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![Mark Shannon](/assets/img/avatar_default.png)
"Zero cost" exception handling. * Uses a lookup table to determine how to handle exceptions. * Removes SETUP_FINALLY and POP_TOP block instructions, eliminating (most of) the runtime overhead of try statements. * Reduces the size of the frame object by about 60%.
180 lines
5.8 KiB
Plaintext
180 lines
5.8 KiB
Plaintext
Description of exception handling in Python 3.11
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------------------------------------------------
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Python 3.11 uses what is known as "zero-cost" exception handling.
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Prior to 3.11, exceptions were handled by a runtime stack of "blocks".
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In zero-cost exception handling, the cost of supporting exceptions is minimized.
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In the common case (where no exception is raised) the cost is reduced
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to zero (or close to zero).
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The cost of raising an exception is increased, but not by much.
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The following code:
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def f():
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try:
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g(0)
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except:
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return "fail"
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compiles as follows in 3.10:
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2 0 SETUP_FINALLY 7 (to 16)
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3 2 LOAD_GLOBAL 0 (g)
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4 LOAD_CONST 1 (0)
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6 CALL_FUNCTION 1
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8 POP_TOP
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10 POP_BLOCK
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12 LOAD_CONST 0 (None)
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14 RETURN_VALUE
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4 >> 16 POP_TOP
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18 POP_TOP
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20 POP_TOP
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5 22 POP_EXCEPT
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24 LOAD_CONST 3 ('fail')
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26 RETURN_VALUE
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Note the explicit instructions to push and pop from the "block" stack:
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SETUP_FINALLY and POP_BLOCK.
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In 3.11, the SETUP_FINALLY and POP_BLOCK are eliminated, replaced with
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a table to determine where to jump to when an exception is raised.
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2 0 NOP
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3 2 LOAD_GLOBAL 0 (g)
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4 LOAD_CONST 1 (0)
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6 CALL_FUNCTION 1
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8 POP_TOP
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10 LOAD_CONST 0 (None)
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12 RETURN_VALUE
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>> 14 PUSH_EXC_INFO
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4 16 POP_TOP
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18 POP_TOP
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20 POP_TOP
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5 22 POP_EXCEPT
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24 LOAD_CONST 2 ('fail')
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26 RETURN_VALUE
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>> 28 POP_EXCEPT_AND_RERAISE
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ExceptionTable:
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2 to 8 -> 14 [0]
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14 to 20 -> 28 [3] lasti
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(Note this code is from an early 3.11 alpha, the NOP may well have be removed before release).
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If an instruction raises an exception then its offset is used to find the target to jump to.
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For example, the CALL_FUNCTION at offset 6, falls into the range 2 to 8.
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So, if g() raises an exception, then control jumps to offset 14.
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Unwinding
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---------
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When an exception is raised, the current instruction offset is used to find following:
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target to jump to, stack depth, and 'lasti', which determines whether the instruction
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offset of the raising instruction should be pushed.
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This information is stored in the exception table, described below.
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If there is no relevant entry, the exception bubbles up to the caller.
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If there is an entry, then:
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1. pop values from the stack until it matches the stack depth for the handler,
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2. if 'lasti' is true, then push the offset that the exception was raised at.
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3. push the exception to the stack as three values: traceback, value, type,
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4. jump to the target offset and resume execution.
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Format of the exception table
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-----------------------------
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Conceptually, the exception table consists of a sequence of 5-tuples:
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1. start-offset (inclusive)
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2. end-offset (exclusive)
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3. target
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4. stack-depth
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5. push-lasti (boolean)
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All offsets and lengths are in instructions, not bytes.
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We want the format to be compact, but quickly searchable.
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For it to be compact, it needs to have variable sized entries so that we can store common (small) offsets compactly, but handle large offsets if needed.
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For it to be searchable quickly, we need to support binary search giving us log(n) performance in all cases.
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Binary search typically assumes fixed size entries, but that is not necesary, as long as we can identify the start of an entry.
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It is worth noting that the size (end-start) is always smaller than the end, so we encode the entries as:
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start, size, target, depth, push-lasti
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Also, sizes are limited to 2**30 as the code length cannot exceed 2**31 and each instruction takes 2 bytes.
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It also happens that depth is generally quite small.
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So, we need to encode:
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start (up to 30 bits)
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size (up to 30 bits)
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target (up to 30 bits)
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depth (up to ~8 bits)
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lasti (1 bit)
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We need a marker for the start of the entry, so the first byte of entry will have the most significant bit set.
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Since the most significant bit is reserved for marking the start of an entry, we have 7 bits per byte to encode offsets.
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Encoding uses a standard varint encoding, but with only 7 bits instead of the usual 8.
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The 8 bits of a bit are (msb left) SXdddddd where S is the start bit. X is the extend bit meaning that the next byte is required to extend the offset.
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In addition, we will combine depth and lasti into a single value, ((depth<<1)+lasti), before encoding.
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For example, the exception entry:
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start: 20
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end: 28
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target: 100
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depth: 3
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lasti: False
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is encoded first by converting to the more compact four value form:
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start: 20
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size: 8
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target: 100
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depth<<1+lasti: 6
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which is then encoded as:
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148 (MSB + 20 for start)
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8 (size)
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65 (Extend bit + 1)
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36 (Remainder of target, 100 == (1<<6)+36)
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6
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for a total of five bytes.
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Script to parse the exception table
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-----------------------------------
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def parse_varint(iterator):
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b = next(iterator)
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val = b & 63
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while b&64:
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val <<= 6
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b = next(iterator)
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val |= b&63
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return val
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def parse_exception_table(code):
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iterator = iter(code.co_exceptiontable)
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try:
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while True:
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start = parse_varint(iterator)*2
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length = parse_varint(iterator)*2
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end = start + length - 2 # Present as inclusive, not exclusive
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target = parse_varint(iterator)*2
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dl = parse_varint(iterator)
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depth = dl >> 1
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lasti = bool(dl&1)
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yield start, end, target, depth, lasti
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except StopIteration:
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return
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