1 //===- SpeculativeExecution.cpp ---------------------------------*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass hoists instructions to enable speculative execution on
11 // targets where branches are expensive. This is aimed at GPUs. It
12 // currently works on simple if-then and if-then-else
13 // patterns.
14 //
15 // Removing branches is not the only motivation for this
16 // pass. E.g. consider this code and assume that there is no
17 // addressing mode for multiplying by sizeof(*a):
18 //
19 //   if (b > 0)
20 //     c = a[i + 1]
21 //   if (d > 0)
22 //     e = a[i + 2]
23 //
24 // turns into
25 //
26 //   p = &a[i + 1];
27 //   if (b > 0)
28 //     c = *p;
29 //   q = &a[i + 2];
30 //   if (d > 0)
31 //     e = *q;
32 //
33 // which could later be optimized to
34 //
35 //   r = &a[i];
36 //   if (b > 0)
37 //     c = r[1];
38 //   if (d > 0)
39 //     e = r[2];
40 //
41 // Later passes sink back much of the speculated code that did not enable
42 // further optimization.
43 //
44 // This pass is more aggressive than the function SpeculativeyExecuteBB in
45 // SimplifyCFG. SimplifyCFG will not speculate if no selects are introduced and
46 // it will speculate at most one instruction. It also will not speculate if
47 // there is a value defined in the if-block that is only used in the then-block.
48 // These restrictions make sense since the speculation in SimplifyCFG seems
49 // aimed at introducing cheap selects, while this pass is intended to do more
50 // aggressive speculation while counting on later passes to either capitalize on
51 // that or clean it up.
52 //
53 // If the pass was created by calling
54 // createSpeculativeExecutionIfHasBranchDivergencePass or the
55 // -spec-exec-only-if-divergent-target option is present, this pass only has an
56 // effect on targets where TargetTransformInfo::hasBranchDivergence() is true;
57 // on other targets, it is a nop.
58 //
59 // This lets you include this pass unconditionally in the IR pass pipeline, but
60 // only enable it for relevant targets.
61 //
62 //===----------------------------------------------------------------------===//
63 
64 #include "llvm/Transforms/Scalar/SpeculativeExecution.h"
65 #include "llvm/ADT/SmallSet.h"
66 #include "llvm/Analysis/GlobalsModRef.h"
67 #include "llvm/Analysis/ValueTracking.h"
68 #include "llvm/IR/Instructions.h"
69 #include "llvm/IR/Module.h"
70 #include "llvm/IR/Operator.h"
71 #include "llvm/Support/CommandLine.h"
72 #include "llvm/Support/Debug.h"
73 
74 using namespace llvm;
75 
76 #define DEBUG_TYPE "speculative-execution"
77 
78 // The risk that speculation will not pay off increases with the
79 // number of instructions speculated, so we put a limit on that.
80 static cl::opt<unsigned> SpecExecMaxSpeculationCost(
81     "spec-exec-max-speculation-cost", cl::init(7), cl::Hidden,
82     cl::desc("Speculative execution is not applied to basic blocks where "
83              "the cost of the instructions to speculatively execute "
84              "exceeds this limit."));
85 
86 // Speculating just a few instructions from a larger block tends not
87 // to be profitable and this limit prevents that. A reason for that is
88 // that small basic blocks are more likely to be candidates for
89 // further optimization.
90 static cl::opt<unsigned> SpecExecMaxNotHoisted(
91     "spec-exec-max-not-hoisted", cl::init(5), cl::Hidden,
92     cl::desc("Speculative execution is not applied to basic blocks where the "
93              "number of instructions that would not be speculatively executed "
94              "exceeds this limit."));
95 
96 static cl::opt<bool> SpecExecOnlyIfDivergentTarget(
97     "spec-exec-only-if-divergent-target", cl::init(false), cl::Hidden,
98     cl::desc("Speculative execution is applied only to targets with divergent "
99              "branches, even if the pass was configured to apply only to all "
100              "targets."));
101 
102 namespace {
103 
104 class SpeculativeExecutionLegacyPass : public FunctionPass {
105 public:
106   static char ID;
107   explicit SpeculativeExecutionLegacyPass(bool OnlyIfDivergentTarget = false)
108       : FunctionPass(ID), OnlyIfDivergentTarget(OnlyIfDivergentTarget ||
109                                                 SpecExecOnlyIfDivergentTarget),
110         Impl(OnlyIfDivergentTarget) {}
111 
112   void getAnalysisUsage(AnalysisUsage &AU) const override;
113   bool runOnFunction(Function &F) override;
114 
115   StringRef getPassName() const override {
116     if (OnlyIfDivergentTarget)
117       return "Speculatively execute instructions if target has divergent "
118              "branches";
119     return "Speculatively execute instructions";
120   }
121 
122 private:
123   // Variable preserved purely for correct name printing.
