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   const char *getPassName() const override {
106     return "SI annotate control flow";
107   }
108 
109   void getAnalysisUsage(AnalysisUsage &AU) const override {
110     AU.addRequired<LoopInfoWrapperPass>();
111     AU.addRequired<DominatorTreeWrapperPass>();
112     AU.addRequired<DivergenceAnalysis>();
113     AU.addPreserved<DominatorTreeWrapperPass>();
114     FunctionPass::getAnalysisUsage(AU);
115   }
116 
117 };
118 
119 } // end anonymous namespace
120 
121 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
122                       "Annotate SI Control Flow", false, false)
123 INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
124 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
125                     "Annotate SI Control Flow", false, false)
126 
127 char SIAnnotateControlFlow::ID = 0;
128 
129 /// \brief Initialize all the types and constants used in the pass
130 bool SIAnnotateControlFlow::doInitialization(Module &M) {
131   LLVMContext &Context = M.getContext();
132 
133   Void = Type::getVoidTy(Context);
134   Boolean = Type::getInt1Ty(Context);
135   Int64 = Type::getInt64Ty(Context);
136   ReturnStruct = StructType::get(Boolean, Int64, (Type *)nullptr);
137 
138   BoolTrue = ConstantInt::getTrue(Context);
139   BoolFalse = ConstantInt::getFalse(Context);
140   BoolUndef = UndefValue::get(Boolean);
141   Int64Zero = ConstantInt::get(Int64, 0);
142 
143   If = M.getOrInsertFunction(
144     IfIntrinsic, ReturnStruct, Boolean, (Type *)nullptr);
145 
146   Else = M.getOrInsertFunction(
147     ElseIntrinsic, ReturnStruct, Int64, (Type *)nullptr);
148 
149   Break = M.getOrInsertFunction(
150     BreakIntrinsic, Int64, Int64, (Type *)nullptr);
151   cast<Function>(Break)->setDoesNotAccessMemory();
152 
153   IfBreak = M.getOrInsertFunction(
154     IfBreakIntrinsic, Int64, Boolean, Int64, (Type *)nullptr);
155   cast<Function>(IfBreak)->setDoesNotAccessMemory();;
156 
157   ElseBreak = M.getOrInsertFunction(
158     ElseBreakIntrinsic, Int64, Int64, Int64, (Type *)nullptr);
159   cast<Function>(ElseBreak)->setDoesNotAccessMemory();
160 
161   Loop = M.getOrInsertFunction(
162     LoopIntrinsic, Boolean, Int64, (Type *)nullptr);
163 
164   EndCf = M.getOrInsertFunction(
165     EndCfIntrinsic, Void, Int64, (Type *)nullptr);
166 
167   return false;
168 }
169 
170 /// \brief Is the branch condition uniform or did the StructurizeCFG pass
171 /// consider it as such?
172 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
173   return DA->isUniform(T->getCondition()) ||
174          T->getMetadata("structurizecfg.uniform") != nullptr;
175 }
176 
177 /// \brief Is BB the last block saved on the stack ?
178 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
179   return !Stack.empty() && Stack.back().first == BB;
180 }
181 
182 /// \brief Pop the last saved value from the control flow stack
183 Value *SIAnnotateControlFlow::popSaved() {
184   return Stack.pop_back_val().second;
185 }
186 
187 /// \brief Push a BB and saved value to the control flow stack
188 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
189   Stack.push_back(std::make_pair(BB, Saved));
190 }
191 
192 /// \brief Can the condition represented by this PHI node treated like
193 /// an "Else" block?
194 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
195   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
196   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
197     if (Phi->getIncomingBlock(i) == IDom) {
198 
199       if (Phi->getIncomingValue(i) != BoolTrue)
200         return false;
201 
202     } else {
203       if (Phi->getIncomingValue(i) != BoolFalse)
204         return false;
205 
206     }
207   }
208   return true;
209 }
210 
211 // \brief Erase "Phi" if it is not used any more
212 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
213   if (!Phi->hasNUsesOrMore(1))
214     Phi->eraseFromParent();
215 }
216 
217 /// \brief Open a new "If" block
218 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
219   if (isUniform(Term)) {
220     return;
221   }
222   Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
223   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
224   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
225 }
226 
227 /// \brief Close the last "If" block and open a new "Else" block
228 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
229   if (isUniform(Term)) {
230     return;
231   }
232   Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
233   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
234   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
235 }
236 
237 /// \brief Recursively handle the condition leading to a loop
238 Value *SIAnnotateControlFlow::handleLoopCondition(Value *Cond, PHINode *Broken,
239                                              llvm::Loop *L, BranchInst *Term) {
240 
241   // Only search through PHI nodes which are inside the loop.  If we try this
242   // with PHI nodes that are outside of the loop, we end up inserting new PHI
243   // nodes outside of the loop which depend on values defined inside the loop.
244   // This will break the module with
245   // 'Instruction does not dominate all users!' errors.
