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 "GCNSubtarget.h"
16 #include "llvm/Analysis/LoopInfo.h"
17 #include "llvm/Analysis/UniformityAnalysis.h"
18 #include "llvm/CodeGen/TargetPassConfig.h"
19 #include "llvm/IR/BasicBlock.h"
20 #include "llvm/IR/Constants.h"
21 #include "llvm/IR/Dominators.h"
22 #include "llvm/IR/IRBuilder.h"
23 #include "llvm/IR/IntrinsicsAMDGPU.h"
24 #include "llvm/InitializePasses.h"
25 #include "llvm/Target/TargetMachine.h"
26 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
27 #include "llvm/Transforms/Utils/Local.h"
28 
29 using namespace llvm;
30 
31 #define DEBUG_TYPE "si-annotate-control-flow"
32 
33 namespace {
34 
35 // Complex types used in this pass
36 using StackEntry = std::pair<BasicBlock *, Value *>;
37 using StackVector = SmallVector<StackEntry, 16>;
38 
39 class SIAnnotateControlFlow : public FunctionPass {
40   UniformityInfo *UA;
41 
42   Type *Boolean;
43   Type *Void;
44   Type *IntMask;
45   Type *ReturnStruct;
46 
47   ConstantInt *BoolTrue;
48   ConstantInt *BoolFalse;
49   UndefValue *BoolUndef;
50   Constant *IntMaskZero;
51 
52   Function *If;
53   Function *Else;
54   Function *IfBreak;
55   Function *Loop;
56   Function *EndCf;
57 
58   DominatorTree *DT;
59   StackVector Stack;
60 
61   LoopInfo *LI;
62 
63   void initialize(Module &M, const GCNSubtarget &ST);
64 
65   bool isUniform(BranchInst *T);
66 
67   bool isTopOfStack(BasicBlock *BB);
68 
69   Value *popSaved();
70 
71   void push(BasicBlock *BB, Value *Saved);
72 
73   bool isElse(PHINode *Phi);
74 
75   bool hasKill(const BasicBlock *BB);
76 
77   bool eraseIfUnused(PHINode *Phi);
78 
79   bool openIf(BranchInst *Term);
80 
81   bool insertElse(BranchInst *Term);
82 
83   Value *
84   handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
85                       BranchInst *Term);
86 
87   bool handleLoop(BranchInst *Term);
88 
89   bool closeControlFlow(BasicBlock *BB);
90 
91 public:
92   static char ID;
93 
SIAnnotateControlFlow()94   SIAnnotateControlFlow() : FunctionPass(ID) {}
95 
96   bool runOnFunction(Function &F) override;
97 
getPassName() const98   StringRef getPassName() const override { return "SI annotate control flow"; }
99 
getAnalysisUsage(AnalysisUsage & AU) const100   void getAnalysisUsage(AnalysisUsage &AU) const override {
101     AU.addRequired<LoopInfoWrapperPass>();
102     AU.addRequired<DominatorTreeWrapperPass>();
103     AU.addRequired<UniformityInfoWrapperPass>();
104     AU.addPreserved<LoopInfoWrapperPass>();
105     AU.addPreserved<DominatorTreeWrapperPass>();
106     AU.addRequired<TargetPassConfig>();
107     FunctionPass::getAnalysisUsage(AU);
108   }
109 };
110 
111 } // end anonymous namespace
112 
113 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
114                       "Annotate SI Control Flow", false, false)
115 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
116 INITIALIZE_PASS_DEPENDENCY(UniformityInfoWrapperPass)
117 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
118 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
119                     "Annotate SI Control Flow", false, false)
120 
121 char SIAnnotateControlFlow::ID = 0;
122 
123 /// Initialize all the types and constants used in the pass
initialize(Module & M,const GCNSubtarget & ST)124 void SIAnnotateControlFlow::initialize(Module &M, const GCNSubtarget &ST) {
125   LLVMContext &Context = M.getContext();
126 
127   Void = Type::getVoidTy(Context);
128   Boolean = Type::getInt1Ty(Context);
129   IntMask = ST.isWave32() ? Type::getInt32Ty(Context)
130                            : Type::getInt64Ty(Context);
131   ReturnStruct = StructType::get(Boolean, IntMask);
132 
133   BoolTrue = ConstantInt::getTrue(Context);
134   BoolFalse = ConstantInt::getFalse(Context);
135   BoolUndef = PoisonValue::get(Boolean);
136   IntMaskZero = ConstantInt::get(IntMask, 0);
137 
138   If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if, { IntMask });
139   Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else,
140                                    { IntMask, IntMask });
141   IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break,
142                                       { IntMask });
143   Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop, { IntMask });
144   EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf, { IntMask });
145 }
146 
147 /// Is the branch condition uniform or did the StructurizeCFG pass
148 /// consider it as such?
