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GH-88116 : Use a compact format to represent end line and column offsets. (GH-91666 )

* Stores all location info in linetable to conform to PEP 626.

* Remove column table from code objects.

* Remove end-line table from code objects.

* Document new location table format

2022-04-21 16:10:37 +01:00

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Locations table

For versions up to 3.10 see ./lnotab_notes.txt

In version 3.11 the co_linetable bytes object of code objects contains a compact representation of the positions returned by the co_positions() iterator.

The co_linetable consists of a sequence of location entries. Each entry starts with a byte with the most significant bit set, followed by zero or more bytes with most significant bit unset.

Each entry contains the following information:

The number of code units covered by this entry (length)
The start line
The end line
The start column
The end column

The first byte has the following format:

Bit 7	Bits 3-6	Bits 0-2
1	Code	Length (in code units) - 1

The codes are enumerated in the _PyCodeLocationInfoKind enum.

Variable length integer encodings

Integers are often encoded using a variable length integer encoding

Unsigned integers (varint)

Unsigned integers are encoded in 6 bit chunks, least significant first. Each chunk but the last has bit 6 set. For example:

63 is encoded as 0x3f
200 is encoded as 0x48, 0x03

Signed integers (svarint)

Signed integers are encoded by converting them to unsigned integers, using the following function:

def convert(s):
    if s < 0:
        return ((-s)<<1) | 1
    else:
        return (s<<1)

Location entries

The meaning of the codes and the following bytes are as follows:

Code	Meaning	Start line	End line	Start column	End column
0-9	Short form	Δ 0	Δ 0	See below	See below
10-12	One line form	Δ (code - 10)	Δ 0	unsigned byte	unsigned byte
13	No column info	Δ svarint	Δ 0	None	None
14	Long form	Δ svarint	Δ varint	varint	varint
15	No location	None	None	None	None

The Δ means the value is encoded as a delta from another value:

Start line: Delta from the previous start line, or co_firstlineno for the first entry.
End line: Delta from the start line

The short forms

Codes 0-9 are the short forms. The short form consists of two bytes, the second byte holding additional column information. The code is the start column divided by 8 (and rounded down).

Start column: (code*8) + ((second_byte>>4)&7)
End column: start_column + (second_byte&15)

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