docs/PDB/TpiStream.rst

=====================================
The PDB TPI and IPI Streams
=====================================

.. contents::
   :local:

.. _tpi_intro:

Introduction
============

The PDB TPI Stream (Index 2) and IPI Stream (Index 4) contain information about
all types used in the program.  It is organized as a :ref:`header <tpi_header>`
followed by a list of :doc:`CodeView Type Records <CodeViewTypes>`.  Types are
referenced from various streams and records throughout the PDB by their
:ref:`type index <type_indices>`.  In general, the sequence of type records
following the :ref:`header <tpi_header>` forms a topologically sorted DAG
(directed acyclic graph), which means that a type record B can only refer to
the type A if ``A.TypeIndex < B.TypeIndex``.  While there are rare cases where
this property will not hold (particularly when dealing with object files
compiled with MASM), an implementation should try very hard to make this
property hold, as it means the entire type graph can be constructed in a single
pass.

.. important::
   Type records form a topologically sorted DAG (directed acyclic graph).
   
.. _tpi_ipi:

TPI vs IPI Stream
=================

Recent versions of the PDB format (aka all versions covered by this document)
have 2 streams with identical layout, henceforth referred to as the TPI stream
and IPI stream.  Subsequent contents of this document describing the on-disk
format apply equally whether it is for the TPI Stream or the IPI Stream.  The
only difference between the two is in *which* CodeView records are allowed to
appear in each one, summarized by the following table:

+----------------------+---------------------+
|    TPI Stream        |    IPI Stream       |
+======================+=====================+
|  LF_POINTER          | LF_FUNC_ID          |
+----------------------+---------------------+
|  LF_MODIFIER         | LF_MFUNC_ID         |
+----------------------+---------------------+
|  LF_PROCEDURE        | LF_BUILDINFO        |
+----------------------+---------------------+
|  LF_MFUNCTION        | LF_SUBSTR_LIST      |
+----------------------+---------------------+
|  LF_LABEL            | LF_STRING_ID        |
+----------------------+---------------------+
|  LF_ARGLIST          | LF_UDT_SRC_LINE     |
+----------------------+---------------------+
|  LF_FIELDLIST        | LF_UDT_MOD_SRC_LINE |
+----------------------+---------------------+
|  LF_ARRAY            |                     |
+----------------------+---------------------+
|  LF_CLASS            |                     |
+----------------------+---------------------+
|  LF_STRUCTURE        |                     |
+----------------------+---------------------+
|  LF_INTERFACE        |                     |
+----------------------+---------------------+
|  LF_UNION            |                     |
+----------------------+---------------------+
|  LF_ENUM             |                     |
+----------------------+---------------------+
|  LF_TYPESERVER2      |                     |
+----------------------+---------------------+
|  LF_VFTABLE          |                     |
+----------------------+---------------------+
|  LF_VTSHAPE          |                     |
+----------------------+---------------------+
|  LF_BITFIELD         |                     |
+----------------------+---------------------+
|  LF_METHODLIST       |                     |
+----------------------+---------------------+
|  LF_PRECOMP          |                     |
+----------------------+---------------------+
|  LF_ENDPRECOMP       |                     |
+----------------------+---------------------+

The usage of these records is described in more detail in
:doc:`CodeView Type Records <CodeViewTypes>`.

.. _type_indices:

Type Indices
============

A type index is a 32-bit integer that uniquely identifies a type inside of an
object file's ``.debug$T`` section or a PDB file's TPI or IPI stream.  The
value of the type index for the first type record from the TPI stream is given
by the ``TypeIndexBegin`` member of the :ref:`TPI Stream Header <tpi_header>`
although in practice this value is always equal to 0x1000 (4096).

Any type index with a high bit set is considered to come from the IPI stream,
although this appears to be more of a hack, and LLVM does not generate type
indices of this nature.  They can, however, be observed in Microsoft PDBs
occasionally, so one should be prepared to handle them.  Note that having the
high bit set is not a necessary condition to determine whether a type index
comes from the IPI stream, it is only sufficient.

