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104 lines
3.8 KiB
ReStructuredText
104 lines
3.8 KiB
ReStructuredText
The PDB Serialized Hash Table Format
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====================================
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.. contents::
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:local:
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.. _hash_intro:
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Introduction
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============
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One of the design goals of the PDB format is to provide accelerated access to
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debug information, and for this reason there are several occasions where hash
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tables are serialized and embedded directly to the file, rather than requiring
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a consumer to read a list of values and reconstruct the hash table on the fly.
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The serialization format supports hash tables of arbitrarily large size and
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capacity, as well as value types and hash functions. The only supported key
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value type is a uint32. The only requirement is that the producer and consumer
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agree on the hash function. As such, the hash function can is not discussed
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further in this document, it is assumed that for a particular instance of a PDB
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file hash table, the appropriate hash function is being used.
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On-Disk Format
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==============
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.. code-block:: none
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.--------------------.-- +0
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| Size |
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.--------------------.-- +4
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| Capacity |
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.--------------------.-- +8
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| Present Bit Vector |
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.--------------------.-- +N
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| Deleted Bit Vector |
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.--------------------.-- +M ─╮
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| Key | │
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.--------------------.-- +M+4 │
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| Value | │
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.--------------------.-- +M+4+sizeof(Value) │
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... ├─ |Capacity| Bucket entries
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.--------------------. │
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| Key | │
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.--------------------. │
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| Value | │
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.--------------------. ─╯
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- **Size** - The number of values contained in the hash table.
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- **Capacity** - The number of buckets in the hash table. Producers should
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maintain a load factor of no greater than ``2/3*Capacity+1``.
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- **Present Bit Vector** - A serialized bit vector which contains information
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about which buckets have valid values. If the bucket has a value, the
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corresponding bit will be set, and if the bucket doesn't have a value (either
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because the bucket is empty or because the value is a tombstone value) the bit
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will be unset.
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- **Deleted Bit Vector** - A serialized bit vector which contains information
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about which buckets have tombstone values. If the entry in this bucket is
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deleted, the bit will be set, otherwise it will be unset.
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- **Keys and Values** - A list of ``Capacity`` hash buckets, where the first
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entry is the key (always a uint32), and the second entry is the value. The
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state of each bucket (valid, empty, deleted) can be determined by examining
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the present and deleted bit vectors.
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.. _hash_bit_vectors:
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Present and Deleted Bit Vectors
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===============================
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The bit vectors indicating the status of each bucket are serialized as follows:
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.. code-block:: none
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.--------------------.-- +0
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| Word Count |
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.--------------------.-- +4
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| Word_0 | ─╮
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.--------------------.-- +8 │
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| Word_1 | │
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.--------------------.-- +12 ├─ |Word Count| values
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... │
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.--------------------. │
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| Word_N | │
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.--------------------. ─╯
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The words, when viewed as a contiguous block of bytes, represent a bit vector
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with the following layout:
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.. code-block:: none
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.------------. .------------.------------.
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| Word_N | ... | Word_1 | Word_0 |
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.------------. .------------.------------.
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| | | | |
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+N*32 +(N-1)*32 +64 +32 +0
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where the k'th bit of this bit vector represents the status of the k'th bucket
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in the hash table.
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