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/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/Analysis/LegacyDivergenceAnalysis.h"
20 #include "llvm/Analysis/LoopInfo.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/IR/BasicBlock.h"
23 #include "llvm/IR/CFG.h"
24 #include "llvm/IR/Constant.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/DerivedTypes.h"
27 #include "llvm/IR/Dominators.h"
28 #include "llvm/IR/Function.h"
29 #include "llvm/IR/Instruction.h"
30 #include "llvm/IR/Instructions.h"
31 #include "llvm/IR/Intrinsics.h"
32 #include "llvm/IR/Module.h"
33 #include "llvm/IR/Type.h"
34 #include "llvm/IR/ValueHandle.h"
35 #include "llvm/Pass.h"
36 #include "llvm/Support/Casting.h"
37 #include "llvm/Support/Debug.h"
38 #include "llvm/Support/ErrorHandling.h"
39 #include "llvm/Support/raw_ostream.h"
40 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
41 #include <cassert>
42 #include <utility>
43 
44 using namespace llvm;
45 
46 #define DEBUG_TYPE "si-annotate-control-flow"
47 
48 namespace {
49 
50 // Complex types used in this pass
51 using StackEntry = std::pair<BasicBlock *, Value *>;
52 using StackVector = SmallVector<StackEntry, 16>;
53 
54 class SIAnnotateControlFlow : public FunctionPass {
55   LegacyDivergenceAnalysis *DA;
56 
57   Type *Boolean;
58   Type *Void;
59   Type *Int64;
60   Type *ReturnStruct;
61 
62   ConstantInt *BoolTrue;
63   ConstantInt *BoolFalse;
64   UndefValue *BoolUndef;
65   Constant *Int64Zero;
66 
67   Function *If;
68   Function *Else;
69   Function *Break;
70   Function *IfBreak;
71   Function *ElseBreak;
72   Function *Loop;
73   Function *EndCf;
74 
75   DominatorTree *DT;
76   StackVector Stack;
77 
78   LoopInfo *LI;
79 
80   bool isUniform(BranchInst *T);
81 
82   bool isTopOfStack(BasicBlock *BB);
83 
84   Value *popSaved();
85 
86   void push(BasicBlock *BB, Value *Saved);
87 
88   bool isElse(PHINode *Phi);
89 
90   void eraseIfUnused(PHINode *Phi);
91 
92   void openIf(BranchInst *Term);
93 
94   void insertElse(BranchInst *Term);
95 
96   Value *
97   handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
98                       BranchInst *Term,
99                       SmallVectorImpl<WeakTrackingVH> &LoopPhiConditions);
100 
101   void handleLoop(BranchInst *Term);
102 
103   void closeControlFlow(BasicBlock *BB);
104 
105 public:
106   static char ID;
107 
108   SIAnnotateControlFlow() : FunctionPass(ID) {}
109 
110   bool doInitialization(Module &M) override;
111 
112   bool runOnFunction(Function &F) override;
113 
114   StringRef getPassName() const override { return "SI annotate control flow"; }
115 
116   void getAnalysisUsage(AnalysisUsage &AU) const override {
117     AU.addRequired<LoopInfoWrapperPass>();
118     AU.addRequired<DominatorTreeWrapperPass>();
119     AU.addRequired<LegacyDivergenceAnalysis>();
120     AU.addPreserved<DominatorTreeWrapperPass>();
121     FunctionPass::getAnalysisUsage(AU);
122   }
123 };
124 
125 } // end anonymous namespace
126 
127 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
128                       "Annotate SI Control Flow", false, false)
129 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
130 INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis)
131 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
132                     "Annotate SI Control Flow", false, false)
133 
134 char SIAnnotateControlFlow::ID = 0;
135 
136 /// Initialize all the types and constants used in the pass
137 bool SIAnnotateControlFlow::doInitialization(Module &M) {
138   LLVMContext &Context = M.getContext();
139 
140   Void = Type::getVoidTy(Context);
141   Boolean = Type::getInt1Ty(Context);
142   Int64 = Type::getInt64Ty(Context);
143   ReturnStruct = StructType::get(Boolean, Int64);
144 
145   BoolTrue = ConstantInt::getTrue(Context);
146   BoolFalse = ConstantInt::getFalse(Context);
147   BoolUndef = UndefValue::get(Boolean);
148   Int64Zero = ConstantInt::get(Int64, 0);
149 
150   If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if);
151   Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else);
152   Break = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_break);
153   IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break);
154   ElseBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else_break);
155   Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop);
156   EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf);
157   return false;
158 }
159 
160 /// Is the branch condition uniform or did the StructurizeCFG pass
161 /// consider it as such?
