow : public FunctionPass {
47   DivergenceAnalysis *DA;
48 
49   Type *Boolean;
50   Type *Void;
51   Type *Int64;
52   Type *ReturnStruct;
53 
54   ConstantInt *BoolTrue;
55   ConstantInt *BoolFalse;
56   UndefValue *BoolUndef;
57   Constant *Int64Zero;
58 
59   Constant *If;
60   Constant *Else;
61   Constant *Break;
62   Constant *IfBreak;
63   Constant *ElseBreak;
64   Constant *Loop;
65   Constant *EndCf;
66 
67   DominatorTree *DT;
68   StackVector Stack;
69 
70   LoopInfo *LI;
71 
72   bool isUniform(BranchInst *T);
73 
74   bool isTopOfStack(BasicBlock *BB);
75 
76   Value *popSaved();
77 
78   void push(BasicBlock *BB, Value *Saved);
79 
80   bool isElse(PHINode *Phi);
81 
82   void eraseIfUnused(PHINode *Phi);
83 
84   void openIf(BranchInst *Term);
85 
86   void insertElse(BranchInst *Term);
87 
88   Value *handleLoopCondition(Value *Cond, PHINode *Broken,
89                              llvm::Loop *L, BranchInst *Term);
90 
91   void handleLoop(BranchInst *Term);
92 
93   void closeControlFlow(BasicBlock *BB);
94 
95 public:
96   static char ID;
97 
98   SIAnnotateControlFlow():
99     FunctionPass(ID) { }
100 
101   bool doInitialization(Module &M) override;
102 
103   bool runOnFunction(Function &F) override;
104 
105   StringRef getPassName() const override { return "SI annotate control flow"; }
106 
107   void getAnalysisUsage(AnalysisUsage &AU) const override {
108     AU.addRequired<LoopInfoWrapperPass>();
109     AU.addRequired<DominatorTreeWrapperPass>();
110     AU.addRequired<DivergenceAnalysis>();
111     AU.addPreserved<DominatorTreeWrapperPass>();
112     FunctionPass::getAnalysisUsage(AU);
113   }
114 
115 };
116 
117 } // end anonymous namespace
118 
119 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
120                       "Annotate SI Control Flow", false, false)
121 INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
122 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
123                     "Annotate SI Control Flow", false, false)
124 
125 char SIAnnotateControlFlow::ID = 0;
126 
127 /// \brief Initialize all the types and constants used in the pass
128 bool SIAnnotateControlFlow::doInitialization(Module &M) {
129   LLVMContext &Context = M.getContext();
130 
131   Void = Type::getVoidTy(Context);
132   Boolean = Type::getInt1Ty(Context);
133   Int64 = Type::getInt64Ty(Context);
134   ReturnStruct = StructType::get(Boolean, Int64, (Type *)nullptr);
135 
136   BoolTrue = ConstantInt::getTrue(Context);
137   BoolFalse = ConstantInt::getFalse(Context);
138   BoolUndef = UndefValue::get(Boolean);
139   Int64Zero = ConstantInt::get(Int64, 0);
140 
141   If = M.getOrInsertFunction(
142     IfIntrinsic, ReturnStruct, Boolean, (Type *)nullptr);
143 
144   Else = M.getOrInsertFunction(
145     ElseIntrinsic, ReturnStruct, Int64, (Type *)nullptr);
146 
147   Break = M.getOrInsertFunction(
148     BreakIntrinsic, Int64, Int64, (Type *)nullptr);
149   cast<Function>(Break)->setDoesNotAccessMemory();
150 
151   IfBreak = M.getOrInsertFunction(
152     IfBreakIntrinsic, Int64, Boolean, Int64, (Type *)nullptr);
153   cast<Function>(IfBreak)->setDoesNotAccessMemory();;
154 
155   ElseBreak = M.getOrInsertFunction(
156     ElseBreakIntrinsic, Int64, Int64, Int64, (Type *)nullptr);
157   cast<Function>(ElseBreak)->setDoesNotAccessMemory();
158 
159   Loop = M.getOrInsertFunction(
160     LoopIntrinsic, Boolean, Int64, (Type *)nullptr);
161 
162   EndCf = M.getOrInsertFunction(
163     EndCfIntrinsic, Void, Int64, (Type *)nullptr);
164 
165   return false;
166 }
167 
168 /// \brief Is the branch condition uniform or did the StructurizeCFG pass
169 /// consider it as such?
