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 *IfBreak;
70   Function *Loop;
71   Function *EndCf;
72 
73   DominatorTree *DT;
74   StackVector Stack;
75 
76   LoopInfo *LI;
77 
78   bool isUniform(BranchInst *T);
79 
80   bool isTopOfStack(BasicBlock *BB);
81 
82   Value *popSaved();
83 
84   void push(BasicBlock *BB, Value *Saved);
85 
86   bool isElse(PHINode *Phi);
87 
88   void eraseIfUnused(PHINode *Phi);
89 
90   void openIf(BranchInst *Term);
91 
92   void insertElse(BranchInst *Term);
93 
94   Value *
95   handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
96                       BranchInst *Term);
97 
98   void handleLoop(BranchInst *Term);
99 
100   void closeControlFlow(BasicBlock *BB);
101 
102 public:
103   static char ID;
104 
SIAnnotateControlFlow()105   SIAnnotateControlFlow() : FunctionPass(ID) {}
106 
107   bool doInitialization(Module &M) override;
108 
109   bool runOnFunction(Function &F) override;
110 
getPassName() const111   StringRef getPassName() const override { return "SI annotate control flow"; }
112 
getAnalysisUsage(AnalysisUsage & AU) const113   void getAnalysisUsage(AnalysisUsage &AU) const override {
114     AU.addRequired<LoopInfoWrapperPass>();
115     AU.addRequired<DominatorTreeWrapperPass>();
116     AU.addRequired<LegacyDivergenceAnalysis>();
117     AU.addPreserved<DominatorTreeWrapperPass>();
118     FunctionPass::getAnalysisUsage(AU);
119   }
120 };
121 
122 } // end anonymous namespace
123 
124 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
125                       "Annotate SI Control Flow", false, false)
126 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
127 INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis)
128 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
129                     "Annotate SI Control Flow", false, false)
130 
131 char SIAnnotateControlFlow::ID = 0;
132 
133 /// Initialize all the types and constants used in the pass
doInitialization(Module & M)134 bool SIAnnotateControlFlow::doInitialization(Module &M) {
135   LLVMContext &Context = M.getContext();
136 
137   Void = Type::getVoidTy(Context);
138   Boolean = Type::getInt1Ty(Context);
139   Int64 = Type::getInt64Ty(Context);
140   ReturnStruct = StructType::get(Boolean, Int64);
141 
142   BoolTrue = ConstantInt::getTrue(Context);
143   BoolFalse = ConstantInt::getFalse(Context);
144   BoolUndef = UndefValue::get(Boolean);
145   Int64Zero = ConstantInt::get(Int64, 0);
146 
147   If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if);
148   Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else);
149   IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break);
150   Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop);
151   EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf);
152   return false;
153 }
154 
155 /// Is the branch condition uniform or did the StructurizeCFG pass
156 /// consider it as such?
isUniform(BranchInst * T)157 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
158   return DA->isUniform(T) ||
159          T->getMetadata("structurizecfg.uniform") != nullptr;
160 }
161 
162 /// Is BB the last block saved on the stack ?
isTopOfStack(BasicBlock * BB)163 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
164   return !Stack.empty() && Stack.back().first == BB;
165 }
166 
167 /// Pop the last saved value from the control flow stack
popSaved()168 Value *SIAnnotateControlFlow::popSaved() {
169   return Stack.pop_back_val().second;
170 }
171 
172 /// Push a BB and saved value to the control flow stack
push(BasicBlock * BB,Value * Saved)173 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
174   Stack.push_back(std::make_pair(BB, Saved));
175 }
176 
177 /// Can the condition represented by this PHI node treated like
178 /// an "Else" block?
