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