162 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
163   return DA->isUniform(T->getCondition()) ||
164          T->getMetadata("structurizecfg.uniform") != nullptr;
165 }
166 
167 /// Is BB the last block saved on the stack ?
168 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
169   return !Stack.empty() && Stack.back().first == BB;
170 }
171 
172 /// Pop the last saved value from the control flow stack
173 Value *SIAnnotateControlFlow::popSaved() {
174   return Stack.pop_back_val().second;
175 }
176 
177 /// Push a BB and saved value to the control flow stack
178 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
179   Stack.push_back(std::make_pair(BB, Saved));
180 }
181 
182 /// Can the condition represented by this PHI node treated like
183 /// an "Else" block?
184 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
185   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
186   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
187     if (Phi->getIncomingBlock(i) == IDom) {
188 
189       if (Phi->getIncomingValue(i) != BoolTrue)
190         return false;
191 
192     } else {
193       if (Phi->getIncomingValue(i) != BoolFalse)
194         return false;
195 
196     }
197   }
198   return true;
199 }
200 
201 // Erase "Phi" if it is not used any more
202 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
203   if (RecursivelyDeleteDeadPHINode(Phi)) {
204     LLVM_DEBUG(dbgs() << "Erased unused condition phi\n");
205   }
206 }
207 
208 /// Open a new "If" block
209 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
210   if (isUniform(Term))
211     return;
212 
213   Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
214   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
215   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
216 }
217 
218 /// Close the last "If" block and open a new "Else" block
219 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
220   if (isUniform(Term)) {
221     return;
222   }
223   Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
224   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
225   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
226 }
227 
228 /// Recursively handle the condition leading to a loop
229 Value *SIAnnotateControlFlow::handleLoopCondition(
230     Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term,
231     SmallVectorImpl<WeakTrackingVH> &LoopPhiConditions) {
232   // Only search through PHI nodes which are inside the loop.  If we try this
233   // with PHI nodes that are outside of the loop, we end up inserting new PHI
234   // nodes outside of the loop which depend on values defined inside the loop.
235   // This will break the module with
236   // 'Instruction does not dominate all users!' errors.
237   PHINode *Phi = nullptr;
238   if ((Phi = dyn_cast<PHINode>(Cond)) && L->contains(Phi)) {
239     BasicBlock *Parent = Phi->getParent();
240     PHINode *NewPhi = PHINode::Create(Int64, 0, "loop.phi", &Parent->front());
241     Value *Ret = NewPhi;
242 
243     // Handle all non-constant incoming values first
244     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
245       Value *Incoming = Phi->getIncomingValue(i);
246       BasicBlock *From = Phi->getIncomingBlock(i);
247       if (isa<ConstantInt>(Incoming)) {
248         NewPhi->addIncoming(Broken, From);
249         continue;
250       }
251 
252       Phi->setIncomingValue(i, BoolFalse);
253       Value *PhiArg = handleLoopCondition(Incoming, Broken, L,
254                                           Term, LoopPhiConditions);
255       NewPhi->addIncoming(PhiArg, From);
256     }
257 
258     BasicBlock *IDom = DT->getNode(Parent)->getIDom()->getBlock();
259 
260     for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
261       Value *Incoming = Phi->getIncomingValue(i);
262       if (Incoming != BoolTrue)
263         continue;
264 
265       BasicBlock *From = Phi->getIncomingBlock(i);
266       if (From == IDom) {
267         // We're in the following situation:
268         //   IDom/From
269         //      |   \
270         //      |   If-block
271         //      |   /
272         //     Parent
273         // where we want to break out of the loop if the If-block is not taken.
274         // Due to the depth-first traversal, there should be an end.cf
275         // intrinsic in Parent, and we insert an else.break before it.
276         //
277         // Note that the end.cf need not be the first non-phi instruction
278         // of parent, particularly when we're dealing with a multi-level
279         // break, but it should occur within a group of intrinsic calls
280         // at the beginning of the block.
