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 void eraseIfUnused(PHINode *Phi);
75
76 void openIf(BranchInst *Term);
77
78 void insertElse(BranchInst *Term);
79
80 Value *
81 handleLoopCondition(Value *Cond, PHINode *Broken, llvm::Loop *L,
82 BranchInst *Term);
83
84 void handleLoop(BranchInst *Term);
85
86 void closeControlFlow(BasicBlock *BB);
87
88 public:
89 static char ID;
90
SIAnnotateControlFlow()91 SIAnnotateControlFlow() : FunctionPass(ID) {}
92
93 bool runOnFunction(Function &F) override;
94
getPassName() const95 StringRef getPassName() const override { return "SI annotate control flow"; }
96
getAnalysisUsage(AnalysisUsage & AU) const97 void getAnalysisUsage(AnalysisUsage &AU) const override {
98 AU.addRequired<LoopInfoWrapperPass>();
99 AU.addRequired<DominatorTreeWrapperPass>();
100 AU.addRequired<LegacyDivergenceAnalysis>();
101 AU.addPreserved<DominatorTreeWrapperPass>();
102 AU.addRequired<TargetPassConfig>();
103 FunctionPass::getAnalysisUsage(AU);
104 }
105 };
106
107 } // end anonymous namespace
108
109 INITIALIZE_PASS_BEGIN(SIAnnotateControlFlow, DEBUG_TYPE,
110 "Annotate SI Control Flow", false, false)
111 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
112 INITIALIZE_PASS_DEPENDENCY(LegacyDivergenceAnalysis)
113 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
114 INITIALIZE_PASS_END(SIAnnotateControlFlow, DEBUG_TYPE,
115 "Annotate SI Control Flow", false, false)
116
117 char SIAnnotateControlFlow::ID = 0;
118
119 /// Initialize all the types and constants used in the pass
initialize(Module & M,const GCNSubtarget & ST)120 void SIAnnotateControlFlow::initialize(Module &M, const GCNSubtarget &ST) {
121 LLVMContext &Context = M.getContext();
122
123 Void = Type::getVoidTy(Context);
124 Boolean = Type::getInt1Ty(Context);
125 IntMask = ST.isWave32() ? Type::getInt32Ty(Context)
126 : Type::getInt64Ty(Context);
127 ReturnStruct = StructType::get(Boolean, IntMask);
128
129 BoolTrue = ConstantInt::getTrue(Context);
130 BoolFalse = ConstantInt::getFalse(Context);
131 BoolUndef = UndefValue::get(Boolean);
132 IntMaskZero = ConstantInt::get(IntMask, 0);
133
134 If = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if, { IntMask });
135 Else = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_else,
136 { IntMask, IntMask });
137 IfBreak = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_if_break,
138 { IntMask });
139 Loop = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_loop, { IntMask });
140 EndCf = Intrinsic::getDeclaration(&M, Intrinsic::amdgcn_end_cf, { IntMask });
141 }
142
143 /// Is the branch condition uniform or did the StructurizeCFG pass
144 /// consider it as such?
isUniform(BranchInst * T)145 bool SIAnnotateControlFlow::isUniform(BranchInst *T) {
146 return DA->isUniform(T) ||
147 T->getMetadata("structurizecfg.uniform") != nullptr;
148 }
149
150 /// Is BB the last block saved on the stack ?
isTopOfStack(BasicBlock * BB)151 bool SIAnnotateControlFlow::isTopOfStack(BasicBlock *BB) {
152 return !Stack.empty() && Stack.back().first == BB;
153 }
154
155 /// Pop the last saved value from the control flow stack
popSaved()156 Value *SIAnnotateControlFlow::popSaved() {
157 return Stack.pop_back_val().second;
158 }
159
160 /// Push a BB and saved value to the control flow stack
push(BasicBlock * BB,Value * Saved)161 void SIAnnotateControlFlow::push(BasicBlock *BB, Value *Saved) {
162 Stack.push_back(std::make_pair(BB, Saved));
163 }
164
165 /// Can the condition represented by this PHI node treated like
166 /// an "Else" block?
isElse(PHINode * Phi)167 bool SIAnnotateControlFlow::isElse(PHINode *Phi) {
168 BasicBlock *IDom = DT->getNode(Phi->getParent())->getIDom()->getBlock();
169 for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
170 if (Phi->getIncomingBlock(i) == IDom) {
171
172 if (Phi->getIncomingValue(i) != BoolTrue)
173 return false;
174
175 } else {
176 if (Phi->getIncomingValue(i) != BoolFalse)
177 return false;
178
179 }
180 }
181 return true;
182 }
183
184 // Erase "Phi" if it is not used any more
eraseIfUnused(PHINode * Phi)185 void SIAnnotateControlFlow::eraseIfUnused(PHINode *Phi) {
186 if (RecursivelyDeleteDeadPHINode(Phi)) {
187 LLVM_DEBUG(dbgs() << "Erased unused condition phi\n");
188 }
189 }
190
191 /// Open a new "If" block
openIf(BranchInst * Term)192 void SIAnnotateControlFlow::openIf(BranchInst *Term) {
193 if (isUniform(Term))
194 return;
195
196 Value *Ret = CallInst::Create(If, Term->getCondition(), "", Term);
197 Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
198 push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
199 }
200
201 /// Close the last "If" block and open a new "Else" block
insertElse(BranchInst * Term)202 void SIAnnotateControlFlow::insertElse(BranchInst *Term) {
203 if (isUniform(Term)) {
204 return;
205 }
206 Value *Ret = CallInst::Create(Else, popSaved(), "", Term);
207 Term->setCondition(ExtractValueInst::Create(Ret, 0, "", Term));
208 push(Term->getSuccessor(1), ExtractValueInst::Create(Ret, 1, "", Term));
209 }
210
211 /// Recursively handle the condition leading to a loop
handleLoopCondition(Value * Cond,PHINode * Broken,llvm::Loop * L,BranchInst * Term)212 Value *SIAnnotateControlFlow::handleLoopCondition(
213 Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term) {
214 if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
215 BasicBlock *Parent = Inst->getParent();
216 Instruction *Insert;
217 if (L->contains(Inst)) {
218 Insert = Parent->getTerminator();
219 } else {
220 Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
221 }
222
223 Value *Args[] = { Cond, Broken };
224 return CallInst::Create(IfBreak, Args, "", Insert);
225 }
226
227 // Insert IfBreak in the loop header TERM for constant COND other than true.
