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) || T->hasMetadata("structurizecfg.uniform");
151 }
152
153 /// Is BB the last block saved on the stack ?
isTopOfStack(BasicBlock * BB)154 bool 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
popSaved()159 Value *SIAnnotateControlFlow::popSaved() {
160 return Stack.pop_back_val().second;
161 }
162
163 /// Push a BB and saved value to the control flow stack
push(BasicBlock * BB,Value * Saved)164 void 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?
isElse(PHINode * Phi)170 bool 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
hasKill(const BasicBlock * BB)187 bool 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.
eraseIfUnused(PHINode * Phi)197 bool 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
openIf(BranchInst * Term)205 bool SIAnnotateControlFlow::openIf(BranchInst *Term) {
206 if (isUniform(Term))
207 return false;
208
209 IRBuilder<> IRB(Term);
210 Value *IfCall = IRB.CreateCall(If, {Term->getCondition()});
211 Value *Cond = IRB.CreateExtractValue(IfCall, {0});
212 Value *Mask = IRB.CreateExtractValue(IfCall, {1});
213 Term->setCondition(Cond);
214 push(Term->getSuccessor(1), Mask);
215 return true;
216 }
217
218 /// Close the last "If" block and open a new "Else" block
insertElse(BranchInst * Term)219 bool SIAnnotateControlFlow::insertElse(BranchInst *Term) {
220 if (isUniform(Term)) {
221 return false;
222 }
223
224 IRBuilder<> IRB(Term);
225 Value *ElseCall = IRB.CreateCall(Else, {popSaved()});
226 Value *Cond = IRB.CreateExtractValue(ElseCall, {0});
227 Value *Mask = IRB.CreateExtractValue(ElseCall, {1});
228 Term->setCondition(Cond);
229 push(Term->getSuccessor(1), Mask);
230 return true;
231 }
232
233 /// Recursively handle the condition leading to a loop
handleLoopCondition(Value * Cond,PHINode * Broken,llvm::Loop * L,BranchInst * Term)234 Value *SIAnnotateControlFlow::handleLoopCondition(
235 Value *Cond, PHINode *Broken, llvm::Loop *L, BranchInst *Term) {
236
237 auto CreateBreak = [this, Cond, Broken](Instruction *I) -> CallInst * {
238 return IRBuilder<>(I).CreateCall(IfBreak, {Cond, Broken});
239 };
240
241 if (Instruction *Inst = dyn_cast<Instruction>(Cond)) {
242 BasicBlock *Parent = Inst->getParent();
243 Instruction *Insert;
244 if (L->contains(Inst)) {
245 Insert = Parent->getTerminator();
246 } else {
247 Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
248 }
249
250 return CreateBreak(Insert);
251 }
252
253 // Insert IfBreak in the loop header TERM for constant COND other than true.
254 if (isa<Constant>(Cond)) {
255 Instruction *Insert = Cond == BoolTrue ?
256 Term : L->getHeader()->getTerminator();
257
258 return CreateBreak(Insert);
259 }
260
261 if (isa<Argument>(Cond)) {
262 Instruction *Insert = L->getHeader()->getFirstNonPHIOrDbgOrLifetime();
263 return CreateBreak(Insert);
264 }
265
266 llvm_unreachable("Unhandled loop condition!");
267 }
268
269 /// Handle a back edge (loop)
handleLoop(BranchInst * Term)270 bool SIAnnotateControlFlow::handleLoop(BranchInst *Term) {
271 if (isUniform(Term))
272 return false;
273
274 BasicBlock *BB = Term->getParent();
275 llvm::Loop *L = LI->getLoopFor(BB);
276 if (!L)
277 return false;
278
279 BasicBlock *Target = Term->getSuccessor(1);
280 PHINode *Broken = PHINode::Create(IntMask, 0, "phi.broken");
281 Broken->insertBefore(Target->begin());
282
283 Value *Cond = Term->getCondition();
284 Term->setCondition(BoolTrue);
285 Value *Arg = handleLoopCondition(Cond, Broken, L, Term);
286
287 for (BasicBlock *Pred : predecessors(Target)) {
288 Value *PHIValue = IntMaskZero;
289 if (Pred == BB) // Remember the value of the previous iteration.
290 PHIValue = Arg;
291 // If the backedge from Pred to Target could be executed before the exit
292 // of the loop at BB, it should not reset or change "Broken", which keeps
293 // track of the number of threads exited the loop at BB.
