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/Module.h"
  22 #include "llvm/IR/PseudoProbe.h"
  23 #include "llvm/InitializePasses.h"
  24
  25 #define DEBUG_TYPE "pseudo-probe-inserter"
  26
  27 using namespace llvm;
  28
  29 namespace {
  30 class PseudoProbeInserter : public MachineFunctionPass {
  31 public:
  32   static char ID;
  33
  34   PseudoProbeInserter() : MachineFunctionPass(ID) {
  35     initializePseudoProbeInserterPass(*PassRegistry::getPassRegistry());
  36   }
  37
  38   StringRef getPassName() const override { return "Pseudo Probe Inserter"; }
  39
  40   void getAnalysisUsage(AnalysisUsage &AU) const override {
  41     AU.setPreservesAll();
  42     MachineFunctionPass::getAnalysisUsage(AU);
  43   }
  44
  45   bool doInitialization(Module &M) override {
  46     ShouldRun = M.getNamedMetadata(PseudoProbeDescMetadataName);
  47     return false;
  48   }
  49
  50   bool runOnMachineFunction(MachineFunction &MF) override {
  51     if (!ShouldRun)
  52       return false;
  53     const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
  54     bool Changed = false;
  55     for (MachineBasicBlock &MBB : MF) {
  56       MachineInstr *FirstInstr = nullptr;
  57       for (MachineInstr &MI : MBB) {
  58         if (!MI.isPseudo())
  59           FirstInstr = &MI;
  60         if (MI.isCall()) {
  61           if (DILocation *DL = MI.getDebugLoc()) {
  62             auto Value = DL->getDiscriminator();
  63             if (DILocation::isPseudoProbeDiscriminator(Value)) {
  64               BuildMI(MBB, MI, DL, TII->get(TargetOpcode::PSEUDO_PROBE))
  65                   .addImm(getFuncGUID(MF.getFunction().getParent(), DL))
  66                   .addImm(
  67                       PseudoProbeDwarfDiscriminator::extractProbeIndex(Value))
  68                   .addImm(
  69                       PseudoProbeDwarfDiscriminator::extractProbeType(Value))
  70                   .addImm(PseudoProbeDwarfDiscriminator::extractProbeAttributes(
  71                       Value));
  72               Changed = true;
  73             }
  74           }
  75         }
  76       }
  77
  78       // Walk the block backwards, move PSEUDO_PROBE before the first real
  79       // instruction to fix out-of-order probes. There is a problem with probes
  80       // as the terminator of the block. During the offline counts processing,
  81       // the samples collected on the first physical instruction following a
  82       // probe will be counted towards the probe. This logically equals to
  83       // treating the instruction next to a probe as if it is from the same
  84       // block of the probe. This is accurate most of the time unless the
  85       // instruction can be reached from multiple flows, which means it actually
  86       // starts a new block. Samples collected on such probes may cause
  87       // imprecision with the counts inference algorithm. Fortunately, if
  88       // there are still other native instructions preceding the probe we can
  89       // use them as a place holder to collect samples for the probe.
  90       if (FirstInstr) {
  91         auto MII = MBB.rbegin();
  92         while (MII != MBB.rend()) {
  93           // Skip all pseudo probes followed by a real instruction since they
  94           // are not dangling.
  95           if (!MII->isPseudo())
  96             break;
  97           auto Cur = MII++;
  98           if (Cur->getOpcode() != TargetOpcode::PSEUDO_PROBE)
  99             continue;
 100           // Move the dangling probe before FirstInstr.
 101           auto *ProbeInstr = &*Cur;
 102           MBB.remove(ProbeInstr);
 103           MBB.insert(FirstInstr, ProbeInstr);
 104           Changed = true;
 105         }
 106       } else {
 107         // Probes not surrounded by any real instructions in the same block are
 108         // called dangling probes. Since there's no good way to pick up a sample
 109         // collection point for dangling probes at compile time, they are being
 110         // removed so that the profile correlation tool will not report any
 111         // samples collected for them and it's up to the counts inference tool
 112         // to get them a reasonable count.
 113         SmallVector<MachineInstr *, 4> ToBeRemoved;
 114         for (MachineInstr &MI : MBB) {
 115           if (MI.isPseudoProbe())
 116             ToBeRemoved.push_back(&MI);
 117         }
 118
 119         for (auto *MI : ToBeRemoved)
 120           MI->eraseFromParent();
 121
 122         Changed |= !ToBeRemoved.empty();
 123       }
 124     }
 125
 126     return Changed;
 127   }
 128
 129 private:
 130   uint64_t getFuncGUID(Module *M, DILocation *DL) {
 131     auto Name = DL->getSubprogramLinkageName();
 132     return Function::getGUID(Name);
 133   }
 134
 135   bool ShouldRun = false;
 136 };
 137 } // namespace
 138
 139 char PseudoProbeInserter::ID = 0;
 140 INITIALIZE_PASS_BEGIN(PseudoProbeInserter, DEBUG_TYPE,
 141                       "Insert pseudo probe annotations for value profiling",
 142                       false, false)
 143 INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
 144 INITIALIZE_PASS_END(PseudoProbeInserter, DEBUG_TYPE,
 145                     "Insert pseudo probe annotations for value profiling",
 146                     false, false)
 147
 148 FunctionPass *llvm::createPseudoProbeInserter() {
 149   return new PseudoProbeInserter();
 150 }