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/Local.h"
26 #include "llvm/Transforms/Utils/SSAUpdater.h"
27 
28 using namespace llvm;
29 
30 #define DEBUG_TYPE "si-annotate-control-flow"
31 
32 namespace {
33 
34 // Complex types used in this pass
35 typedef std::pair<BasicBlock *, Value *> StackEntry;
36 typedef SmallVector<StackEntry, 16> StackVector;
37 
38 class SIAnnotateControlFlow : public FunctionPass {
39   DivergenceAnalysis *DA;
40 
41   Type *Boolean;
42   Type *Void;
43   Type *Int64;
44   Type *ReturnStruct;
45 
46   ConstantInt *BoolTrue;
47   ConstantInt *BoolFalse;
48   UndefValue *BoolUndef;
49   Constant *Int64Zero;
50 
51   Function *If;
52   Function *Else;
53   Function *Break;
54   Function *IfBreak;
55   Function *ElseBreak;
56   Function *Loop;
57   Function *EndCf;
58 
59   DominatorTree *DT;
60   StackVector Stack;
61 
62   LoopInfo *LI;
63 
64   bool isUniform(BranchInst *T);
65 
66   bool isTopOfStack(BasicBlock *BB);
67 
68   Value *popSaved();
69 
70   void push(BasicBlock *BB, Value *Saved);
71 
72   bool isElse(PHINode *Phi);
73 
74   void eraseIfUnused(PHINode *Phi);
75 
76   void openIf(BranchInst *Term);
77 
78   void insertElse(BranchInst *Term);
79 
80   Value *handleLoopCondition(Value *Cond, PHINode *Broken,
81                              llvm::Loop *L, BranchInst *Term,
82                              SmallVectorImpl<WeakVH> &LoopPhiConditions);
83 
84   void handleLoop(BranchInst *Term);
85 
86   void closeControlFlow(BasicBlock *BB);
87 
88 public:
89   static char ID;
90 
91   SIAnnotateControlFlow():
92     FunctionPass(ID) { }
93 
94   bool doInitialization(Module &M) override;
95 
96   bool runOnFunction(Function &F) override;
97 
98   StringRef getPassName() const override { return "SI annotate control flow"; }
99 
100   void getAnalysisUsage(AnalysisUsage &AU) const override {
101     AU.addRequired<LoopInfoWrapperPass>();
102     AU.addRequired<DominatorTreeWrapperPass>();
103     AU.addRequired<DivergenceAnalysis>();
104     AU.addPreserved<DominatorTreeWrapperPass>();
105     FunctionPass::getAnalysisUsage(AU);
106   }
107 
108 };
109 
110 } // end anonymous namespace
111 
112 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
113                       "Annotate SI Control Flow", false, false)
114 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
115 INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
116 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
117                     "Annotate SI Control Flow", false, false)
118 
119 char SIAnnotateControlFlow::ID = 0;
120 
121 /// \brief Initialize all the types and constants used in the pass
122 bool SIAnnotateControlFlow::doInitialization(Module &M) {
123   LLVMContext &Context = M.getContext();
124 
125   Void = Type::getVoidTy(Context);
126   Boolean = Type::getInt1Ty(Context);
127   Int64 = Type::getInt64Ty(Context);
128   ReturnStruct = StructType::get(Boolean, Int64, (Type *)nullptr);
129 
130   BoolTrue = ConstantInt::getTrue(Context);
131   BoolFalse = ConstantInt::getFalse(Context);
132   BoolUndef = UndefValue::get(Boolean);
133   Int64Zero = ConstantInt::get(Int64, 0);
134 
135   If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if);
136   Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else);
137   Break = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_break);
138   IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break);
139   ElseBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else_break);
140   Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop);
141   EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf);
142   return false;
143 }
144 
145 /// \brief Is the branch condition uniform or did the StructurizeCFG pass
146 /// consider it as such?
147 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
148   return DA->isUniform(T->getCondition()) ||
149          T->getMetadata("structurizecfg.uniform") != nullptr;
150 }
151 
152 /// \brief Is BB the last block saved on the stack ?
