1 //===- SIAnnotateControlFlow.cpp ------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 /// \file
10 /// Annotates the control flow with hardware specific intrinsics.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "AMDGPU.h"
15 #include "llvm/ADT/DepthFirstIterator.h"
16 #include "llvm/ADT/STLExtras.h"
17 #include "llvm/ADT/SmallVector.h"
18 #include "llvm/Analysis/LegacyDivergenceAnalysis.h"
19 #include "llvm/Analysis/LoopInfo.h"
20 #include "llvm/Transforms/Utils/Local.h"
21 #include "llvm/IR/BasicBlock.h"
22 #include "llvm/IR/CFG.h"
23 #include "llvm/IR/Constant.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DerivedTypes.h"
26 #include "llvm/IR/Dominators.h"
27 #include "llvm/IR/Function.h"
28 #include "llvm/IR/Instruction.h"
29 #include "llvm/IR/Instructions.h"
30 #include "llvm/IR/Intrinsics.h"
31 #include "llvm/IR/Module.h"
32 #include "llvm/IR/Type.h"
33 #include "llvm/IR/ValueHandle.h"
34 #include "llvm/Pass.h"
35 #include "llvm/Support/Casting.h"
36 #include "llvm/Support/Debug.h"
37 #include "llvm/Support/ErrorHandling.h"
38 #include "llvm/Support/raw_ostream.h"
39 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
40 #include <cassert>
41 #include <utility>
42 
43 using namespace llvm;
44 
45 #define DEBUG_TYPE "si-annotate-control-flow"
46 
47 namespace {
48 
49 // Complex types used in this pass
50 using StackEntry = std::pair<BasicBlock *, Value *>;
51 using StackVector = SmallVector<StackEntry, 16>;
52 
53 class SIAnnotateControlFlow : public FunctionPass {
54   LegacyDivergenceAnalysis *DA;
55 
56   Type *Boolean;
57   Type *Void;
58   Type *Int64;
59   Type *ReturnStruct;
60 
61   ConstantInt *BoolTrue;
62   ConstantInt *BoolFalse;
63   UndefValue *BoolUndef;
64   Constant *Int64Zero;
65 
66   Function *If;
67   Function *Else;
68   Function *IfBreak;
69   Function *Loop;
70   Function *EndCf;
71 
72   DominatorTree *DT;
73   StackVector Stack;
74 
75   LoopInfo *LI;
76 
77   bool isUniform(BranchInst *T);
78 
79   bool isTopOfStack(BasicBlock *BB);
80 
81   Value *popSaved();
82 
83   void push(BasicBlock *BB, Value *Saved);
84 
85   bool isElse(PHINode *Phi);
86 
87   void eraseIfUnused(PHINode *Phi);
88 
89   void openIf(BranchInst *Term);
90 
91   void insertElse(BranchInst *Term);
92 
93   Value *
94   handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
95                       BranchInst *Term);
96 
97   void handleLoop(BranchInst *Term);
98 
99   void closeControlFlow(BasicBlock *BB);
100 
101 public:
102   static char ID;
103 
104   SIAnnotateControlFlow() : FunctionPass(ID) {}
105 
106   bool doInitialization(Module &M) override;
107 
108   bool runOnFunction(Function &F) override;
109 
110   StringRef getPassName() const override { return "SI annotate control flow"; }
111 
112   void getAnalysisUsage(AnalysisUsage &AU) const override {
113     AU.addRequired<LoopInfoWrapperPass>();
114     AU.addRequired<DominatorTreeWrapperPass>();
115     AU.addRequired<LegacyDivergenceAnalysis>();
116     AU.addPreserved<DominatorTreeWrapperPass>();
117     FunctionPass::getAnalysisUsage(AU);
118   }
119 };
120 
121 } // end anonymous namespace
122 
123 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
124                       "Annotate SI Control Flow", false, false)
125 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
126 INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis)
127 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
128                     "Annotate SI Control Flow", false, false)
129 
130 char SIAnnotateControlFlow::ID = 0;
131 
132 /// Initialize all the types and constants used in the pass
133 bool SIAnnotateControlFlow::doInitialization(Module &M) {
134   LLVMContext &Context = M.getContext();
135 
136   Void = Type::getVoidTy(Context);
137   Boolean = Type::getInt1Ty(Context);
138   Int64 = Type::getInt64Ty(Context);
139   ReturnStruct = StructType::get(Boolean, Int64);
140 
141   BoolTrue = ConstantInt::getTrue(Context);
142   BoolFalse = ConstantInt::getFalse(Context);
143   BoolUndef = UndefValue::get(Boolean);
144   Int64Zero = ConstantInt::get(Int64, 0);
145 
146   If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if);
147   Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else);
148   IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break);
149   Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop);
150   EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf);
151   return false;
152 }
153 
154 /// Is the branch condition uniform or did the StructurizeCFG pass
155 /// consider it as such?
156 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
157   return DA->isUniform(T) ||
158          T->getMetadata("structurizecfg.uniform") != nullptr;
159 }
160 
161 /// Is BB the last block saved on the stack ?