294 else if (L->contains(Pred) && DT->dominates(Pred, BB))
295 PHIValue = Broken;
296 Broken->addIncoming(PHIValue, Pred);
297 }
298
299 CallInst *LoopCall = IRBuilder<>(Term).CreateCall(Loop, {Arg});
300 Term->setCondition(LoopCall);
301
302 push(Term->getSuccessor(0), Arg);
303
304 return true;
305 }
306
307 /// Close the last opened control flow
closeControlFlow(BasicBlock * BB)308 bool SIAnnotateControlFlow::closeControlFlow(BasicBlock *BB) {
309 llvm::Loop *L = LI->getLoopFor(BB);
310
311 assert(Stack.back().first == BB);
312
313 if (L && L->getHeader() == BB) {
314 // We can't insert an EndCF call into a loop header, because it will
315 // get executed on every iteration of the loop, when it should be
316 // executed only once before the loop.
317 SmallVector <BasicBlock *, 8> Latches;
318 L->getLoopLatches(Latches);
319
320 SmallVector<BasicBlock *, 2> Preds;
321 for (BasicBlock *Pred : predecessors(BB)) {
322 if (!is_contained(Latches, Pred))
323 Preds.push_back(Pred);
324 }
325
326 BB = SplitBlockPredecessors(BB, Preds, "endcf.split", DT, LI, nullptr,
327 false);
328 }
329
330 Value *Exec = popSaved();
331 BasicBlock::iterator FirstInsertionPt = BB->getFirstInsertionPt();
332 if (!isa<UndefValue>(Exec) && !isa<UnreachableInst>(FirstInsertionPt)) {
333 Instruction *ExecDef = cast<Instruction>(Exec);
334 BasicBlock *DefBB = ExecDef->getParent();
335 if (!DT->dominates(DefBB, BB)) {
336 // Split edge to make Def dominate Use
337 FirstInsertionPt = SplitEdge(DefBB, BB, DT, LI)->getFirstInsertionPt();
338 }
339 IRBuilder<> IRB(FirstInsertionPt->getParent(), FirstInsertionPt);
340 // TODO: StructurizeCFG 'Flow' blocks have debug locations from the
341 // condition, for now just avoid copying these DebugLocs so that stepping
342 // out of the then/else block in a debugger doesn't step to the condition.
343 IRB.SetCurrentDebugLocation(DebugLoc());
344 IRB.CreateCall(EndCf, {Exec});
345 }
346
347 return true;
348 }
349
350 /// Annotate the control flow with intrinsics so the backend can
351 /// recognize if/then/else and loops.
runOnFunction(Function & F)352 bool SIAnnotateControlFlow::runOnFunction(Function &F) {
353 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
354 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
355 UA = &getAnalysis<UniformityInfoWrapperPass>().getUniformityInfo();
356 TargetPassConfig &TPC = getAnalysis<TargetPassConfig>();
357 const TargetMachine &TM = TPC.getTM<TargetMachine>();
358
359 bool Changed = false;
360 initialize(*F.getParent(), TM.getSubtarget<GCNSubtarget>(F));
361 for (df_iterator<BasicBlock *> I = df_begin(&F.getEntryBlock()),
362 E = df_end(&F.getEntryBlock()); I != E; ++I) {
363 BasicBlock *BB = *I;
364 BranchInst *Term = dyn_cast<BranchInst>(BB->getTerminator());
365
366 if (!Term || Term->isUnconditional()) {
367 if (isTopOfStack(BB))
368 Changed |= closeControlFlow(BB);
369
370 continue;
371 }
372
373 if (I.nodeVisited(Term->getSuccessor(1))) {
374 if (isTopOfStack(BB))
375 Changed |= closeControlFlow(BB);
376
377 if (DT->dominates(Term->getSuccessor(1), BB))
378 Changed |= handleLoop(Term);
379 continue;
380 }
381
382 if (isTopOfStack(BB)) {
383 PHINode *Phi = dyn_cast<PHINode>(Term->getCondition());
384 if (Phi && Phi->getParent() == BB && isElse(Phi) && !hasKill(BB)) {
385 Changed |= insertElse(Term);
386 Changed |= eraseIfUnused(Phi);
387 continue;
388 }
389
390 Changed |= closeControlFlow(BB);
391 }
392
393 Changed |= openIf(Term);
394 }
395
396 if (!Stack.empty()) {
397 // CFG was probably not structured.
398 report_fatal_error("failed to annotate CFG");
399 }
400
401 return Changed;
402 }
403
404 /// Create the annotation pass
createSIAnnotateControlFlowPass()405 FunctionPass *llvm::createSIAnnotateControlFlowPass() {
406 return new SIAnnotateControlFlow();
407 }
408