lib/xray/xray_fdr_logging.h

//===-- xray_fdr_logging.h ------------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file is a part of XRay, a function call tracing system.
//
//===----------------------------------------------------------------------===//
#ifndef XRAY_XRAY_FDR_LOGGING_H
#define XRAY_XRAY_FDR_LOGGING_H

#include "xray/xray_log_interface.h"

// FDR (Flight Data Recorder) Mode
// ===============================
//
// The XRay whitepaper describes a mode of operation for function call trace
// logging that involves writing small records into an in-memory circular
// buffer, that then gets logged to disk on demand. To do this efficiently and
// capture as much data as we can, we use smaller records compared to the
// default mode of always writing fixed-size records.

namespace __xray {

enum class RecordType : uint8_t {
  Function, Metadata
};

// A MetadataRecord encodes the kind of record in its first byte, and have 15
// additional bytes in the end to hold free-form data.
struct alignas(16) MetadataRecord {
  // A MetadataRecord must always have a type of 1.
  /* RecordType */ uint8_t Type : 1;

  // Each kind of record is represented as a 7-bit value (even though we use an
  // unsigned 8-bit enum class to do so).
  enum class RecordKinds : uint8_t {
    NewBuffer,
    EndOfBuffer,
    NewCPUId,
    TSCWrap,
    WalltimeMarker,
  };
  // Use 7 bits to identify this record type.
  /* RecordKinds */ uint8_t RecordKind : 7;
  char Data[15];
} __attribute__((packed));

static_assert(sizeof(MetadataRecord) == 16, "Wrong size for MetadataRecord.");

struct alignas(8) FunctionRecord {
  // A FunctionRecord must always have a type of 0.
  /* RecordType */ uint8_t Type : 1;
  enum class RecordKinds {
    FunctionEnter = 0x00,
    FunctionExit = 0x01,
    FunctionTailExit = 0x02,
  };
  /* RecordKinds */ uint8_t RecordKind : 3;

  // We only use 28 bits of the function ID, so that we can use as few bytes as
  // possible. This means we only support 2^28 (268,435,456) unique function ids
  // in a single binary.
  int FuncId : 28;

  // We use another 4 bytes to hold the delta between the previous entry's TSC.
  // In case we've found that the distance is greater than the allowable 32 bits
  // (either because we are running in a different CPU and the TSC might be
  // different then), we should use a MetadataRecord before this FunctionRecord
  // that will contain the full TSC for that CPU, and keep this to 0.
  uint32_t TSCDelta;
} __attribute__((packed));

static_assert(sizeof(FunctionRecord) == 8, "Wrong size for FunctionRecord.");

// Options used by the FDR implementation.
struct FDRLoggingOptions {
  bool ReportErrors = false;
  int Fd = -1;
};

// Flight Data Recorder mode implementation interfaces.
XRayLogInitStatus fdrLoggingInit(size_t BufferSize, size_t BufferMax,
                                  void *Options, size_t OptionsSize);
XRayLogInitStatus fdrLoggingFinalize();
void fdrLoggingHandleArg0(int32_t FuncId, XRayEntryType Entry);
XRayLogFlushStatus fdrLoggingFlush();
XRayLogInitStatus fdrLoggingReset();

} // namespace __xray

#endif // XRAY_XRAY_FDR_LOGGING_H