170 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
171   return DA->isUniform(T->getCondition()) ||
172          T->getMetadata("structurizecfg.uniform") != nullptr;
173 }
174 
175 /// \brief Is BB the last block saved on the stack ?
176 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
177   return !Stack.empty() && Stack.back().first == BB;
178 }
179 
180 /// \brief Pop the last saved value from the control flow stack
181 Value *SIAnnotateControlFlow::popSaved() {
182   return Stack.pop_back_val().second;
183 }
184 
185 /// \brief Push a BB and saved value to the control flow stack
186 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
187   Stack.push_back(std::make_pair(BB, Saved));
188 }
189 
190 /// \brief Can the condition represented by this PHI node treated like
191 /// an "Else" block?
192 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
193   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
194   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
195     if (Phi->getIncomingBlock(i) == IDom) {
196 
197       if (Phi->getIncomingValue(i) != BoolTrue)
198         return false;
199 
200     } else {
201       if (Phi->getIncomingValue(i) != BoolFalse)
202         return false;
203 
204     }
205   }
206   return true;
207 }
208 
209 // \brief Erase "Phi" if it is not used any more
210 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
211   if (!Phi->hasNUsesOrMore(1))
212     Phi->eraseFromParent();
213 }
214 
215 /// \brief Open a new "If" block
216 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
217   if (isUniform(Term)) {
218     return;
219   }
220   Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
221   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
222   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
223 }
224 
225 /// \brief Close the last "If" block and open a new "Else" block
226 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
227   if (isUniform(Term)) {
228     return;
229   }
230   Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
231   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
232   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
233 }
234 
235 /// \brief Recursively handle the condition leading to a loop
236 Value *SIAnnotateControlFlow::handleLoopCondition(Value *Cond, PHINode *Broken,
237                                              llvm::Loop *L, BranchInst *Term) {
238 
239   // Only search through PHI nodes which are inside the loop.  If we try this
240   // with PHI nodes that are outside of the loop, we end up inserting new PHI
241   // nodes outside of the loop which depend on values defined inside the loop.
242   // This will break the module with
243   // 'Instruction does not dominate all users!' errors.
244   PHINode *Phi = nullptr;
245   if ((Phi = dyn_cast<PHINode>(Cond)) && L->contains(Phi)) {
246 
247     BasicBlock *Parent = Phi->getParent();
248     PHINode *NewPhi = PHINode::Create(Int64, 0, "", &Parent->front());
249     Value *Ret = NewPhi;
250 
251     // Handle all non-constant incoming values first
252     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
253       Value *Incoming = Phi->getIncomingValue(i);
254       BasicBlock *From = Phi->getIncomingBlock(i);
255       if (isa<ConstantInt>(Incoming)) {
256         NewPhi->addIncoming(Broken, From);
257         continue;
258       }
259 
260       Phi->setIncomingValue(i, BoolFalse);
261       Value *PhiArg = handleLoopCondition(Incoming, Broken, L, Term);
262       NewPhi->addIncoming(PhiArg, From);
263     }
264 
265     BasicBlock *IDom = DT->getNode(Parent)->getIDom()->getBlock();
266 
267     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
268 
269       Value *Incoming = Phi->getIncomingValue(i);
270       if (Incoming != BoolTrue)
271         continue;
272 
273       BasicBlock *From = Phi->getIncomingBlock(i);
274       if (From == IDom) {
275         // We're in the following situation:
276         //   IDom/From
277         //      |   \
278         //      |   If-block
279         //      |   /
280         //     Parent
281         // where we want to break out of the loop if the If-block is not taken.
282         // Due to the depth-first traversal, there should be an end.cf
283         // intrinsic in Parent, and we insert an else.break before it.
284         //
285         // Note that the end.cf need not be the first non-phi instruction
286         // of parent, particularly when we're dealing with a multi-level
287         // break, but it should occur within a group of intrinsic calls
288         // at the beginning of the block.