228 if (isa<Constant>(Cond)) {
229 Instruction *Insert = Cond == BoolTrue ?
230 Term : L->getHeader()->getTerminator();
231
232 Value *Args[] = { Cond, Broken };
233 return CallInst::Create(IfBreak, Args, "", Insert);
234 }
235
236 llvm_unreachable("Unhandled loop condition!");
237 }
238
239 /// Handle a back edge (loop)
handleLoop(BranchInst * Term)240 void SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
241 if (isUniform(Term))
242 return;
243
244 BasicBlock *BB = Term->getParent();
245 llvm::Loop *L = LI->getLoopFor(BB);
246 if (!L)
247 return;
248
249 BasicBlock *Target = Term->getSuccessor(1);
250 PHINode *Broken = PHINode::Create(IntMask, 0, "phi.broken", &Target->front());
251
252 Value *Cond = Term->getCondition();
253 Term->setCondition(BoolTrue);
254 Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
255
256 for (BasicBlock *Pred : predecessors(Target)) {
257 Value *PHIValue = IntMaskZero;
258 if (Pred == BB) // Remember the value of the previous iteration.
259 PHIValue = Arg;
260 // If the backedge from Pred to Target could be executed before the exit
261 // of the loop at BB, it should not reset or change "Broken", which keeps
262 // track of the number of threads exited the loop at BB.
263 else if (L->contains(Pred) && DT->dominates(Pred, BB))
264 PHIValue = Broken;
265 Broken->addIncoming(PHIValue, Pred);
266 }
267
268 Term->setCondition(CallInst::Create(Loop, Arg, "", Term));
269
270 push(Term->getSuccessor(0), Arg);
271 }
272
273 /// Close the last opened control flow
closeControlFlow(BasicBlock * BB)274 void SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
275 llvm::Loop *L = LI->getLoopFor(BB);
276
277 assert(Stack.back().first == BB);
278
279 if (L && L->getHeader() == BB) {
280 // We can't insert an EndCF call into a loop header, because it will
281 // get executed on every iteration of the loop, when it should be
282 // executed only once before the loop.
283 SmallVector <BasicBlock *, 8> Latches;
284 L->getLoopLatches(Latches);
285
286 SmallVector<BasicBlock *, 2> Preds;
287 for (BasicBlock *Pred : predecessors(BB)) {
288 if (!is_contained(Latches, Pred))
289 Preds.push_back(Pred);
290 }
291
292 BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
293 false);
294 }
295
296 Value *Exec = popSaved();
297 Instruction *FirstInsertionPt = &*BB->getFirstInsertionPt();
298 if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt)) {
299 Instruction *ExecDef = cast<Instruction>(Exec);
300 BasicBlock *DefBB = ExecDef->getParent();
301 if (!DT->dominates(DefBB, BB)) {
302 // Split edge to make Def dominate Use
303 FirstInsertionPt = &*SplitEdge(DefBB, BB, DT, LI)->getFirstInsertionPt();
304 }
305 CallInst::Create(EndCf, Exec, "", FirstInsertionPt);
306 }
307 }
308
309 /// Annotate the control flow with intrinsics so the backend can
310 /// recognize if/then/else and loops.
runOnFunction(Function & F)311 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
312 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
313 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
314 DA = &getAnalysis<LegacyDivergenceAnalysis>();
315 TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
316 const TargetMachine &TM = TPC.getTM<TargetMachine>();
317
318 initialize(*F.getParent(), TM.getSubtarget<GCNSubtarget>(F));
319 for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
320 E = df_end(&F.getEntryBlock()); I != E; ++I) {
321 BasicBlock *BB = *I;
322 BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
323
324 if (!Term || Term->isUnconditional()) {
325 if (isTopOfStack(BB))
326 closeControlFlow(BB);
327
328 continue;
329 }
330
331 if (I.nodeVisited(Term->getSuccessor(1))) {
332 if (isTopOfStack(BB))
333 closeControlFlow(BB);
334
335 if (DT->dominates(Term->getSuccessor(1), BB))
336 handleLoop(Term);
337 continue;
338 }
339
340 if (isTopOfStack(BB)) {
341 PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
342 if (Phi && Phi->getParent() == BB && isElse(Phi)) {
343 insertElse(Term);
344 eraseIfUnused(Phi);
345 continue;
346 }
347
348 closeControlFlow(BB);
349 }
350
351 openIf(Term);
352 }
353
354 if (!Stack.empty()) {
355 // CFG was probably not structured.
356 report_fatal_error("failed to annotate CFG");
357 }
358
359 return true;
360 }
361
362 /// Create the annotation pass
createSIAnnotateControlFlowPass()363 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
364 return new SIAnnotateControlFlow();
365 }
366