162 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
163   return !Stack.empty() && Stack.back().first == BB;
164 }
165 
166 /// Pop the last saved value from the control flow stack
167 Value *SIAnnotateControlFlow::popSaved() {
168   return Stack.pop_back_val().second;
169 }
170 
171 /// Push a BB and saved value to the control flow stack
172 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
173   Stack.push_back(std::make_pair(BB, Saved));
174 }
175 
176 /// Can the condition represented by this PHI node treated like
177 /// an "Else" block?
178 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
179   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
180   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
181     if (Phi->getIncomingBlock(i) == IDom) {
182 
183       if (Phi->getIncomingValue(i) != BoolTrue)
184         return false;
185 
186     } else {
187       if (Phi->getIncomingValue(i) != BoolFalse)
188         return false;
189 
190     }
191   }
192   return true;
193 }
194 
195 // Erase "Phi" if it is not used any more
196 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
197   if (RecursivelyDeleteDeadPHINode(Phi)) {
198     LLVM_DEBUG(dbgs() << "Erased unused condition phi\n");
199   }
200 }
201 
202 /// Open a new "If" block
203 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
204   if (isUniform(Term))
205     return;
206 
207   Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
208   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
209   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
210 }
211 
212 /// Close the last "If" block and open a new "Else" block
213 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
214   if (isUniform(Term)) {
215     return;
216   }
217   Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
218   Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
219   push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
220 }
221 
222 /// Recursively handle the condition leading to a loop
223 Value *SIAnnotateControlFlow::handleLoopCondition(
224     Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term) {
225   if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
226     BasicBlock *Parent = Inst->getParent();
227     Instruction *Insert;
228     if (L->contains(Inst)) {
229       Insert = Parent->getTerminator();
230     } else {
231       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
232     }
233 
234     Value *Args[] = { Cond, Broken };
235     return CallInst::Create(IfBreak, Args, "", Insert);
236   }
237 
238   // Insert IfBreak in the loop header TERM for constant COND other than true.
239   if (isa<Constant>(Cond)) {
240     Instruction *Insert = Cond == BoolTrue ?
241       Term : L->getHeader()->getTerminator();
242 
243     Value *Args[] = { Cond, Broken };
244     return CallInst::Create(IfBreak, Args, "", Insert);
245   }
246 
247   llvm_unreachable("Unhandled loop condition!");
248 }
249 
250 /// Handle a back edge (loop)
251 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
252   if (isUniform(Term))
253     return;
254 
255   BasicBlock *BB = Term->getParent();
256   llvm::Loop *L = LI->getLoopFor(BB);
257   if (!L)
258     return;
259 
260   BasicBlock *Target = Term->getSuccessor(1);
261   PHINode *Broken = PHINode::Create(Int64, 0, "phi.broken", &Target->front());
262 
263   Value *Cond = Term->getCondition();
264   Term->setCondition(BoolTrue);
265   Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
266 
267   for (BasicBlock *Pred : predecessors(Target))
268     Broken->addIncoming(Pred == BB ? Arg : Int64Zero, Pred);
269 
270   Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
271 
272   push(Term->getSuccessor(0), Arg);
273 }
274 
275 /// Close the last opened control flow
276 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
277   llvm::Loop *L = LI->getLoopFor(BB);
278 
279   assert(Stack.back().first == BB);
280 
281   if (L && L->getHeader() == BB) {
282     // We can't insert an EndCF call into a loop header, because it will
283     // get executed on every iteration of the loop, when it should be
284     // executed only once before the loop.
285     SmallVector <BasicBlock *, 8> Latches;
286     L->getLoopLatches(Latches);
287 
288     SmallVector<BasicBlock *, 2> Preds;
289     for (BasicBlock *Pred : predecessors(BB)) {
290       if (!is_contained(Latches, Pred))
291         Preds.push_back(Pred);
292     }
293 
294     BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
295                                 false);
296   }
297 
298   Value *Exec = popSaved();
299   Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
300   if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt))
301     CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
302 }
303 
304 /// Annotate the control flow with intrinsics so the backend can
305 /// recognize if/then/else and loops.
306 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
307   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
308   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
309   DA = &getAnalysis<LegacyDivergenceAnalysis>();
310 
311   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
312        E = df_end(&F.getEntryBlock()); I != E; ++I) {
313     BasicBlock *BB = *I;
314     BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
315 
316     if (!Term || Term->isUnconditional()) {
317       if (isTopOfStack(BB))
318         closeControlFlow(BB);
319 
320       continue;
321     }
322 
323     if (I.nodeVisited(Term->getSuccessor(1))) {
324       if (isTopOfStack(BB))
325         closeControlFlow(BB);
326 
327       handleLoop(Term);
328       continue;
329     }
330 
331     if (isTopOfStack(BB)) {
332       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
333       if (Phi && Phi->getParent() == BB && isElse(Phi)) {
334         insertElse(Term);
335         eraseIfUnused(Phi);
336         continue;
337       }
338 
339       closeControlFlow(BB);
340     }
341 
342     openIf(Term);
343   }
344 
345   if (!Stack.empty()) {
346     // CFG was probably not structured.
347     report_fatal_error("failed to annotate CFG");
348   }
349 
350   return true;
351 }
352 
353 /// Create the annotation pass
354 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
355   return new SIAnnotateControlFlow();
356 }
357