isElse(PHINode * Phi)179 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
180   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
181   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
182     if (Phi->getIncomingBlock(i) == IDom) {
183 
184       if (Phi->getIncomingValue(i) != BoolTrue)
185         return false;
186 
187     } else {
188       if (Phi->getIncomingValue(i) != BoolFalse)
189         return false;
190 
191     }
192   }
193   return true;
194 }
195 
196 // Erase "Phi" if it is not used any more
eraseIfUnused(PHINode * Phi)197 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
198   if (RecursivelyDeleteDeadPHINode(Phi)) {
199     LLVM_DEBUG(dbgs() << "Erased unused condition phi\n");
200   }
201 }
202 
203 /// Open a new "If" block
openIf(BranchInst * Term)204 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
205   if (isUniform(Term))
206     return;
207 
208   Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
209   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
210   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
211 }
212 
213 /// Close the last "If" block and open a new "Else" block
insertElse(BranchInst * Term)214 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
215   if (isUniform(Term)) {
216     return;
217   }
218   Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
219   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
220   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
221 }
222 
223 /// Recursively handle the condition leading to a loop
handleLoopCondition(Value * Cond,PHINode * Broken,llvm::Loop * L,BranchInst * Term)224 Value *SIAnnotateControlFlow::handleLoopCondition(
225     Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term) {
226   if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
227     BasicBlock *Parent = Inst->getParent();
228     Instruction *Insert;
229     if (L->contains(Inst)) {
230       Insert = Parent->getTerminator();
231     } else {
232       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
233     }
234 
235     Value *Args[] = { Cond, Broken };
236     return CallInst::Create(IfBreak, Args, "", Insert);
237   }
238 
239   // Insert IfBreak in the loop header TERM for constant COND other than true.
240   if (isa<Constant>(Cond)) {
241     Instruction *Insert = Cond == BoolTrue ?
242       Term : L->getHeader()->getTerminator();
243 
244     Value *Args[] = { Cond, Broken };
245     return CallInst::Create(IfBreak, Args, "", Insert);
246   }
247 
248   llvm_unreachable("Unhandled loop condition!");
249 }
250 
251 /// Handle a back edge (loop)
handleLoop(BranchInst * Term)252 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
253   if (isUniform(Term))
254     return;
255 
256   BasicBlock *BB = Term->getParent();
257   llvm::Loop *L = LI->getLoopFor(BB);
258   if (!L)
259     return;
260 
261   BasicBlock *Target = Term->getSuccessor(1);
262   PHINode *Broken = PHINode::Create(Int64, 0, "phi.broken", &Target->front());
263 
264   Value *Cond = Term->getCondition();
265   Term->setCondition(BoolTrue);
266   Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
267 
268   for (BasicBlock *Pred : predecessors(Target))
269     Broken->addIncoming(Pred == BB ? Arg : Int64Zero, Pred);
270 
271   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
272 
273   push(Term->getSuccessor(0), Arg);
274 }
275 
276 /// Close the last opened control flow
closeControlFlow(BasicBlock * BB)277 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
278   llvm::Loop *L = LI->getLoopFor(BB);
279 
280   assert(Stack.back().first == BB);
281 
282   if (L && L->getHeader() == BB) {
283     // We can't insert an EndCF call into a loop header, because it will
284     // get executed on every iteration of the loop, when it should be
285     // executed only once before the loop.
286     SmallVector <BasicBlock *, 8> Latches;
287     L->getLoopLatches(Latches);
288 
289     SmallVector<BasicBlock *, 2> Preds;
290     for (BasicBlock *Pred : predecessors(BB)) {
291       if (!is_contained(Latches, Pred))
292         Preds.push_back(Pred);
293     }
294 
295     BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
296                                 false);
297   }
298 
299   Value *Exec = popSaved();
300   Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
301   if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt))
302     CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
303 }
304 
305 /// Annotate the control flow with intrinsics so the backend can
306 /// recognize if/then/else and loops.
runOnFunction(Function & F)307 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
308   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
309   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
310   DA = &getAnalysis<LegacyDivergenceAnalysis>();
311 
312   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
313        E = df_end(&F.getEntryBlock()); I != E; ++I) {
314     BasicBlock *BB = *I;
315     BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
316 
317     if (!Term || Term->isUnconditional()) {
318       if (isTopOfStack(BB))
319         closeControlFlow(BB);
320 
321       continue;
322     }
323 
324     if (I.nodeVisited(Term->getSuccessor(1))) {
325       if (isTopOfStack(BB))
326         closeControlFlow(BB);
327 
328       handleLoop(Term);
329       continue;
330     }
331 
332     if (isTopOfStack(BB)) {
333       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
334       if (Phi && Phi->getParent() == BB && isElse(Phi)) {
335         insertElse(Term);
336         eraseIfUnused(Phi);
337         continue;
338       }
339 
340       closeControlFlow(BB);
341     }
342 
343     openIf(Term);
344   }
345 
346   if (!Stack.empty()) {
347     // CFG was probably not structured.
348     report_fatal_error("failed to annotate CFG");
349   }
350 
351   return true;
352 }
353 
354 /// Create the annotation pass
createSIAnnotateControlFlowPass()355 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
356   return new SIAnnotateControlFlow();
357 }
358