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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 //===----------------------------------------------------------------------===//
     54 
     55 #include "llvm/ADT/SmallSet.h"
     56 #include "llvm/Analysis/TargetTransformInfo.h"
     57 #include "llvm/Analysis/ValueTracking.h"
     58 #include "llvm/IR/Instructions.h"
     59 #include "llvm/IR/Module.h"
     60 #include "llvm/IR/Operator.h"
     61 #include "llvm/Support/CommandLine.h"
     62 #include "llvm/Support/Debug.h"
     63 
     64 using namespace llvm;
     65 
     66 #define DEBUG_TYPE "speculative-execution"
     67 
     68 // The risk that speculation will not pay off increases with the
     69 // number of instructions speculated, so we put a limit on that.
     70 static cl::opt<unsigned> SpecExecMaxSpeculationCost(
     71     "spec-exec-max-speculation-cost", cl::init(7), cl::Hidden,
     72     cl::desc("Speculative execution is not applied to basic blocks where "
     73              "the cost of the instructions to speculatively execute "
     74              "exceeds this limit."));
     75 
     76 // Speculating just a few instructions from a larger block tends not
     77 // to be profitable and this limit prevents that. A reason for that is
     78 // that small basic blocks are more likely to be candidates for
     79 // further optimization.
     80 static cl::opt<unsigned> SpecExecMaxNotHoisted(
     81     "spec-exec-max-not-hoisted", cl::init(5), cl::Hidden,
     82     cl::desc("Speculative execution is not applied to basic blocks where the "
     83              "number of instructions that would not be speculatively executed "
     84              "exceeds this limit."));
     85 
     86 namespace {
     87 class SpeculativeExecution : public FunctionPass {
     88  public:
     89   static char ID;
     90   SpeculativeExecution(): FunctionPass(ID) {}
     91 
     92   void getAnalysisUsage(AnalysisUsage &AU) const override;
     93   bool runOnFunction(Function &F) override;
     94 
     95  private:
     96   bool runOnBasicBlock(BasicBlock &B);
     97   bool considerHoistingFromTo(BasicBlock &FromBlock, BasicBlock &ToBlock);
     98 
     99   const TargetTransformInfo *TTI = nullptr;
    100 };
    101 } // namespace
    102 
    103 char SpeculativeExecution::ID = 0;
    104 INITIALIZE_PASS_BEGIN(SpeculativeExecution, "speculative-execution",
    105                       "Speculatively execute instructions", false, false)
    106 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
    107 INITIALIZE_PASS_END(SpeculativeExecution, "speculative-execution",
    108                       "Speculatively execute instructions", false, false)
    109 
    110 void SpeculativeExecution::getAnalysisUsage(AnalysisUsage &AU) const {
    111   AU.addRequired<TargetTransformInfoWrapperPass>();
    112 }
    113 
    114 bool SpeculativeExecution::runOnFunction(Function &F) {
    115   if (skipOptnoneFunction(F))
    116     return false;
    117 
    118   TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
    119 
    120   bool Changed = false;
    121   for (auto& B : F) {
    122     Changed |= runOnBasicBlock(B);
    123   }
    124   return Changed;
    125 }
    126 
    127 bool SpeculativeExecution::runOnBasicBlock(BasicBlock &B) {
    128   BranchInst *BI = dyn_cast<BranchInst>(B.getTerminator());
    129   if (BI == nullptr)
    130     return false;
    131 
    132   if (BI->getNumSuccessors() != 2)
    133     return false;
    134   BasicBlock &Succ0 = *BI->getSuccessor(0);
    135   BasicBlock &Succ1 = *BI->getSuccessor(1);
    136 
    137   if (&B == &Succ0 || &B == &Succ1 || &Succ0 == &Succ1) {
    138     return false;
    139   }
    140 
    141   // Hoist from if-then (triangle).
    142   if (Succ0.getSinglePredecessor() != nullptr &&
    143       Succ0.getSingleSuccessor() == &Succ1) {
    144     return considerHoistingFromTo(Succ0, B);
    145   }
    146 
    147   // Hoist from if-else (triangle).