124   const bool OnlyIfDivergentTarget;
125 
126   SpeculativeExecutionPass Impl;
127 };
128 } // namespace
129 
130 char SpeculativeExecutionLegacyPass::ID = 0;
131 INITIALIZE_PASS_BEGIN(SpeculativeExecutionLegacyPass, "speculative-execution",
132                       "Speculatively execute instructions", false, false)
133 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
134 INITIALIZE_PASS_END(SpeculativeExecutionLegacyPass, "speculative-execution",
135                     "Speculatively execute instructions", false, false)
136 
137 void SpeculativeExecutionLegacyPass::getAnalysisUsage(AnalysisUsage &AU) const {
138   AU.addRequired<TargetTransformInfoWrapperPass>();
139   AU.addPreserved<GlobalsAAWrapperPass>();
140 }
141 
142 bool SpeculativeExecutionLegacyPass::runOnFunction(Function &F) {
143   if (skipFunction(F))
144     return false;
145 
146   auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
147   return Impl.runImpl(F, TTI);
148 }
149 
150 namespace llvm {
151 
152 bool SpeculativeExecutionPass::runImpl(Function &F, TargetTransformInfo *TTI) {
153   if (OnlyIfDivergentTarget && !TTI->hasBranchDivergence()) {
154     DEBUG(dbgs() << "Not running SpeculativeExecution because "
155                     "TTI->hasBranchDivergence() is false.\n");
156     return false;
157   }
158 
159   this->TTI = TTI;
160   bool Changed = false;
161   for (auto& B : F) {
162     Changed |= runOnBasicBlock(B);
163   }
164   return Changed;
165 }
166 
167 bool SpeculativeExecutionPass::runOnBasicBlock(BasicBlock &B) {
168   BranchInst *BI = dyn_cast<BranchInst>(B.getTerminator());
169   if (BI == nullptr)
170     return false;
171 
172   if (BI->getNumSuccessors() != 2)
173     return false;
174   BasicBlock &Succ0 = *BI->getSuccessor(0);
175   BasicBlock &Succ1 = *BI->getSuccessor(1);
176 
177   if (&B == &Succ0 || &B == &Succ1 || &Succ0 == &Succ1) {
178     return false;
179   }
180 
181   // Hoist from if-then (triangle).
182   if (Succ0.getSinglePredecessor() != nullptr &&
183       Succ0.getSingleSuccessor() == &Succ1) {
184     return considerHoistingFromTo(Succ0, B);
185   }
186 
187   // Hoist from if-else (triangle).
188   if (Succ1.getSinglePredecessor() != nullptr &&
189       Succ1.getSingleSuccessor() == &Succ0) {
190     return considerHoistingFromTo(Succ1, B);
191   }
192 
193   // Hoist from if-then-else (diamond), but only if it is equivalent to
194   // an if-else or if-then due to one of the branches doing nothing.
195   if (Succ0.getSinglePredecessor() != nullptr &&
196       Succ1.getSinglePredecessor() != nullptr &&
197       Succ1.getSingleSuccessor() != nullptr &&
198       Succ1.getSingleSuccessor() != &B &&
199       Succ1.getSingleSuccessor() == Succ0.getSingleSuccessor()) {
200     // If a block has only one instruction, then that is a terminator
201     // instruction so that the block does nothing. This does happen.
