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