PseudoProbeInserter.cpp

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//===- PseudoProbeInserter.cpp - Insert annotation for callsite profiling -===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements PseudoProbeInserter pass, which inserts pseudo probe
// annotations for call instructions with a pseudo-probe-specific dwarf
// discriminator. such discriminator indicates that the call instruction comes
// with a pseudo probe, and the discriminator value holds information to
// identify the corresponding counter.
//===----------------------------------------------------------------------===//
 
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PseudoProbe.h"
#include "llvm/InitializePasses.h"
 
#define DEBUG_TYPE "pseudo-probe-inserter"
 
using namespace llvm;
 
namespace {
class PseudoProbeInserter : public MachineFunctionPass {
public:
  static char ID;
 
  PseudoProbeInserter() : MachineFunctionPass(ID) {
    initializePseudoProbeInserterPass(*PassRegistry::getPassRegistry());
  }
 
  StringRef getPassName() const override { return "Pseudo Probe Inserter"; }
 
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.setPreservesAll();
    MachineFunctionPass::getAnalysisUsage(AU);
  }
 
  bool doInitialization(Module &M) override {
    ShouldRun = M.getNamedMetadata(PseudoProbeDescMetadataName);
    return false;
  }
 
  bool runOnMachineFunction(MachineFunction &MF) override {
    if (!ShouldRun)
      return false;
    const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
    bool Changed = false;
    for (MachineBasicBlock &MBB : MF) {
      MachineInstr *FirstInstr = nullptr;
      for (MachineInstr &MI : MBB) {
        if (!MI.isPseudo())
          FirstInstr = &MI;
        if (MI.isCall()) {
          if (DILocation *DL = MI.getDebugLoc()) {
            auto Value = DL->getDiscriminator();
            if (DILocation::isPseudoProbeDiscriminator(Value)) {
              BuildMI(MBB, MI, DL, TII->get(TargetOpcode::PSEUDO_PROBE))
                  .addImm(getFuncGUID(MF.getFunction().getParent(), DL))
                  .addImm(
                      PseudoProbeDwarfDiscriminator::extractProbeIndex(Value))
                  .addImm(
                      PseudoProbeDwarfDiscriminator::extractProbeType(Value))
                  .addImm(PseudoProbeDwarfDiscriminator::extractProbeAttributes(
                      Value));
              Changed = true;
            }
          }
        }
      }
 
      // Walk the block backwards, move PSEUDO_PROBE before the first real
      // instruction to fix out-of-order probes. There is a problem with probes
      // as the terminator of the block. During the offline counts processing,
      // the samples collected on the first physical instruction following a
      // probe will be counted towards the probe. This logically equals to
      // treating the instruction next to a probe as if it is from the same
      // block of the probe. This is accurate most of the time unless the
      // instruction can be reached from multiple flows, which means it actually
      // starts a new block. Samples collected on such probes may cause
      // imprecision with the counts inference algorithm. Fortunately, if
      // there are still other native instructions preceding the probe we can
      // use them as a place holder to collect samples for the probe.
      if (FirstInstr) {
        auto MII = MBB.rbegin();
        while (MII != MBB.rend()) {
          // Skip all pseudo probes followed by a real instruction since they
          // are not dangling.
          if (!MII->isPseudo())
            break;
          auto Cur = MII++;
          if (Cur->getOpcode() != TargetOpcode::PSEUDO_PROBE)
            continue;
          // Move the dangling probe before FirstInstr.
          auto *ProbeInstr = &*Cur;
          MBB.remove(ProbeInstr);
          MBB.insert(FirstInstr, ProbeInstr);
          Changed = true;
        }
      } else {
        // Probes not surrounded by any real instructions in the same block are
        // called dangling probes. Since there's no good way to pick up a sample
        // collection point for dangling probes at compile time, they are being
        // removed so that the profile correlation tool will not report any
        // samples collected for them and it's up to the counts inference tool
        // to get them a reasonable count.
        SmallVector<MachineInstr *, 4> ToBeRemoved;
        for (MachineInstr &MI : MBB) {
          if (MI.isPseudoProbe())
            ToBeRemoved.push_back(&MI);
        }
 
        for (auto *MI : ToBeRemoved)
          MI->eraseFromParent();
 
        Changed |= !ToBeRemoved.empty();
      }
    }
 
    return Changed;
  }
 
private:
  uint64_t getFuncGUID(Module *M, DILocation *DL) {
    auto Name = DL->getSubprogramLinkageName();
    return Function::getGUIDAssumingExternalLinkage(Name);
  }
 
  bool ShouldRun = false;
};
} // namespace
 
char PseudoProbeInserter::ID = 0;
INITIALIZE_PASS_BEGIN(PseudoProbeInserter, DEBUG_TYPE,
                      "Insert pseudo probe annotations for value profiling",
                      false, false)
INITIALIZE_PASS_DEPENDENCY(TargetPassConfig)
INITIALIZE_PASS_END(PseudoProbeInserter, DEBUG_TYPE,
                    "Insert pseudo probe annotations for value profiling",
                    false, false)
 
FunctionPass *llvm::createPseudoProbeInserter() {
  return new PseudoProbeInserter();
}