202     if (Succ1.size() == 1) // equivalent to if-then
203       return considerHoistingFromTo(Succ0, B);
204     if (Succ0.size() == 1) // equivalent to if-else
205       return considerHoistingFromTo(Succ1, B);
206   }
207 
208   return false;
209 }
210 
211 static unsigned ComputeSpeculationCost(const Instruction *I,
212                                        const TargetTransformInfo &TTI) {
213   switch (Operator::getOpcode(I)) {
214     case Instruction::GetElementPtr:
215     case Instruction::Add:
216     case Instruction::Mul:
217     case Instruction::And:
218     case Instruction::Or:
219     case Instruction::Select:
220     case Instruction::Shl:
221     case Instruction::Sub:
222     case Instruction::LShr:
223     case Instruction::AShr:
224     case Instruction::Xor:
225     case Instruction::ZExt:
226     case Instruction::SExt:
227     case Instruction::Call:
228     case Instruction::BitCast:
229     case Instruction::PtrToInt:
230     case Instruction::IntToPtr:
231     case Instruction::AddrSpaceCast:
232     case Instruction::FPToUI:
233     case Instruction::FPToSI:
234     case Instruction::UIToFP:
235     case Instruction::SIToFP:
236     case Instruction::FPExt:
237     case Instruction::FPTrunc:
238     case Instruction::FAdd:
239     case Instruction::FSub:
240     case Instruction::FMul:
241     case Instruction::FDiv:
242     case Instruction::FRem:
243     case Instruction::ICmp:
244     case Instruction::FCmp:
245       return TTI.getUserCost(I);
246 
247     default:
248       return UINT_MAX; // Disallow anything not whitelisted.
249   }
250 }
251 
252 bool SpeculativeExecutionPass::considerHoistingFromTo(
253     BasicBlock &FromBlock, BasicBlock &ToBlock) {
254   SmallSet<const Instruction *, 8> NotHoisted;
255   const auto AllPrecedingUsesFromBlockHoisted = [&NotHoisted](User *U) {
256     for (Value* V : U->operand_values()) {
257       if (Instruction *I = dyn_cast<Instruction>(V)) {
258         if (NotHoisted.count(I) > 0)
259           return false;
260       }
261     }
262     return true;
263   };
264 
265   unsigned TotalSpeculationCost = 0;
266   for (auto& I : FromBlock) {
267     const unsigned Cost = ComputeSpeculationCost(&I, *TTI);
268     if (Cost != UINT_MAX && isSafeToSpeculativelyExecute(&I) &&
269         AllPrecedingUsesFromBlockHoisted(&I)) {
270       TotalSpeculationCost += Cost;
271       if (TotalSpeculationCost > SpecExecMaxSpeculationCost)
272         return false;  // too much to hoist
273     } else {
274       NotHoisted.insert(&I);
275       if (NotHoisted.size() > SpecExecMaxNotHoisted)
276         return false; // too much left behind
277     }
278   }
279 
280   if (TotalSpeculationCost == 0)
281     return false; // nothing to hoist
282 
283   for (auto I = FromBlock.begin(); I != FromBlock.end();) {
284     // We have to increment I before moving Current as moving Current
285     // changes the list that I is iterating through.
286     auto Current = I;
287     ++I;
288     if (!NotHoisted.count(&*Current)) {
289       Current->moveBefore(ToBlock.getTerminator());
290     }
291   }
292   return true;
293 }
294 
295 FunctionPass *createSpeculativeExecutionPass() {
296   return new SpeculativeExecutionLegacyPass();
297 }
298 
299 FunctionPass *createSpeculativeExecutionIfHasBranchDivergencePass() {
300   return new SpeculativeExecutionLegacyPass(/* OnlyIfDivergentTarget = */ true);
301 }
302 
303 SpeculativeExecutionPass::SpeculativeExecutionPass(bool OnlyIfDivergentTarget)
304     : OnlyIfDivergentTarget(OnlyIfDivergentTarget ||
305                             SpecExecOnlyIfDivergentTarget) {}
306 
307 PreservedAnalyses SpeculativeExecutionPass::run(Function &F,
308                                                 FunctionAnalysisManager &AM) {
309   auto *TTI = &AM.getResult<TargetIRAnalysis>(F);
310 
311   bool Changed = runImpl(F, TTI);
312 
313   if (!Changed)
314     return PreservedAnalyses::all();
315   PreservedAnalyses PA;
316   PA.preserve<GlobalsAA>();
317   return PA;
318 }
319 }  // namespace llvm
320