Once the high bit is cleared, any type index >= ``TypeIndexBegin`` is presumed
to come from the appropriate stream, and any type index less than this is a
bitmask which can be decomposed as follows:

.. code-block:: none

  .---------------------------.------.----------.
  |           Unused          | Mode |   Kind   |
  '---------------------------'------'----------'
  |+32                        |+12   |+8        |+0


- **Kind** - A value from the following enum:

.. code-block:: c++

  enum class SimpleTypeKind : uint32_t {
    None = 0x0000,          // uncharacterized type (no type)
    Void = 0x0003,          // void
    NotTranslated = 0x0007, // type not translated by cvpack
    HResult = 0x0008,       // OLE/COM HRESULT

    SignedCharacter = 0x0010,   // 8 bit signed
    UnsignedCharacter = 0x0020, // 8 bit unsigned
    NarrowCharacter = 0x0070,   // really a char
    WideCharacter = 0x0071,     // wide char
    Character16 = 0x007a,       // char16_t
    Character32 = 0x007b,       // char32_t

    SByte = 0x0068,       // 8 bit signed int
    Byte = 0x0069,        // 8 bit unsigned int
    Int16Short = 0x0011,  // 16 bit signed
    UInt16Short = 0x0021, // 16 bit unsigned
    Int16 = 0x0072,       // 16 bit signed int
    UInt16 = 0x0073,      // 16 bit unsigned int
    Int32Long = 0x0012,   // 32 bit signed
    UInt32Long = 0x0022,  // 32 bit unsigned
    Int32 = 0x0074,       // 32 bit signed int
    UInt32 = 0x0075,      // 32 bit unsigned int
    Int64Quad = 0x0013,   // 64 bit signed
    UInt64Quad = 0x0023,  // 64 bit unsigned
    Int64 = 0x0076,       // 64 bit signed int
    UInt64 = 0x0077,      // 64 bit unsigned int
    Int128Oct = 0x0014,   // 128 bit signed int
    UInt128Oct = 0x0024,  // 128 bit unsigned int
    Int128 = 0x0078,      // 128 bit signed int
    UInt128 = 0x0079,     // 128 bit unsigned int

    Float16 = 0x0046,                 // 16 bit real
    Float32 = 0x0040,                 // 32 bit real
    Float32PartialPrecision = 0x0045, // 32 bit PP real
    Float48 = 0x0044,                 // 48 bit real
    Float64 = 0x0041,                 // 64 bit real
    Float80 = 0x0042,                 // 80 bit real
    Float128 = 0x0043,                // 128 bit real

    Complex16 = 0x0056,                 // 16 bit complex
    Complex32 = 0x0050,                 // 32 bit complex
    Complex32PartialPrecision = 0x0055, // 32 bit PP complex
    Complex48 = 0x0054,                 // 48 bit complex
    Complex64 = 0x0051,                 // 64 bit complex
    Complex80 = 0x0052,                 // 80 bit complex
    Complex128 = 0x0053,                // 128 bit complex

    Boolean8 = 0x0030,   // 8 bit boolean
    Boolean16 = 0x0031,  // 16 bit boolean
    Boolean32 = 0x0032,  // 32 bit boolean
    Boolean64 = 0x0033,  // 64 bit boolean
    Boolean128 = 0x0034, // 128 bit boolean
  };

- **Mode** - A value from the following enum:

.. code-block:: c++

  enum class SimpleTypeMode : uint32_t {
    Direct = 0,        // Not a pointer
    NearPointer = 1,   // Near pointer
    FarPointer = 2,    // Far pointer
    HugePointer = 3,   // Huge pointer
    NearPointer32 = 4, // 32 bit near pointer
    FarPointer32 = 5,  // 32 bit far pointer
    NearPointer64 = 6, // 64 bit near pointer
    NearPointer128 = 7 // 128 bit near pointer
  };
  
Note that for pointers, the bitness is represented in the mode.  So a ``void*``
would have a type index with ``Mode=NearPointer32, Kind=Void`` if built for 32-bits
but a type index with ``Mode=NearPointer64, Kind=Void`` if built for 64-bits.

By convention, the type index for ``std::nullptr_t`` is constructed the same way
as the type index for ``void*``, but using the bitless enumeration value
``NearPointer``.