281         CallInst *OldEnd = dyn_cast<CallInst>(Parent->getFirstInsertionPt());
282         while (OldEnd && OldEnd->getCalledFunction() != EndCf)
283           OldEnd = dyn_cast<CallInst>(OldEnd->getNextNode());
284         if (OldEnd && OldEnd->getCalledFunction() == EndCf) {
285           Value *Args[] = { OldEnd->getArgOperand(0), NewPhi };
286           Ret = CallInst::Create(ElseBreak, Args, "", OldEnd);
287           continue;
288         }
289       }
290 
291       TerminatorInst *Insert = From->getTerminator();
292       Value *PhiArg = CallInst::Create(Break, Broken, "", Insert);
293       NewPhi->setIncomingValue(i, PhiArg);
294     }
295 
296     LoopPhiConditions.push_back(WeakTrackingVH(Phi));
297     return Ret;
298   }
299 
300   if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
301     BasicBlock *Parent = Inst->getParent();
302     Instruction *Insert;
303     if (L->contains(Inst)) {
304       Insert = Parent->getTerminator();
305     } else {
306       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
307     }
308 
309     Value *Args[] = { Cond, Broken };
310     return CallInst::Create(IfBreak, Args, "", Insert);
311   }
312 
313   // Insert IfBreak in the loop header TERM for constant COND other than true.
314   if (isa<Constant>(Cond)) {
315     Instruction *Insert = Cond == BoolTrue ?
316       Term : L->getHeader()->getTerminator();
317 
318     Value *Args[] = { Cond, Broken };
319     return CallInst::Create(IfBreak, Args, "", Insert);
320   }
321 
322   llvm_unreachable("Unhandled loop condition!");
323 }
324 
325 /// Handle a back edge (loop)
326 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
327   if (isUniform(Term))
328     return;
329 
330   BasicBlock *BB = Term->getParent();
331   llvm::Loop *L = LI->getLoopFor(BB);
332   if (!L)
333     return;
334 
335   BasicBlock *Target = Term->getSuccessor(1);
336   PHINode *Broken = PHINode::Create(Int64, 0, "phi.broken", &Target->front());
337 
338   SmallVector<WeakTrackingVH, 8> LoopPhiConditions;
339   Value *Cond = Term->getCondition();
340   Term->setCondition(BoolTrue);
341   Value *Arg = handleLoopCondition(Cond, Broken, L, Term, LoopPhiConditions);
342 
343   for (BasicBlock *Pred : predecessors(Target))
344     Broken->addIncoming(Pred == BB ? Arg : Int64Zero, Pred);
345 
346   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
347 
348   for (WeakTrackingVH Val : llvm::reverse(LoopPhiConditions)) {
349     if (PHINode *Cond = cast_or_null<PHINode>(Val))
350       eraseIfUnused(Cond);
351   }
352 
353   push(Term->getSuccessor(0), Arg);
354 }
355 
356 /// Close the last opened control flow
357 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
358   llvm::Loop *L = LI->getLoopFor(BB);
359 
360   assert(Stack.back().first == BB);
361 
362   if (L && L->getHeader() == BB) {
363     // We can't insert an EndCF call into a loop header, because it will
364     // get executed on every iteration of the loop, when it should be
365     // executed only once before the loop.
366     SmallVector <BasicBlock *, 8> Latches;
367     L->getLoopLatches(Latches);
368 
369     SmallVector<BasicBlock *, 2> Preds;
370     for (BasicBlock *Pred : predecessors(BB)) {
371       if (!is_contained(Latches, Pred))
372         Preds.push_back(Pred);
373     }
374 
375     BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
376                                 false);
377   }
378 
379   Value *Exec = popSaved();
380   Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
381   if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt))
382     CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
383 }
384 
385 /// Annotate the control flow with intrinsics so the backend can
386 /// recognize if/then/else and loops.
387 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
388   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
389   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
390   DA = &getAnalysis<LegacyDivergenceAnalysis>();
391 
392   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
393        E = df_end(&F.getEntryBlock()); I != E; ++I) {
394     BasicBlock *BB = *I;
395     BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
396 
397     if (!Term || Term->isUnconditional()) {
398       if (isTopOfStack(BB))
399         closeControlFlow(BB);
400 
401       continue;
402     }
403 
404     if (I.nodeVisited(Term->getSuccessor(1))) {
405       if (isTopOfStack(BB))
406         closeControlFlow(BB);
407 
408       handleLoop(Term);
409       continue;
410     }
411 
412     if (isTopOfStack(BB)) {
413       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
414       if (Phi && Phi->getParent() == BB && isElse(Phi)) {
415         insertElse(Term);
416         eraseIfUnused(Phi);
417         continue;
418       }
419 
420       closeControlFlow(BB);
421     }
422 
423     openIf(Term);
424   }
425 
426   if (!Stack.empty()) {
427     // CFG was probably not structured.
428     report_fatal_error("failed to annotate CFG");
429   }
430 
431   return true;
432 }
433 
434 /// Create the annotation pass
435 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
436   return new SIAnnotateControlFlow();
437 }
438