llvm/lib/CodeGen/PseudoProbeInserter.cpp

   1 //===- PseudoProbeInserter.cpp - Insert annotation for callsite profiling -===//
   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 // This file implements PseudoProbeInserter pass, which inserts pseudo probe
  10 // annotations for call instructions with a pseudo-probe-specific dwarf
  11 // discriminator. such discriminator indicates that the call instruction comes
  12 // with a pseudo probe, and the discriminator value holds information to
  13 // identify the corresponding counter.
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/CodeGen/MachineBasicBlock.h"
  17 #include "llvm/CodeGen/MachineFunctionPass.h"
  18 #include "llvm/CodeGen/MachineInstr.h"
  19 #include "llvm/CodeGen/TargetInstrInfo.h"
  20 #include "llvm/IR/DebugInfoMetadata.h"
  21 #include "llvm/IR/PseudoProbe.h"
  22 #include "llvm/InitializePasses.h"
  23 #include "llvm/MC/MCPseudoProbe.h"
  24 #include "llvm/Target/TargetMachine.h"
  25 #include <unordered_set>
  26
  27 #define DEBUG_TYPE "pseudo-probe-inserter"
  28
  29 using namespace llvm;
  30
  31 namespace {
  32 class PseudoProbeInserter : public MachineFunctionPass {
  33 public:
  34   static char ID;
  35
  36   PseudoProbeInserter() : MachineFunctionPass(ID) {
  37     initializePseudoProbeInserterPass(*PassRegistry::getPassRegistry());
  38   }
  39
  40   StringRef getPassName() const override { return "Pseudo Probe Inserter"; }
  41
  42   void getAnalysisUsage(AnalysisUsage &AU) const override {
  43     AU.setPreservesAll();
  44     MachineFunctionPass::getAnalysisUsage(AU);
  45   }
  46
  47   bool runOnMachineFunction(MachineFunction &MF) override {
  48     const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
  49     bool Changed = false;
  50     for (MachineBasicBlock &MBB : MF) {
  51       MachineInstr *FirstInstr = nullptr;
  52       for (MachineInstr &MI : MBB) {
  53         if (!MI.isPseudo())
  54           FirstInstr = &MI;
  55         if (MI.isCall()) {
  56           if (DILocation *DL = MI.getDebugLoc()) {
  57             auto Value = DL->getDiscriminator();
  58             if (DILocation::isPseudoProbeDiscriminator(Value)) {
  59               BuildMI(MBB, MI, DL, TII->get(TargetOpcode::PSEUDO_PROBE))
  60                   .addImm(getFuncGUID(MF.getFunction().getParent(), DL))
  61                   .addImm(
  62                       PseudoProbeDwarfDiscriminator::extractProbeIndex(Value))
  63                   .addImm(
  64                       PseudoProbeDwarfDiscriminator::extractProbeType(Value))
  65                   .addImm(PseudoProbeDwarfDiscriminator::extractProbeAttributes(
  66                       Value));
  67               Changed = true;
  68             }
  69           }
  70         }
  71       }
  72
  73       // Walk the block backwards, move PSEUDO_PROBE before the first real
  74       // instruction to fix out-of-order probes. There is a problem with probes
  75       // as the terminator of the block. During the offline counts processing,
  76       // the samples collected on the first physical instruction following a
  77       // probe will be counted towards the probe. This logically equals to
  78       // treating the instruction next to a probe as if it is from the same
  79       // block of the probe. This is accurate most of the time unless the
  80       // instruction can be reached from multiple flows, which means it actually
  81       // starts a new block. Samples collected on such probes may cause
  82       // imprecision with the counts inference algorithm. Fortunately, if
  83       // there are still other native instructions preceding the probe we can
  84       // use them as a place holder to collect samples for the probe.
  85       if (FirstInstr) {
  86         auto MII = MBB.rbegin();
  87         while (MII != MBB.rend()) {
  88           // Skip all pseudo probes followed by a real instruction since they
  89           // are not dangling.
  90           if (!MII->isPseudo())
  91             break;
  92           auto Cur = MII++;
  93           if (Cur->getOpcode() != TargetOpcode::PSEUDO_PROBE)
  94             continue;
  95           // Move the dangling probe before FirstInstr.
  96           auto *ProbeInstr = &*Cur;
  97           MBB.remove(ProbeInstr);
  98           MBB.insert(FirstInstr, ProbeInstr);
  99           Changed = true;
 100         }
 101       } else {
 102         // Probes not surrounded by any real instructions in the same block are
 103         // called dangling probes. Since there's no good way to pick up a sample
 104         // collection point for dangling probes at compile time, they are being
 105         // removed so that the profile correlation tool will not report any
 106         // samples collected for them and it's up to the counts inference tool
 107         // to get them a reasonable count.
 108         SmallVector<MachineInstr *, 4> ToBeRemoved;
 109         for (MachineInstr &MI : MBB) {
 110           if (MI.isPseudoProbe())
 111             ToBeRemoved.push_back(&MI);
 112         }
 113
 114         for (auto *MI : ToBeRemoved)
 115           MI->eraseFromParent();
 116
 117         Changed |= !ToBeRemoved.empty();
 118       }
 119     }
 120
 121     return Changed;
 122   }
 123
 124 private:
 125   uint64_t getFuncGUID(Module *M, DILocation *DL) {
 126     auto *SP = DL->getScope()->getSubprogram();
 127     auto Name = SP->getLinkageName();
 128     if (Name.empty())
 129       Name = SP->getName();
 130     return Function::getGUID(Name);
 131   }
 132 };
 133 } // namespace
 134
 135 char PseudoProbeInserter::ID = 0;
 136 INITIALIZE_PASS_BEGIN(PseudoProbeInserter, DEBUG_TYPE,
 137                       "Insert pseudo probe annotations for value profiling",
 138                       false, false)
 139 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
 140 INITIALIZE_PASS_END(PseudoProbeInserter, DEBUG_TYPE,
 141                     "Insert pseudo probe annotations for value profiling",
 142                     false, false)
 143
 144 FunctionPass *llvm::createPseudoProbeInserter() {
 145   return new PseudoProbeInserter();
 146 }