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/LegacyDivergenceAnalysis.h"
17#include "llvm/Analysis/LoopInfo.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/IntrinsicsAMDGPU.h"
23#include "llvm/InitializePasses.h"
24#include "llvm/Target/TargetMachine.h"
25#include "llvm/Transforms/Utils/BasicBlockUtils.h"
26#include "llvm/Transforms/Utils/Local.h"
27
28using namespace llvm;
29
30#define DEBUG_TYPE "si-annotate-control-flow"
31
32namespace {
33
34// Complex types used in this pass
35using StackEntry = std::pair<BasicBlock *, Value *>;
36using StackVector = SmallVector<StackEntry, 16>;
37
38class SIAnnotateControlFlow : public FunctionPass {
39  LegacyDivergenceAnalysis *DA;
40
41  Type *Boolean;
42  Type *Void;
43  Type *IntMask;
44  Type *ReturnStruct;
45
46  ConstantInt *BoolTrue;
47  ConstantInt *BoolFalse;
48  UndefValue *BoolUndef;
49  Constant *IntMaskZero;
50
51  Function *If;
52  Function *Else;
53  Function *IfBreak;
54  Function *Loop;
55  Function *EndCf;
56
57  DominatorTree *DT;
58  StackVector Stack;
59
60  LoopInfo *LI;
61
62  void initialize(Module &M, const GCNSubtarget &ST);
63
64  bool isUniform(BranchInst *T);
65
66  bool isTopOfStack(BasicBlock *BB);
67
68  Value *popSaved();
69
70  void push(BasicBlock *BB, Value *Saved);
71
72  bool isElse(PHINode *Phi);
73
74  bool hasKill(const BasicBlock *BB);
75
76  bool eraseIfUnused(PHINode *Phi);
77
78  bool openIf(BranchInst *Term);
79
80  bool insertElse(BranchInst *Term);
81
82  Value *
83  handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
84                      BranchInst *Term);
85
86  bool handleLoop(BranchInst *Term);
87
88  bool closeControlFlow(BasicBlock *BB);
89
90public:
91  static char ID;
92
93  SIAnnotateControlFlow() : FunctionPass(ID) {}
94
95  bool runOnFunction(Function &F) override;
96
97  StringRef getPassName() const override { return "SI annotate control flow"; }
98
99  void getAnalysisUsage(AnalysisUsage &AU) const override {
100    AU.addRequired<LoopInfoWrapperPass>();
101    AU.addRequired<DominatorTreeWrapperPass>();
102    AU.addRequired<LegacyDivergenceAnalysis>();
103    AU.addPreserved<LoopInfoWrapperPass>();
104    AU.addPreserved<DominatorTreeWrapperPass>();
105    AU.addRequired<TargetPassConfig>();
106    FunctionPass::getAnalysisUsage(AU);
107  }
108};
109
110} // end anonymous namespace
111
112INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
113                      "Annotate SI Control Flow", false, false)
114INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
115INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis)
116INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
117INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
118                    "Annotate SI Control Flow", false, false)
119
120char SIAnnotateControlFlow::ID = 0;
121
122/// Initialize all the types and constants used in the pass
123void SIAnnotateControlFlow::initialize(Module &M, const GCNSubtarget &ST) {
124  LLVMContext &Context = M.getContext();
125
126  Void = Type::getVoidTy(Context);
127  Boolean = Type::getInt1Ty(Context);
128  IntMask = ST.isWave32() ? Type::getInt32Ty(Context)
129                           : Type::getInt64Ty(Context);
130  ReturnStruct = StructType::get(Boolean, IntMask);
131
132  BoolTrue = ConstantInt::getTrue(Context);
133  BoolFalse = ConstantInt::getFalse(Context);
134  BoolUndef = PoisonValue::get(Boolean);
135  IntMaskZero = ConstantInt::get(IntMask, 0);
136
137  If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if, { IntMask });
138  Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else,
139                                   { IntMask, IntMask });
140  IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break,
141                                      { IntMask });
142  Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop, { IntMask });
143  EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf, { IntMask });
144}
145
146/// Is the branch condition uniform or did the StructurizeCFG pass
147/// consider it as such?
148bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
149  return DA->isUniform(T) ||
150         T->getMetadata("structurizecfg.uniform") != nullptr;
151}
152
153/// Is BB the last block saved on the stack ?
154bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
155  return !Stack.empty() && Stack.back().first == BB;
156}
157
158/// Pop the last saved value from the control flow stack
159Value *SIAnnotateControlFlow::popSaved() {
160  return Stack.pop_back_val().second;
161}
162
163/// Push a BB and saved value to the control flow stack
164void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
165  Stack.push_back(std::pair(BB, Saved));
166}
167
168/// Can the condition represented by this PHI node treated like
169/// an "Else" block?
170bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
171  BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
172  for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
173    if (Phi->getIncomingBlock(i) == IDom) {
174
175      if (Phi->getIncomingValue(i) != BoolTrue)
176        return false;
177
178    } else {
179      if (Phi->getIncomingValue(i) != BoolFalse)
180        return false;
181
182    }
183  }
184  return true;
185}
186
187bool SIAnnotateControlFlow::hasKill(const BasicBlock *BB) {
188  for (const Instruction &I : *BB) {
189    if (const CallInst *CI = dyn_cast<CallInst>(&I))
190      if (CI->getIntrinsicID() == Intrinsic::amdgcn_kill)
191        return true;
192  }
193  return false;
194}
195
196// Erase "Phi" if it is not used any more. Return true if any change was made.
197bool SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
198  bool Changed = RecursivelyDeleteDeadPHINode(Phi);
199  if (Changed)
200    LLVM_DEBUG(dbgs() << "Erased unused condition phi\n");
201  return Changed;
202}
203
204/// Open a new "If" block
205bool SIAnnotateControlFlow::openIf(BranchInst *Term) {
206  if (isUniform(Term))
207    return false;
208
209  Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
210  Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
211  push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
212  return true;
213}
214
215/// Close the last "If" block and open a new "Else" block
216bool SIAnnotateControlFlow::insertElse(BranchInst *Term) {
217  if (isUniform(Term)) {
218    return false;
219  }
220  Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
221  Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
222  push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
223  return true;
224}
225
226/// Recursively handle the condition leading to a loop
227Value *SIAnnotateControlFlow::handleLoopCondition(
228    Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term) {
229  if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
230    BasicBlock *Parent = Inst->getParent();
231    Instruction *Insert;
232    if (L->contains(Inst)) {
233      Insert = Parent->getTerminator();
234    } else {
235      Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
236    }
237
238    Value *Args[] = { Cond, Broken };
239    return CallInst::Create(IfBreak, Args, "", Insert);
240  }
241
242  // Insert IfBreak in the loop header TERM for constant COND other than true.
243  if (isa<Constant>(Cond)) {
244    Instruction *Insert = Cond == BoolTrue ?
245      Term : L->getHeader()->getTerminator();
246
247    Value *Args[] = { Cond, Broken };
248    return CallInst::Create(IfBreak, Args, "", Insert);
249  }
250
251  if (isa<Argument>(Cond)) {
252    Instruction *Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
253    Value *Args[] = { Cond, Broken };
254    return CallInst::Create(IfBreak, Args, "", Insert);
255  }
256
257  llvm_unreachable("Unhandled loop condition!");
258}
259
260/// Handle a back edge (loop)
261bool SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
262  if (isUniform(Term))
263    return false;
264
265  BasicBlock *BB = Term->getParent();
266  llvm::Loop *L = LI->getLoopFor(BB);
267  if (!L)
268    return false;
269
270  BasicBlock *Target = Term->getSuccessor(1);
271  PHINode *Broken = PHINode::Create(IntMask, 0, "phi.broken", &Target->front());
272
273  Value *Cond = Term->getCondition();
274  Term->setCondition(BoolTrue);
275  Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
276
277  for (BasicBlock *Pred : predecessors(Target)) {
278    Value *PHIValue = IntMaskZero;
279    if (Pred == BB) // Remember the value of the previous iteration.
