lib/MC/MCPseudoProbe.cpp

//===- lib/MC/MCPseudoProbe.cpp - Pseudo probe encoding support ----------===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "llvm/MC/MCPseudoProbe.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCObjectFileInfo.h"
#include "llvm/MC/MCObjectStreamer.h"
#include "llvm/MC/MCStreamer.h"

#define DEBUG_TYPE "mcpseudoprobe"

using namespace llvm;

#ifndef NDEBUG
int MCPseudoProbeTable::DdgPrintIndent = 0;
#endif

static const MCExpr *buildSymbolDiff(MCObjectStreamer *MCOS, const MCSymbol *A,
                                     const MCSymbol *B) {
  MCContext &Context = MCOS->getContext();
  MCSymbolRefExpr::VariantKind Variant = MCSymbolRefExpr::VK_None;
  const MCExpr *ARef = MCSymbolRefExpr::create(A, Variant, Context);
  const MCExpr *BRef = MCSymbolRefExpr::create(B, Variant, Context);
  const MCExpr *AddrDelta =
      MCBinaryExpr::create(MCBinaryExpr::Sub, ARef, BRef, Context);
  return AddrDelta;
}

void MCPseudoProbe::emit(MCObjectStreamer *MCOS,
                         const MCPseudoProbe *LastProbe) const {
  // Emit Index
  MCOS->emitULEB128IntValue(Index);
  // Emit Type and the flag:
  // Type (bit 0 to 3), with bit 4 to 6 for attributes.
  // Flag (bit 7, 0 - code address, 1 - address delta). This indicates whether
  // the following field is a symbolic code address or an address delta.
  assert(Type <= 0xF && "Probe type too big to encode, exceeding 15");
  assert(Attributes <= 0x7 &&
         "Probe attributes too big to encode, exceeding 7");
  uint8_t PackedType = Type | (Attributes << 4);
  uint8_t Flag = LastProbe ? ((int8_t)MCPseudoProbeFlag::AddressDelta << 7) : 0;
  MCOS->emitInt8(Flag | PackedType);

  if (LastProbe) {
    // Emit the delta between the address label and LastProbe.
    const MCExpr *AddrDelta =
        buildSymbolDiff(MCOS, Label, LastProbe->getLabel());
    int64_t Delta;
    if (AddrDelta->evaluateAsAbsolute(Delta, MCOS->getAssemblerPtr())) {
      MCOS->emitSLEB128IntValue(Delta);
    } else {
      MCOS->insert(new MCPseudoProbeAddrFragment(AddrDelta));
    }
  } else {
    // Emit label as a symbolic code address.
    MCOS->emitSymbolValue(
        Label, MCOS->getContext().getAsmInfo()->getCodePointerSize());
  }

  LLVM_DEBUG({
    dbgs().indent(MCPseudoProbeTable::DdgPrintIndent);
    dbgs() << "Probe: " << Index << "\n";
  });
}

MCPseudoProbeInlineTree::~MCPseudoProbeInlineTree() {
  for (auto &Inlinee : Inlinees)
    delete Inlinee.second;
}

MCPseudoProbeInlineTree *
MCPseudoProbeInlineTree::getOrAddNode(InlineSite Site) {
  auto Iter = Inlinees.find(Site);
  if (Iter == Inlinees.end()) {
    auto *Node = new MCPseudoProbeInlineTree(std::get<0>(Site));
    Inlinees[Site] = Node;
    return Node;
  } else {
    return Iter->second;
  }
}

void MCPseudoProbeInlineTree::addPseudoProbe(
    const MCPseudoProbe &Probe, const MCPseudoProbeInlineStack &InlineStack) {
  // The function should not be called on the root.
  assert(isRoot() && "Should not be called on root");

  // When it comes here, the input look like:
  //    Probe: GUID of C, ...
  //    InlineStack: [88, A], [66, B]
  // which means, Function A inlines function B at call site with a probe id of
  // 88, and B inlines C at probe 66. The tri-tree expects a tree path like {[0,
  // A], [88, B], [66, C]} to locate the tree node where the probe should be
  // added. Note that the edge [0, A] means A is the top-level function we are
  // emitting probes for.