isUniform(BranchInst * T)149 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
150   return UA->isUniform(T) ||
151          T->getMetadata("structurizecfg.uniform") != nullptr;
152 }
153 
154 /// Is BB the last block saved on the stack ?
isTopOfStack(BasicBlock * BB)155 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
156   return !Stack.empty() && Stack.back().first == BB;
157 }
158 
159 /// Pop the last saved value from the control flow stack
popSaved()160 Value *SIAnnotateControlFlow::popSaved() {
161   return Stack.pop_back_val().second;
162 }
163 
164 /// Push a BB and saved value to the control flow stack
push(BasicBlock * BB,Value * Saved)165 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
166   Stack.push_back(std::pair(BB, Saved));
167 }
168 
169 /// Can the condition represented by this PHI node treated like
170 /// an "Else" block?
isElse(PHINode * Phi)171 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
172   BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
173   for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
174     if (Phi->getIncomingBlock(i) == IDom) {
175 
176       if (Phi->getIncomingValue(i) != BoolTrue)
177         return false;
178 
179     } else {
180       if (Phi->getIncomingValue(i) != BoolFalse)
181         return false;
182 
183     }
184   }
185   return true;
186 }
187 
hasKill(const BasicBlock * BB)188 bool SIAnnotateControlFlow::hasKill(const BasicBlock *BB) {
189   for (const Instruction &I : *BB) {
190     if (const CallInst *CI = dyn_cast<CallInst>(&I))
191       if (CI->getIntrinsicID() == Intrinsic::amdgcn_kill)
192         return true;
193   }
194   return false;
195 }
196 
197 // Erase "Phi" if it is not used any more. Return true if any change was made.
eraseIfUnused(PHINode * Phi)198 bool SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
199   bool Changed = RecursivelyDeleteDeadPHINode(Phi);
200   if (Changed)
201     LLVM_DEBUG(dbgs() << "Erased unused condition phi\n");
202   return Changed;
203 }
204 
205 /// Open a new "If" block
openIf(BranchInst * Term)206 bool SIAnnotateControlFlow::openIf(BranchInst *Term) {
207   if (isUniform(Term))
208     return false;
209 
210   IRBuilder<> IRB(Term);
211   Value *IfCall = IRB.CreateCall(If, {Term->getCondition()});
212   Value *Cond = IRB.CreateExtractValue(IfCall, {0});
213   Value *Mask = IRB.CreateExtractValue(IfCall, {1});
214   Term->setCondition(Cond);
215   push(Term->getSuccessor(1), Mask);
216   return true;
217 }
218 
219 /// Close the last "If" block and open a new "Else" block
insertElse(BranchInst * Term)220 bool SIAnnotateControlFlow::insertElse(BranchInst *Term) {
221   if (isUniform(Term)) {
222     return false;
223   }
224 
225   IRBuilder<> IRB(Term);
226   Value *ElseCall = IRB.CreateCall(Else, {popSaved()});
227   Value *Cond = IRB.CreateExtractValue(ElseCall, {0});
228   Value *Mask = IRB.CreateExtractValue(ElseCall, {1});
229   Term->setCondition(Cond);
230   push(Term->getSuccessor(1), Mask);
231   return true;
232 }
233 
234 /// Recursively handle the condition leading to a loop
handleLoopCondition(Value * Cond,PHINode * Broken,llvm::Loop * L,BranchInst * Term)235 Value *SIAnnotateControlFlow::handleLoopCondition(
236     Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term) {
237 
238   auto CreateBreak = [this, Cond, Broken](Instruction *I) -> CallInst * {
239     return IRBuilder<>(I).CreateCall(IfBreak, {Cond, Broken});
240   };
241 
242   if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
243     BasicBlock *Parent = Inst->getParent();
244     Instruction *Insert;
245     if (L->contains(Inst)) {
246       Insert = Parent->getTerminator();
247     } else {
248       Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
249     }
250 
251     return CreateBreak(Insert);
252   }
253 
254   // Insert IfBreak in the loop header TERM for constant COND other than true.