246   PHINode *Phi = nullptr;
247   if ((Phi = dyn_cast<PHINode>(Cond)) && L->contains(Phi)) {
248 
249     BasicBlock *Parent = Phi->getParent();
250     PHINode *NewPhi = PHINode::Create(Int64, 0, "", &Parent->front());
251     Value *Ret = NewPhi;
252 
253     // Handle all non-constant incoming values first
254     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
255       Value *Incoming = Phi->getIncomingValue(i);
256       BasicBlock *From = Phi->getIncomingBlock(i);
257       if (isa<ConstantInt>(Incoming)) {
258         NewPhi->addIncoming(Broken, From);
259         continue;
260       }
261 
262       Phi->setIncomingValue(i, BoolFalse);
263       Value *PhiArg = handleLoopCondition(Incoming, Broken, L, Term);
264       NewPhi->addIncoming(PhiArg, From);
265     }
266 
267     BasicBlock *IDom = DT->getNode(Parent)->getIDom()->getBlock();
268 
269     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
270 
271       Value *Incoming = Phi->getIncomingValue(i);
272       if (Incoming != BoolTrue)
273         continue;
274 
275       BasicBlock *From = Phi->getIncomingBlock(i);
276       if (From == IDom) {
277         // We're in the following situation:
278         //   IDom/From
279         //      |   \
280         //      |   If-block
281         //      |   /
282         //     Parent
283         // where we want to break out of the loop if the If-block is not taken.
284         // Due to the depth-first traversal, there should be an end.cf
285         // intrinsic in Parent, and we insert an else.break before it.
286         //
287         // Note that the end.cf need not be the first non-phi instruction
288         // of parent, particularly when we're dealing with a multi-level
289         // break, but it should occur within a group of intrinsic calls
290         // at the beginning of the block.
291         CallInst *OldEnd = dyn_cast<CallInst>(Parent->getFirstInsertionPt());
292         while (OldEnd && OldEnd->getCalledFunction() != EndCf)
293           OldEnd = dyn_cast<CallInst>(OldEnd->getNextNode());
294         if (OldEnd && OldEnd->getCalledFunction() == EndCf) {
295           Value *Args[] = { OldEnd->getArgOperand(0), NewPhi };
296           Ret = CallInst::Create(ElseBreak, Args, "", OldEnd);
297           continue;
298         }
299       }
300       TerminatorInst *Insert = From->getTerminator();
301       Value *PhiArg = CallInst::Create(Break, Broken, "", Insert);
302       NewPhi->setIncomingValue(i, PhiArg);
303     }
304     eraseIfUnused(Phi);
305     return Ret;
306 
307   } else if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
308     BasicBlock *Parent = Inst->getParent();
309     Instruction *Insert;
310     if (L->contains(Inst)) {
311       Insert = Parent->getTerminator();
312     } else {
313       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
314     }
315     Value *Args[] = { Cond, Broken };
316     return CallInst::Create(IfBreak, Args, "", Insert);
317 
318   // Insert IfBreak before TERM for constant COND.
319   } else if (isa<ConstantInt>(Cond)) {
320     Value *Args[] = { Cond, Broken };
321     return CallInst::Create(IfBreak, Args, "", Term);
322 
323   } else {
324     llvm_unreachable("Unhandled loop condition!");
325   }
326   return nullptr;
327 }
328 
329 /// \brief Handle a back edge (loop)
330 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
331   if (isUniform(Term)) {
332     return;
333   }
334 
335   BasicBlock *BB = Term->getParent();
336   llvm::Loop *L = LI->getLoopFor(BB);
337   if (!L)
338     return;
339   BasicBlock *Target = Term->getSuccessor(1);
340   PHINode *Broken = PHINode::Create(Int64, 0, "", &Target->front());
341 
342   Value *Cond = Term->getCondition();
343   Term->setCondition(BoolTrue);
344   Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
345 
346   for (pred_iterator PI = pred_begin(Target), PE = pred_end(Target);
347        PI != PE; ++PI) {
348 
349     Broken->addIncoming(*PI == BB ? Arg : Int64Zero, *PI);
350   }
351 
352   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
353   push(Term->getSuccessor(0), Arg);
354 }/// \brief Close the last opened control flow
355 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
356   llvm::Loop *L = LI->getLoopFor(BB);
357 
358   assert(Stack.back().first == BB);
359 
360   if (L && L->getHeader() == BB) {
361     // We can't insert an EndCF call into a loop header, because it will
362     // get executed on every iteration of the loop, when it should be
363     // executed only once before the loop.
364     SmallVector <BasicBlock*, 8> Latches;
365     L->getLoopLatches(Latches);
366 
367     std::vector<BasicBlock*> Preds;
368     for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI) {
369       if (!is_contained(Latches, *PI))
370         Preds.push_back(*PI);
371     }
372     BB = llvm::SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, false);
373   }
374 
375   Value *Exec = popSaved();
376   if (!isa<UndefValue>(Exec))
377     CallInst::Create(EndCf, Exec, "", &*BB->getFirstInsertionPt());
378 }
379 
380 /// \brief Annotate the control flow with intrinsics so the backend can
381 /// recognize if/then/else and loops.
382 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
383 
384   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
385   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
386   DA = &getAnalysis<DivergenceAnalysis>();
387 
388   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
389        E = df_end(&F.getEntryBlock()); I != E; ++I) {
390 
391     BranchInst *Term = dyn_cast<BranchInst>((*I)->getTerminator());
392 
393     if (!Term || Term->isUnconditional()) {
394       if (isTopOfStack(*I))
395         closeControlFlow(*I);
396 
397       continue;
398     }
399 
400     if (I.nodeVisited(Term->getSuccessor(1))) {
401       if (isTopOfStack(*I))
402         closeControlFlow(*I);
403 
404       handleLoop(Term);
405       continue;
406     }
407 
408     if (isTopOfStack(*I)) {
409       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
410       if (Phi && Phi->getParent() == *I && isElse(Phi)) {
411         insertElse(Term);
412         eraseIfUnused(Phi);
413         continue;
414       }
415       closeControlFlow(*I);
416     }
417     openIf(Term);
418   }
419 
420   assert(Stack.empty());
421   return true;
422 }
423 
424 /// \brief Create the annotation pass
425 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
426   return new SIAnnotateControlFlow();
427 }
428