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