153 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
154   return !Stack.empty() && Stack.back().first == BB;
155 }
156 
157 /// \brief Pop the last saved value from the control flow stack
158 Value *SIAnnotateControlFlow::popSaved() {
159   return Stack.pop_back_val().second;
160 }
161 
162 /// \brief Push a BB and saved value to the control flow stack
163 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
164   Stack.push_back(std::make_pair(BB, Saved));
165 }
166 
167 /// \brief Can the condition represented by this PHI node treated like
168 /// an "Else" block?
169 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
170   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
171   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
172     if (Phi->getIncomingBlock(i) == IDom) {
173 
174       if (Phi->getIncomingValue(i) != BoolTrue)
175         return false;
176 
177     } else {
178       if (Phi->getIncomingValue(i) != BoolFalse)
179         return false;
180 
181     }
182   }
183   return true;
184 }
185 
186 // \brief Erase "Phi" if it is not used any more
187 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
188   if (llvm::RecursivelyDeleteDeadPHINode(Phi)) {
189     DEBUG(dbgs() << "Erased unused condition phi\n");
190   }
191 }
192 
193 /// \brief Open a new "If" block
194 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
195   if (isUniform(Term))
196     return;
197 
198   Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
199   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
200   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
201 }
202 
203 /// \brief Close the last "If" block and open a new "Else" block
204 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
205   if (isUniform(Term)) {
206     return;
207   }
208   Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
209   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
210   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
211 }
212 
213 /// \brief Recursively handle the condition leading to a loop
214 Value *SIAnnotateControlFlow::handleLoopCondition(
215   Value *Cond, PHINode *Broken,
216   llvm::Loop *L, BranchInst *Term,
217   SmallVectorImpl<WeakVH> &LoopPhiConditions) {
218 
219   // Only search through PHI nodes which are inside the loop.  If we try this
220   // with PHI nodes that are outside of the loop, we end up inserting new PHI
221   // nodes outside of the loop which depend on values defined inside the loop.
222   // This will break the module with
223   // 'Instruction does not dominate all users!' errors.
224   PHINode *Phi = nullptr;
225   if ((Phi = dyn_cast<PHINode>(Cond)) && L->contains(Phi)) {
226 
227     BasicBlock *Parent = Phi->getParent();
228     PHINode *NewPhi = PHINode::Create(Int64, 0, "loop.phi", &Parent->front());
229     Value *Ret = NewPhi;
230 
231     // Handle all non-constant incoming values first
232     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
233       Value *Incoming = Phi->getIncomingValue(i);
234       BasicBlock *From = Phi->getIncomingBlock(i);
235       if (isa<ConstantInt>(Incoming)) {
236         NewPhi->addIncoming(Broken, From);
237         continue;
238       }
239 
240       Phi->setIncomingValue(i, BoolFalse);
241       Value *PhiArg = handleLoopCondition(Incoming, Broken, L,
242                                           Term, LoopPhiConditions);
243       NewPhi->addIncoming(PhiArg, From);
244     }
245 
246     BasicBlock *IDom = DT->getNode(Parent)->getIDom()->getBlock();
247 
248     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
249       Value *Incoming = Phi->getIncomingValue(i);
250       if (Incoming != BoolTrue)
251         continue;
252 
253       BasicBlock *From = Phi->getIncomingBlock(i);
254       if (From == IDom) {
255         // We're in the following situation:
256         //   IDom/From
257         //      |   \
258         //      |   If-block
259         //      |   /
260         //     Parent
261         // where we want to break out of the loop if the If-block is not taken.
262         // Due to the depth-first traversal, there should be an end.cf
263         // intrinsic in Parent, and we insert an else.break before it.
264         //
265         // Note that the end.cf need not be the first non-phi instruction
266         // of parent, particularly when we're dealing with a multi-level
267         // break, but it should occur within a group of intrinsic calls
268         // at the beginning of the block.
269         CallInst *OldEnd = dyn_cast<CallInst>(Parent->getFirstInsertionPt());
270         while (OldEnd && OldEnd->getCalledFunction() != EndCf)
271           OldEnd = dyn_cast<CallInst>(OldEnd->getNextNode());
272         if (OldEnd && OldEnd->getCalledFunction() == EndCf) {
273           Value *Args[] = { OldEnd->getArgOperand(0), NewPhi };
274           Ret = CallInst::Create(ElseBreak, Args, "", OldEnd);
275           continue;
276         }
277       }
278 
279       TerminatorInst *Insert = From->getTerminator();
280       Value *PhiArg = CallInst::Create(Break, Broken, "", Insert);
281       NewPhi->setIncomingValue(i, PhiArg);
282     }
283 
284     LoopPhiConditions.push_back(WeakVH(Phi));
285     return Ret;
286   }
287 
288   if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
289     BasicBlock *Parent = Inst->getParent();
290     Instruction *Insert;
291     if (L->contains(Inst)) {
292       Insert = Parent->getTerminator();
293     } else {
294       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
295     }
296 
297     Value *Args[] = { Cond, Broken };
298     return CallInst::Create(IfBreak, Args, "", Insert);
299   }
300 
301   // Insert IfBreak in the loop header TERM for constant COND other than true.
