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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/ADT/SmallSet.h"
65 #include "llvm/Analysis/GlobalsModRef.h"
66 #include "llvm/Analysis/TargetTransformInfo.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 SpeculativeExecution : public FunctionPass {
105  public:
106    static char ID;
SpeculativeExecution(bool OnlyIfDivergentTarget=false)107    explicit SpeculativeExecution(bool OnlyIfDivergentTarget = false)
108        : FunctionPass(ID),
109          OnlyIfDivergentTarget(OnlyIfDivergentTarget ||
110                                SpecExecOnlyIfDivergentTarget) {}
111 
112    void getAnalysisUsage(AnalysisUsage &AU) const override;
113    bool runOnFunction(Function &F) override;
114 
getPassName() const115    const char *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   bool runOnBasicBlock(BasicBlock &B);
124   bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock);
125 
126   // If true, this pass is a nop unless the target architecture has branch
127   // divergence.
128   const bool OnlyIfDivergentTarget;
129   const TargetTransformInfo *TTI = nullptr;
130 };
131 } // namespace
132 
133 char SpeculativeExecution::ID = 0;
134 INITIALIZE_PASS_BEGIN(SpeculativeExecution, "speculative-execution",
135                       "Speculatively execute instructions", false, false)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)136 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
137 INITIALIZE_PASS_END(SpeculativeExecution, "speculative-execution",
138                     "Speculatively execute instructions", false, false)
139 
140 void SpeculativeExecution::getAnalysisUsage(AnalysisUsage &AU) const {
141   AU.addRequired<TargetTransformInfoWrapperPass>();
142   AU.addPreserved<GlobalsAAWrapperPass>();
143 }
144 
runOnFunction(Function & F)145 bool SpeculativeExecution::runOnFunction(Function &F) {
146   if (skipFunction(F))
147     return false;
148 
149   TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
150   if (OnlyIfDivergentTarget && !TTI->hasBranchDivergence()) {
151     DEBUG(dbgs() << "Not running SpeculativeExecution because "
152                     "TTI->hasBranchDivergence() is false.\n");
153     return false;
154   }
155 
156   bool Changed = false;
157   for (auto& B : F) {
158     Changed |= runOnBasicBlock(B);
159   }
160   return Changed;
161 }
162 
runOnBasicBlock(BasicBlock & B)163 bool SpeculativeExecution::runOnBasicBlock(BasicBlock &B) {
164   BranchInst *BI = dyn_cast<BranchInst>(B.getTerminator());
165   if (BI == nullptr)
166     return false;
167 
168   if (BI->getNumSuccessors() != 2)
169     return false;
170   BasicBlock &Succ0 = *BI->getSuccessor(0);
171   BasicBlock &Succ1 = *BI->getSuccessor(1);
172 
173   if (&B == &Succ0 || &B == &Succ1 || &Succ0 == &Succ1) {
174     return false;
175   }
176 
177   // Hoist from if-then (triangle).
178   if (Succ0.getSinglePredecessor() != nullptr &&
179       Succ0.getSingleSuccessor() == &Succ1) {
180     return considerHoistingFromTo(Succ0, B);
181   }
182 
183   // Hoist from if-else (triangle).
184   if (Succ1.getSinglePredecessor() != nullptr &&
185       Succ1.getSingleSuccessor() == &Succ0) {
186     return considerHoistingFromTo(Succ1, B);
187   }
188 
189   // Hoist from if-then-else (diamond), but only if it is equivalent to
190   // an if-else or if-then due to one of the branches doing nothing.
191   if (Succ0.getSinglePredecessor() != nullptr &&
192       Succ1.getSinglePredecessor() != nullptr &&
193       Succ1.getSingleSuccessor() != nullptr &&
194       Succ1.getSingleSuccessor() != &B &&
195       Succ1.getSingleSuccessor() == Succ0.getSingleSuccessor()) {
196     // If a block has only one instruction, then that is a terminator
197     // instruction so that the block does nothing. This does happen.
198     if (Succ1.size() == 1) // equivalent to if-then
199       return considerHoistingFromTo(Succ0, B);
200     if (Succ0.size() == 1) // equivalent to if-else
201       return considerHoistingFromTo(Succ1, B);
202   }
203 
204   return false;
205 }
206 
ComputeSpeculationCost(const Instruction * I,const TargetTransformInfo & TTI)207 static unsigned ComputeSpeculationCost(const Instruction *I,
208                                        const TargetTransformInfo &TTI) {
209   switch (Operator::getOpcode(I)) {
210     case Instruction::GetElementPtr:
211     case Instruction::Add:
212     case Instruction::Mul:
213     case Instruction::And:
214     case Instruction::Or:
215     case Instruction::Select:
216     case Instruction::Shl:
217     case Instruction::Sub:
218     case Instruction::LShr:
219     case Instruction::AShr:
220     case Instruction::Xor:
221     case Instruction::ZExt:
222     case Instruction::SExt:
223       return TTI.getUserCost(I);
224 
225     default:
226       return UINT_MAX; // Disallow anything not whitelisted.
227   }
228 }
229 
considerHoistingFromTo(BasicBlock & FromBlock,BasicBlock & ToBlock)230 bool SpeculativeExecution::considerHoistingFromTo(BasicBlock &FromBlock,
231                                                   BasicBlock &ToBlock) {
232   SmallSet<const Instruction *, 8> NotHoisted;
233   const auto AllPrecedingUsesFromBlockHoisted = [&NotHoisted](User *U) {
234     for (Value* V : U->operand_values()) {
235       if (Instruction *I = dyn_cast<Instruction>(V)) {
236         if (NotHoisted.count(I) > 0)
237           return false;
238       }
239     }
240     return true;
241   };
242 
243   unsigned TotalSpeculationCost = 0;
244   for (auto& I : FromBlock) {
245     const unsigned Cost = ComputeSpeculationCost(&I, *TTI);
246     if (Cost != UINT_MAX && isSafeToSpeculativelyExecute(&I) &&
247         AllPrecedingUsesFromBlockHoisted(&I)) {
248       TotalSpeculationCost += Cost;
249       if (TotalSpeculationCost > SpecExecMaxSpeculationCost)
250         return false;  // too much to hoist
251     } else {
252       NotHoisted.insert(&I);
253       if (NotHoisted.size() > SpecExecMaxNotHoisted)
254         return false; // too much left behind
255     }
256   }
257 
258   if (TotalSpeculationCost == 0)
259     return false; // nothing to hoist
260 
261   for (auto I = FromBlock.begin(); I != FromBlock.end();) {
262     // We have to increment I before moving Current as moving Current
263     // changes the list that I is iterating through.
264     auto Current = I;
265     ++I;
266     if (!NotHoisted.count(&*Current)) {
267       Current->moveBefore(ToBlock.getTerminator());
268     }
269   }
270   return true;
271 }
272 
273 namespace llvm {
274 
createSpeculativeExecutionPass()275 FunctionPass *createSpeculativeExecutionPass() {
276   return new SpeculativeExecution();
277 }
278 
createSpeculativeExecutionIfHasBranchDivergencePass()279 FunctionPass *createSpeculativeExecutionIfHasBranchDivergencePass() {
280   return new SpeculativeExecution(/* OnlyIfDivergentTarget = */ true);
281 }
282 
283 }  // namespace llvm
284