    148   if (Succ1.getSinglePredecessor() != nullptr &&
    149       Succ1.getSingleSuccessor() == &Succ0) {
    150     return considerHoistingFromTo(Succ1, B);
    151   }
    152 
    153   // Hoist from if-then-else (diamond), but only if it is equivalent to
    154   // an if-else or if-then due to one of the branches doing nothing.
    155   if (Succ0.getSinglePredecessor() != nullptr &&
    156       Succ1.getSinglePredecessor() != nullptr &&
    157       Succ1.getSingleSuccessor() != nullptr &&
    158       Succ1.getSingleSuccessor() != &B &&
    159       Succ1.getSingleSuccessor() == Succ0.getSingleSuccessor()) {
    160     // If a block has only one instruction, then that is a terminator
    161     // instruction so that the block does nothing. This does happen.
    162     if (Succ1.size() == 1) // equivalent to if-then
    163       return considerHoistingFromTo(Succ0, B);
    164     if (Succ0.size() == 1) // equivalent to if-else
    165       return considerHoistingFromTo(Succ1, B);
    166   }
    167 
    168   return false;
    169 }
    170 
    171 static unsigned ComputeSpeculationCost(const Instruction *I,
    172                                        const TargetTransformInfo &TTI) {
    173   switch (Operator::getOpcode(I)) {
    174     case Instruction::GetElementPtr:
    175     case Instruction::Add:
    176     case Instruction::Mul:
    177     case Instruction::And:
    178     case Instruction::Or:
    179     case Instruction::Select:
    180     case Instruction::Shl:
    181     case Instruction::Sub:
    182     case Instruction::LShr:
    183     case Instruction::AShr:
    184     case Instruction::Xor:
    185     case Instruction::ZExt:
    186     case Instruction::SExt:
    187       return TTI.getUserCost(I);
    188 
    189     default:
    190       return UINT_MAX; // Disallow anything not whitelisted.
    191   }
    192 }
    193 
    194 bool SpeculativeExecution::considerHoistingFromTo(BasicBlock &FromBlock,
    195                                                   BasicBlock &ToBlock) {
    196   SmallSet<const Instruction *, 8> NotHoisted;
    197   const auto AllPrecedingUsesFromBlockHoisted = [&NotHoisted](User *U) {
    198     for (Value* V : U->operand_values()) {
    199       if (Instruction *I = dyn_cast<Instruction>(V)) {
    200         if (NotHoisted.count(I) > 0)
    201           return false;
    202       }
    203     }
    204     return true;
    205   };
    206 
    207   unsigned TotalSpeculationCost = 0;
    208   for (auto& I : FromBlock) {
    209     const unsigned Cost = ComputeSpeculationCost(&I, *TTI);
    210     if (Cost != UINT_MAX && isSafeToSpeculativelyExecute(&I) &&
    211         AllPrecedingUsesFromBlockHoisted(&I)) {
    212       TotalSpeculationCost += Cost;
    213       if (TotalSpeculationCost > SpecExecMaxSpeculationCost)
    214         return false;  // too much to hoist
    215     } else {
    216       NotHoisted.insert(&I);
    217       if (NotHoisted.size() > SpecExecMaxNotHoisted)
    218         return false; // too much left behind
    219     }
    220   }
    221 
    222   if (TotalSpeculationCost == 0)
    223     return false; // nothing to hoist
    224 
    225   for (auto I = FromBlock.begin(); I != FromBlock.end();) {
    226     // We have to increment I before moving Current as moving Current
    227     // changes the list that I is iterating through.
    228     auto Current = I;
    229     ++I;
    230     if (!NotHoisted.count(&*Current)) {
    231       Current->moveBefore(ToBlock.getTerminator());
    232     }
    233   }
    234   return true;
    235 }
    236 
    237 namespace llvm {
    238 
    239 FunctionPass *createSpeculativeExecutionPass() {
    240   return new SpeculativeExecution();
    241 }
    242 
    243 }  // namespace llvm
    244