  // Make a [0, A] edge.
  // An empty inline stack means the function that the probe originates from
  // is a top-level function.
  InlineSite Top;
  if (InlineStack.empty()) {
    Top = InlineSite(Probe.getGuid(), 0);
  } else {
    Top = InlineSite(std::get<0>(InlineStack.front()), 0);
  }

  auto *Cur = getOrAddNode(Top);

  // Make interior edges by walking the inline stack. Once it's done, Cur should
  // point to the node that the probe originates from.
  if (!InlineStack.empty()) {
    auto Iter = InlineStack.begin();
    auto Index = std::get<1>(*Iter);
    Iter++;
    for (; Iter != InlineStack.end(); Iter++) {
      // Make an edge by using the previous probe id and current GUID.
      Cur = Cur->getOrAddNode(InlineSite(std::get<0>(*Iter), Index));
      Index = std::get<1>(*Iter);
    }
    Cur = Cur->getOrAddNode(InlineSite(Probe.getGuid(), Index));
  }

  Cur->Probes.push_back(Probe);
}

void MCPseudoProbeInlineTree::emit(MCObjectStreamer *MCOS,
                                   const MCPseudoProbe *&LastProbe) {
  LLVM_DEBUG({
    dbgs().indent(MCPseudoProbeTable::DdgPrintIndent);
    dbgs() << "Group [\n";
    MCPseudoProbeTable::DdgPrintIndent += 2;
  });
  // Emit probes grouped by GUID.
  if (Guid != 0) {
    LLVM_DEBUG({
      dbgs().indent(MCPseudoProbeTable::DdgPrintIndent);
      dbgs() << "GUID: " << Guid << "\n";
    });
    // Emit Guid
    MCOS->emitInt64(Guid);
    // Emit number of probes in this node
    MCOS->emitULEB128IntValue(Probes.size());
    // Emit number of direct inlinees
    MCOS->emitULEB128IntValue(Inlinees.size());
    // Emit probes in this group
    for (const auto &Probe : Probes) {
      Probe.emit(MCOS, LastProbe);
      LastProbe = &Probe;
    }
  } else {
    assert(Probes.empty() && "Root should not have probes");
  }

  // Emit descendent
  for (const auto &Inlinee : Inlinees) {
    if (Guid) {
      // Emit probe index
      MCOS->emitULEB128IntValue(std::get<1>(Inlinee.first));
      LLVM_DEBUG({
        dbgs().indent(MCPseudoProbeTable::DdgPrintIndent);
        dbgs() << "InlineSite: " << std::get<1>(Inlinee.first) << "\n";
      });
    }
    // Emit the group
    Inlinee.second->emit(MCOS, LastProbe);
  }

  LLVM_DEBUG({
    MCPseudoProbeTable::DdgPrintIndent -= 2;
    dbgs().indent(MCPseudoProbeTable::DdgPrintIndent);
    dbgs() << "]\n";
  });
}

void MCPseudoProbeSection::emit(MCObjectStreamer *MCOS) {
  MCContext &Ctx = MCOS->getContext();

  for (auto &ProbeSec : MCProbeDivisions) {
    const MCPseudoProbe *LastProbe = nullptr;
    if (auto *S =
            Ctx.getObjectFileInfo()->getPseudoProbeSection(ProbeSec.first)) {
      // Switch to the .pseudoprobe section or a comdat group.
      MCOS->SwitchSection(S);
      // Emit probes grouped by GUID.
      ProbeSec.second.emit(MCOS, LastProbe);
    }
  }
}

//
// This emits the pseudo probe tables.
//
void MCPseudoProbeTable::emit(MCObjectStreamer *MCOS) {
  MCContext &Ctx = MCOS->getContext();
  auto &ProbeTable = Ctx.getMCPseudoProbeTable();

  // Bail out early so we don't switch to the pseudo_probe section needlessly
  // and in doing so create an unnecessary (if empty) section.
  auto &ProbeSections = ProbeTable.getProbeSections();
  if (ProbeSections.empty())
    return;

  LLVM_DEBUG(MCPseudoProbeTable::DdgPrintIndent = 0);

  // Put out the probe.
  ProbeSections.emit(MCOS);
}