289         CallInst *OldEnd = dyn_cast<CallInst>(Parent->getFirstInsertionPt());
290         while (OldEnd && OldEnd->getCalledFunction() != EndCf)
291           OldEnd = dyn_cast<CallInst>(OldEnd->getNextNode());
292         if (OldEnd && OldEnd->getCalledFunction() == EndCf) {
293           Value *Args[] = { OldEnd->getArgOperand(0), NewPhi };
294           Ret = CallInst::Create(ElseBreak, Args, "", OldEnd);
295           continue;
296         }
297       }
298       TerminatorInst *Insert = From->getTerminator();
299       Value *PhiArg = CallInst::Create(Break, Broken, "", Insert);
300       NewPhi->setIncomingValue(i, PhiArg);
301     }
302     eraseIfUnused(Phi);
303     return Ret;
304 
305   } else if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
306     BasicBlock *Parent = Inst->getParent();
307     Instruction *Insert;
308     if (L->contains(Inst)) {
309       Insert = Parent->getTerminator();
310     } else {
311       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
312     }
313     Value *Args[] = { Cond, Broken };
314     return CallInst::Create(IfBreak, Args, "", Insert);
315 
316   // Insert IfBreak before TERM for constant COND.
317   } else if (isa<ConstantInt>(Cond)) {
318     Value *Args[] = { Cond, Broken };
319     return CallInst::Create(IfBreak, Args, "", Term);
320 
321   } else {
322     llvm_unreachable("Unhandled loop condition!");
323   }
324   return nullptr;
325 }
326 
327 /// \brief Handle a back edge (loop)
328 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
329   if (isUniform(Term)) {
330     return;
331   }
332 
333   BasicBlock *BB = Term->getParent();
334   llvm::Loop *L = LI->getLoopFor(BB);
335   if (!L)
336     return;
337   BasicBlock *Target = Term->getSuccessor(1);
338   PHINode *Broken = PHINode::Create(Int64, 0, "", &Target->front());
339 
340   Value *Cond = Term->getCondition();
341   Term->setCondition(BoolTrue);
342   Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
343 
344   for (pred_iterator PI = pred_begin(Target), PE = pred_end(Target);
345        PI != PE; ++PI) {
346 
347     Broken->addIncoming(*PI == BB ? Arg : Int64Zero, *PI);
348   }
349 
350   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
351   push(Term->getSuccessor(0), Arg);
352 }/// \brief Close the last opened control flow
353 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
354   llvm::Loop *L = LI->getLoopFor(BB);
355 
356   assert(Stack.back().first == BB);
357 
358   if (L && L->getHeader() == BB) {
359     // We can't insert an EndCF call into a loop header, because it will
360     // get executed on every iteration of the loop, when it should be
361     // executed only once before the loop.
362     SmallVector <BasicBlock*, 8> Latches;
363     L->getLoopLatches(Latches);
364 
365     std::vector<BasicBlock*> Preds;
366     for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI) {
367       if (!is_contained(Latches, *PI))
368         Preds.push_back(*PI);
369     }
370     BB = llvm::SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, false);
371   }
372 
373   Value *Exec = popSaved();
374   if (!isa<UndefValue>(Exec))
375     CallInst::Create(EndCf, Exec, "", &*BB->getFirstInsertionPt());
376 }
377 
378 /// \brief Annotate the control flow with intrinsics so the backend can
379 /// recognize if/then/else and loops.
380 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
381 
382   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
383   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
384   DA = &getAnalysis<DivergenceAnalysis>();
385 
386   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
387        E = df_end(&F.getEntryBlock()); I != E; ++I) {
388 
389     BranchInst *Term = dyn_cast<BranchInst>((*I)->getTerminator());
390 
391     if (!Term || Term->isUnconditional()) {
392       if (isTopOfStack(*I))
393         closeControlFlow(*I);
394 
395       continue;
396     }
397 
398     if (I.nodeVisited(Term->getSuccessor(1))) {
399       if (isTopOfStack(*I))
400         closeControlFlow(*I);
401 
402       handleLoop(Term);
403       continue;
404     }
405 
406     if (isTopOfStack(*I)) {
407       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
408       if (Phi && Phi->getParent() == *I && isElse(Phi)) {
409         insertElse(Term);
410         eraseIfUnused(Phi);
411         continue;
412       }
413       closeControlFlow(*I);
414     }
415     openIf(Term);
416   }
417 
418   assert(Stack.empty());
419   return true;
420 }
421 
422 /// \brief Create the annotation pass
423 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
424   return new SIAnnotateControlFlow();
425 }
426