255   if (isa<Constant>(Cond)) {
256     Instruction *Insert = Cond == BoolTrue ?
257       Term : L->getHeader()->getTerminator();
258 
259     return CreateBreak(Insert);
260   }
261 
262   if (isa<Argument>(Cond)) {
263     Instruction *Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
264     return CreateBreak(Insert);
265   }
266 
267   llvm_unreachable("Unhandled loop condition!");
268 }
269 
270 /// Handle a back edge (loop)
handleLoop(BranchInst * Term)271 bool SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
272   if (isUniform(Term))
273     return false;
274 
275   BasicBlock *BB = Term->getParent();
276   llvm::Loop *L = LI->getLoopFor(BB);
277   if (!L)
278     return false;
279 
280   BasicBlock *Target = Term->getSuccessor(1);
281   PHINode *Broken = PHINode::Create(IntMask, 0, "phi.broken");
282   Broken->insertBefore(Target->begin());
283 
284   Value *Cond = Term->getCondition();
285   Term->setCondition(BoolTrue);
286   Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
287 
288   for (BasicBlock *Pred : predecessors(Target)) {
289     Value *PHIValue = IntMaskZero;
290     if (Pred == BB) // Remember the value of the previous iteration.
291       PHIValue = Arg;
292     // If the backedge from Pred to Target could be executed before the exit
293     // of the loop at BB, it should not reset or change "Broken", which keeps
294     // track of the number of threads exited the loop at BB.
295     else if (L->contains(Pred) && DT->dominates(Pred, BB))
296       PHIValue = Broken;
297     Broken->addIncoming(PHIValue, Pred);
298   }
299 
300   CallInst *LoopCall = IRBuilder<>(Term).CreateCall(Loop, {Arg});
301   Term->setCondition(LoopCall);
302 
303   push(Term->getSuccessor(0), Arg);
304 
305   return true;
306 }
307 
308 /// Close the last opened control flow
closeControlFlow(BasicBlock * BB)309 bool SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
310   llvm::Loop *L = LI->getLoopFor(BB);
311 
312   assert(Stack.back().first == BB);
313 
314   if (L && L->getHeader() == BB) {
315     // We can't insert an EndCF call into a loop header, because it will
316     // get executed on every iteration of the loop, when it should be
317     // executed only once before the loop.
318     SmallVector <BasicBlock *, 8> Latches;
319     L->getLoopLatches(Latches);
320 
321     SmallVector<BasicBlock *, 2> Preds;
322     for (BasicBlock *Pred : predecessors(BB)) {
323       if (!is_contained(Latches, Pred))
324         Preds.push_back(Pred);
325     }
326 
327     BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
328                                 false);
329   }
330 
331   Value *Exec = popSaved();
332   BasicBlock::iterator FirstInsertionPt = BB->getFirstInsertionPt();
333   if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt)) {
334     Instruction *ExecDef = cast<Instruction>(Exec);
335     BasicBlock *DefBB = ExecDef->getParent();
336     if (!DT->dominates(DefBB, BB)) {
337       // Split edge to make Def dominate Use
338       FirstInsertionPt = SplitEdge(DefBB, BB, DT, LI)->getFirstInsertionPt();
339     }
340     IRBuilder<>(FirstInsertionPt->getParent(), FirstInsertionPt)
341         .CreateCall(EndCf, {Exec});
342   }
343 
344   return true;
345 }
346 
347 /// Annotate the control flow with intrinsics so the backend can
348 /// recognize if/then/else and loops.
runOnFunction(Function & F)349 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
350   DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
351   LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
352   UA = &getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
353   TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
354   const TargetMachine &TM = TPC.getTM<TargetMachine>();
355 
356   bool Changed = false;
357   initialize(*F.getParent(), TM.getSubtarget<GCNSubtarget>(F));
358   for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
359        E = df_end(&F.getEntryBlock()); I != E; ++I) {
360     BasicBlock *BB = *I;
361     BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
362 
363     if (!Term || Term->isUnconditional()) {
364       if (isTopOfStack(BB))
365         Changed |= closeControlFlow(BB);
366 
367       continue;
368     }
369 
370     if (I.nodeVisited(Term->getSuccessor(1))) {
371       if (isTopOfStack(BB))
372         Changed |= closeControlFlow(BB);
373 
374       if (DT->dominates(Term->getSuccessor(1), BB))
375         Changed |= handleLoop(Term);
376       continue;
377     }
378 
379     if (isTopOfStack(BB)) {
380       PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
381       if (Phi && Phi->getParent() == BB && isElse(Phi) && !hasKill(BB)) {
382         Changed |= insertElse(Term);
383         Changed |= eraseIfUnused(Phi);
384         continue;
385       }
386 
387       Changed |= closeControlFlow(BB);
388     }
389 
390     Changed |= openIf(Term);
391   }
392 
393   if (!Stack.empty()) {
394     // CFG was probably not structured.
395     report_fatal_error("failed to annotate CFG");
396   }
397 
398   return Changed;
399 }
400 
401 /// Create the annotation pass
createSIAnnotateControlFlowPass()402 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
403   return new SIAnnotateControlFlow();
404 }
405