280      PHIValue = Arg;
281    // If the backedge from Pred to Target could be executed before the exit
282    // of the loop at BB, it should not reset or change "Broken", which keeps
283    // track of the number of threads exited the loop at BB.
284    else if (L->contains(Pred) && DT->dominates(Pred, BB))
285      PHIValue = Broken;
286    Broken->addIncoming(PHIValue, Pred);
287  }
288
289  Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
290
291  push(Term->getSuccessor(0), Arg);
292
293  return true;
294}
295
296/// Close the last opened control flow
297bool SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
298  llvm::Loop *L = LI->getLoopFor(BB);
299
300  assert(Stack.back().first == BB);
301
302  if (L && L->getHeader() == BB) {
303    // We can't insert an EndCF call into a loop header, because it will
304    // get executed on every iteration of the loop, when it should be
305    // executed only once before the loop.
306    SmallVector <BasicBlock *, 8> Latches;
307    L->getLoopLatches(Latches);
308
309    SmallVector<BasicBlock *, 2> Preds;
310    for (BasicBlock *Pred : predecessors(BB)) {
311      if (!is_contained(Latches, Pred))
312        Preds.push_back(Pred);
313    }
314
315    BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
316                                false);
317  }
318
319  Value *Exec = popSaved();
320  Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
321  if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt)) {
322    Instruction *ExecDef = cast<Instruction>(Exec);
323    BasicBlock *DefBB = ExecDef->getParent();
324    if (!DT->dominates(DefBB, BB)) {
325      // Split edge to make Def dominate Use
326      FirstInsertionPt = &*SplitEdge(DefBB, BB, DT, LI)->getFirstInsertionPt();
327    }
328    CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
329  }
330
331  return true;
332}
333
334/// Annotate the control flow with intrinsics so the backend can
335/// recognize if/then/else and loops.
336bool SIAnnotateControlFlow::runOnFunction(Function &F) {
337  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
338  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
339  DA = &getAnalysis<LegacyDivergenceAnalysis>();
340  TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
341  const TargetMachine &TM = TPC.getTM<TargetMachine>();
342
343  bool Changed = false;
344  initialize(*F.getParent(), TM.getSubtarget<GCNSubtarget>(F));
345  for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
346       E = df_end(&F.getEntryBlock()); I != E; ++I) {
347    BasicBlock *BB = *I;
348    BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
349
350    if (!Term || Term->isUnconditional()) {
351      if (isTopOfStack(BB))
352        Changed |= closeControlFlow(BB);
353
354      continue;
355    }
356
357    if (I.nodeVisited(Term->getSuccessor(1))) {
358      if (isTopOfStack(BB))
359        Changed |= closeControlFlow(BB);
360
361      if (DT->dominates(Term->getSuccessor(1), BB))
362        Changed |= handleLoop(Term);
363      continue;
364    }
365
366    if (isTopOfStack(BB)) {
367      PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
368      if (Phi && Phi->getParent() == BB && isElse(Phi) && !hasKill(BB)) {
369        Changed |= insertElse(Term);
370        Changed |= eraseIfUnused(Phi);
371        continue;
372      }
373
374      Changed |= closeControlFlow(BB);
375    }
376
377    Changed |= openIf(Term);
378  }
379
380  if (!Stack.empty()) {
381    // CFG was probably not structured.
382    report_fatal_error("failed to annotate CFG");
383  }
384
385  return Changed;
386}
387
388/// Create the annotation pass
389FunctionPass *llvm::createSIAnnotateControlFlowPass() {
390  return new SIAnnotateControlFlow();
391}
392