.. _tpi_header:

Stream Header
=============
At offset 0 of the TPI Stream is a header with the following layout:


.. code-block:: c++

  struct TpiStreamHeader {
    uint32_t Version;
    uint32_t HeaderSize;
    uint32_t TypeIndexBegin;
    uint32_t TypeIndexEnd;
    uint32_t TypeRecordBytes;

    uint16_t HashStreamIndex;
    uint16_t HashAuxStreamIndex;
    uint32_t HashKeySize;
    uint32_t NumHashBuckets;

    int32_t HashValueBufferOffset;
    uint32_t HashValueBufferLength;
    
    int32_t IndexOffsetBufferOffset;
    uint32_t IndexOffsetBufferLength;

    int32_t HashAdjBufferOffset;
    uint32_t HashAdjBufferLength;
  };
  
- **Version** - A value from the following enum.

.. code-block:: c++

  enum class TpiStreamVersion : uint32_t {
    V40 = 19950410,
    V41 = 19951122,
    V50 = 19961031,
    V70 = 19990903,
    V80 = 20040203,
  };

Similar to the :doc:`PDB Stream <PdbStream>`, this value always appears to be
``V80``, and no other values have been observed.  It is assumed that should
another value be observed, the layout described by this document may not be
accurate.

- **HeaderSize** - ``sizeof(TpiStreamHeader)``
  
- **TypeIndexBegin** - The numeric value of the type index representing the
  first type record in the TPI stream.  This is usually the value 0x1000 as type
  indices lower than this are reserved (see :ref:`Type Indices <type_indices>` for
  a discussion of reserved type indices).
  
- **TypeIndexEnd** - One greater than the numeric value of the type index
  representing the last type record in the TPI stream.  The total number of type
  records in the TPI stream can be computed as ``TypeIndexEnd - TypeIndexBegin``.
  
- **TypeRecordBytes** - The number of bytes of type record data following the header.
  
- **HashStreamIndex** - The index of a stream which contains a list of hashes for
  every type record.  This value may be -1, indicating that hash information is not
  present.  In practice a valid stream index is always observed, so any producer
  implementation should be prepared to emit this stream to ensure compatibility with
  tools which may expect it to be present.
  
- **HashAuxStreamIndex** - Presumably the index of a stream which contains a separate
  hash table, although this has not been observed in practice and it's unclear what it
  might be used for.
  
- **HashKeySize** - The size of a hash value (usually 4 bytes).

- **NumHashBuckets** - The number of buckets used to generate the hash values in the
  aforementioned hash streams.

- **HashValueBufferOffset / HashValueBufferLength** - The offset and size within
  the TPI Hash Stream of the list of hash values.  It should be assumed that there
  are either 0 hash values, or a number equal to the number of type records in the
  TPI stream (``TypeIndexEnd - TypeEndBegin``).  Thus, if ``HashBufferLength`` is
  not equal to ``(TypeIndexEnd - TypeEndBegin) * HashKeySize`` we can consider the
  PDB malformed.

- **IndexOffsetBufferOffset / IndexOffsetBufferLength** - The offset and size
  within the TPI Hash Stream of the Type Index Offsets Buffer.  This is a list of
  pairs of uint32_t's where the first value is a :ref:`Type Index <type_indices>`
  and the second value is the offset in the type record data of the type with this
  index.  This can be used to do a binary search followed bin a linear search to
  get amortized O(log n) lookup by type index.

- **HashAdjBufferOffset / HashAdjBufferLength** - The offset and size within
  the TPI hash stream of a serialized hash table whose keys are the hash values
  in the hash value buffer and whose values are type indices.  This appears to
  be useful in incremental linking scenarios, so that if a type is modified an
  entry can be created mapping the old hash value to the new type index so that
  a PDB file consumer can always have the most up to date version of the type
  without forcing the incremental linker to garbage collect and update
  references that point to the old version to now point to the new version.
  The layout of this hash table is described in :doc:`HashTable`.

.. _tpi_records:

CodeView Type Record List
=========================
Following the header, there are ``TypeRecordBytes`` bytes of data that represent a
variable length array of :doc:`CodeView type records <CodeViewTypes>`.  The number
of such records (e.g. the length of the array) can be determined by computing the
value ``Header.TypeIndexEnd - Header.TypeIndexBegin``.

log(n) random access is provided by way of the Type Index Offsets array (if present)
described previously.