302   if (isa<Constant>(Cond)) {
303     Instruction *Insert = Cond == BoolTrue ?
304       Term : L->getHeader()->getTerminator();
305 
306     Value *Args[] = { Cond, Broken };
307     return CallInst::Create(IfBreak, Args, "", Insert);
308   }
309 
310   llvm_unreachable("Unhandled loop condition!");
311 }
312 
313 /// \brief Handle a back edge (loop)
314 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
315   if (isUniform(Term))
316     return;
317 
318   BasicBlock *BB = Term->getParent();
319   llvm::Loop *L = LI->getLoopFor(BB);
320   if (!L)
321     return;
322 
323   BasicBlock *Target = Term->getSuccessor(1);
324   PHINode *Broken = PHINode::Create(Int64, 0, "phi.broken", &Target->front());
325 
326   SmallVector<WeakVH, 8> LoopPhiConditions;
327   Value *Cond = Term->getCondition();
328   Term->setCondition(BoolTrue);
329   Value *Arg = handleLoopCondition(Cond, Broken, L, Term, LoopPhiConditions);
330 
331   for (BasicBlock *Pred : predecessors(Target))
332     Broken->addIncoming(Pred == BB ? Arg : Int64Zero, Pred);
333 
334   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
335 
336   for (WeakVH Val : reverse(LoopPhiConditions)) {
337     if (PHINode *Cond = cast_or_null<PHINode>(Val))
338       eraseIfUnused(Cond);
339   }
340 
341   push(Term->getSuccessor(0), Arg);
342 }
343 
344 /// \brief Close the last opened control flow
345 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
346   llvm::Loop *L = LI->getLoopFor(BB);
347 
348   assert(Stack.back().first == BB);
349 
350   if (L && L->getHeader() == BB) {
351     // We can't insert an EndCF call into a loop header, because it will
352     // get executed on every iteration of the loop, when it should be
353     // executed only once before the loop.
354     SmallVector <BasicBlock *, 8> Latches;
355     L->getLoopLatches(Latches);
356 
357     SmallVector<BasicBlock *, 2> Preds;
358     for (BasicBlock *Pred : predecessors(BB)) {
359       if (!is_contained(Latches, Pred))
360         Preds.push_back(Pred);
361     }
362 
363     BB = llvm::SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, false);
364   }
365 
366   Value *Exec = popSaved();
367   Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
368   if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt))
369     CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
370 }
371 
372 /// \brief Annotate the control flow with intrinsics so the backend can
373 /// recognize if/then/else and loops.
374 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
375   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
376   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
377   DA = &getAnalysis<DivergenceAnalysis>();
378 
379   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
380        E = df_end(&F.getEntryBlock()); I != E; ++I) {
381     BasicBlock *BB = *I;
382     BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
383 
384     if (!Term || Term->isUnconditional()) {
385       if (isTopOfStack(BB))
386         closeControlFlow(BB);
387 
388       continue;
389     }
390 
391     if (I.nodeVisited(Term->getSuccessor(1))) {
392       if (isTopOfStack(BB))
393         closeControlFlow(BB);
394 
395       handleLoop(Term);
396       continue;
397     }
398 
399     if (isTopOfStack(BB)) {
400       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
401       if (Phi && Phi->getParent() == BB && isElse(Phi)) {
402         insertElse(Term);
403         eraseIfUnused(Phi);
404         continue;
405       }
406 
407       closeControlFlow(BB);
408     }
409 
410     openIf(Term);
411   }
412 
413   assert(Stack.empty());
414   return true;
415 }
416 
417 /// \brief Create the annotation pass
418 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
419   return new SIAnnotateControlFlow();
420 }
421