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
28 using namespace llvm;
29
30 #define DEBUG_TYPE "si-annotate-control-flow"
31
32 namespace {
33
34 // Complex types used in this pass
35 using StackEntry = std::pair<BasicBlock *, Value *>;
36 using StackVector = SmallVector<StackEntry, 16>;
37
38 class 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 void eraseIfUnused(PHINode *Phi);
77
78 void openIf(BranchInst *Term);
79
80 void insertElse(BranchInst *Term);
81
82 Value *
83 handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
84 BranchInst *Term);
85
86 void handleLoop(BranchInst *Term);
87
88 void closeControlFlow(BasicBlock *BB);
89
90 public:
91 static char ID;
92
SIAnnotateControlFlow()93 SIAnnotateControlFlow() : FunctionPass(ID) {}
94
95 bool runOnFunction(Function &F) override;
96
getPassName() const97 StringRef getPassName() const override { return "SI annotate control flow"; }
98
getAnalysisUsage(AnalysisUsage & AU) const99 void getAnalysisUsage(AnalysisUsage &AU) const override {
100 AU.addRequired<LoopInfoWrapperPass>();
101 AU.addRequired<DominatorTreeWrapperPass>();
102 AU.addRequired<LegacyDivergenceAnalysis>();
103 AU.addPreserved<DominatorTreeWrapperPass>();
104 AU.addRequired<TargetPassConfig>();
105 FunctionPass::getAnalysisUsage(AU);
106 }
107 };
108
109 } // end anonymous namespace
110
111 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
112 "Annotate SI Control Flow", false, false)
113 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
114 INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis)
115 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
116 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
117 "Annotate SI Control Flow", false, false)
118
119 char SIAnnotateControlFlow::ID = 0;
120
121 /// Initialize all the types and constants used in the pass
initialize(Module & M,const GCNSubtarget & ST)122 void SIAnnotateControlFlow::initialize(Module &M, const GCNSubtarget &ST) {
123 LLVMContext &Context = M.getContext();
124
125 Void = Type::getVoidTy(Context);
126 Boolean = Type::getInt1Ty(Context);
127 IntMask = ST.isWave32() ? Type::getInt32Ty(Context)
128 : Type::getInt64Ty(Context);
129 ReturnStruct = StructType::get(Boolean, IntMask);
130
131 BoolTrue = ConstantInt::getTrue(Context);
132 BoolFalse = ConstantInt::getFalse(Context);
133 BoolUndef = UndefValue::get(Boolean);
134 IntMaskZero = ConstantInt::get(IntMask, 0);
135
136 If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if, { IntMask });
137 Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else,
138 { IntMask, IntMask });
139 IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break,
140 { IntMask });
141 Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop, { IntMask });
142 EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf, { IntMask });
143 }
144
145 /// Is the branch condition uniform or did the StructurizeCFG pass
146 /// consider it as such?
isUniform(BranchInst * T)147 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
148 return DA->isUniform(T) ||
149 T->getMetadata("structurizecfg.uniform") != nullptr;
150 }
151
152 /// Is BB the last block saved on the stack ?
isTopOfStack(BasicBlock * BB)153 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
154 return !Stack.empty() && Stack.back().first == BB;
155 }
156
157 /// Pop the last saved value from the control flow stack
popSaved()158 Value *SIAnnotateControlFlow::popSaved() {
159 return Stack.pop_back_val().second;
160 }
161
162 /// Push a BB and saved value to the control flow stack
push(BasicBlock * BB,Value * Saved)163 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
164 Stack.push_back(std::make_pair(BB, Saved));
165 }
166
167 /// Can the condition represented by this PHI node treated like
168 /// an "Else" block?
isElse(PHINode * Phi)169 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
170 BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
171 for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
172 if (Phi->getIncomingBlock(i) == IDom) {
173
174 if (Phi->getIncomingValue(i) != BoolTrue)
175 return false;
176
177 } else {
178 if (Phi->getIncomingValue(i) != BoolFalse)
179 return false;
180
181 }
182 }
183 return true;
184 }
185
hasKill(const BasicBlock * BB)186 bool SIAnnotateControlFlow::hasKill(const BasicBlock *BB) {
187 for (const Instruction &I : *BB) {
188 if (const CallInst *CI = dyn_cast<CallInst>(&I))
189 if (CI->getIntrinsicID() == Intrinsic::amdgcn_kill)
190 return true;
191 }
192 return false;
193 }
194
195 // Erase "Phi" if it is not used any more
eraseIfUnused(PHINode * Phi)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
openIf(BranchInst * Term)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
insertElse(BranchInst * Term)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
handleLoopCondition(Value * Cond,PHINode * Broken,llvm::Loop * L,BranchInst * Term)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)
handleLoop(BranchInst * Term)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(IntMask, 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 Value *PHIValue = IntMaskZero;
269 if (Pred == BB) // Remember the value of the previous iteration.
270 PHIValue = Arg;
271 // If the backedge from Pred to Target could be executed before the exit
272 // of the loop at BB, it should not reset or change "Broken", which keeps
273 // track of the number of threads exited the loop at BB.
274 else if (L->contains(Pred) && DT->dominates(Pred, BB))
275 PHIValue = Broken;
276 Broken->addIncoming(PHIValue, Pred);
277 }
278
279 Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
280
281 push(Term->getSuccessor(0), Arg);
282 }
283
284 /// Close the last opened control flow
closeControlFlow(BasicBlock * BB)285 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
286 llvm::Loop *L = LI->getLoopFor(BB);
287
288 assert(Stack.back().first == BB);
289
290 if (L && L->getHeader() == BB) {
291 // We can't insert an EndCF call into a loop header, because it will
292 // get executed on every iteration of the loop, when it should be
293 // executed only once before the loop.
294 SmallVector <BasicBlock *, 8> Latches;
295 L->getLoopLatches(Latches);
296
297 SmallVector<BasicBlock *, 2> Preds;
298 for (BasicBlock *Pred : predecessors(BB)) {
299 if (!is_contained(Latches, Pred))
300 Preds.push_back(Pred);
301 }
302
303 BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
304 false);
305 }
306
307 Value *Exec = popSaved();
308 Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
309 if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt)) {
310 Instruction *ExecDef = cast<Instruction>(Exec);
311 BasicBlock *DefBB = ExecDef->getParent();
312 if (!DT->dominates(DefBB, BB)) {
313 // Split edge to make Def dominate Use
314 FirstInsertionPt = &*SplitEdge(DefBB, BB, DT, LI)->getFirstInsertionPt();
315 }
316 CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
317 }
318 }
319
320 /// Annotate the control flow with intrinsics so the backend can
321 /// recognize if/then/else and loops.
runOnFunction(Function & F)322 bool 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 for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
331 E = df_end(&F.getEntryBlock()); I != E; ++I) {
332 BasicBlock *BB = *I;
333 BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
334
335 if (!Term || Term->isUnconditional()) {
336 if (isTopOfStack(BB))
337 closeControlFlow(BB);
338
339 continue;
340 }
341
342 if (I.nodeVisited(Term->getSuccessor(1))) {
343 if (isTopOfStack(BB))
344 closeControlFlow(BB);
345
346 if (DT->dominates(Term->getSuccessor(1), BB))
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) && !hasKill(BB)) {
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
createSIAnnotateControlFlowPass()374 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
375 return new SIAnnotateControlFlow();
376 }
377