8000 gh-119786: move exception handling doc to InternalDocs by iritkatriel · Pull Request #119815 · python/cpython · GitHub
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Merged
merged 10 commits into from
Jun 3, 2024
Merged

gh-119786: move exception handling doc to InternalDocs #119815

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81a0e13
rename index->README and move exception handling doc to new folder
iritkatriel May 30, 2024
cd7725f
add link to exception handling file from index
iritkatriel May 30, 2024
7719234
edit exception handling
iritkatriel May 30, 2024
023ad9d
whitespace
iritkatriel May 30, 2024
fc5d217
formatting
iritkatriel May 30, 2024
3436cdf
formatting
iritkatriel May 31, 2024
49af193
add explanation of reraise and lasti. fix typos
iritkatriel May 31, 2024
d299a3e
address review - clarify pseudo-instructions/intermediate code
iritkatriel May 31, 2024
e93a253
fix markup
iritkatriel May 31, 2024
a3da517
typo
iritkatriel Jun 3, 2024
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@iritkatriel
iritkatriel committed May 31, 2024
commit 3436cdf7d7ee0f003fa22cde38620ecfc6e0390b
36 changes: 20 additions & 16 deletions InternalDocs/exception_handling.md
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Expand Up @@ -8,34 +8,34 @@ The cost of raising an exception is increased, but not by much.

The following code:

<code>
```
try:
g(0)
except:
res = "fail"

</code>
```

compiles into pseudo-code like the following:

```
`RESUME` 0
RESUME 0

1 `SETUP_FINALLY` 8 (to L1)
1 SETUP_FINALLY 8 (to L1)

2 `LOAD_NAME` 0 (g)
`PUSH_NULL`
`LOAD_CONST` 0 (0)
`CALL` 1
`POP_TOP`
`POP_BLOCK`
2 LOAD_NAME 0 (g)
PUSH_NULL
LOAD_CONST 0 (0)
CALL 1
POP_TOP
POP_BLOCK

-- L1: `PUSH_EXC_INFO`
-- L1: PUSH_EXC_INFO

3 `POP_TOP`
3 POP_TOP

4 `LOAD_CONST` 1 ('fail')
`STORE_NAME` 1 (res)
4 LOAD_CONST 1 ('fail')
STORE_NAME 1 (res)
```

The `SETUP_FINALLY` instruction specifies that henceforth, exceptions
Expand Down Expand Up @@ -91,8 +91,8 @@ of the following steps:
Format of the exception table
-----------------------------

```
Conceptually, the exception table consists of a sequence of 5-tuples:
```
1. `start-offset` (inclusive)
2. `end-offset` (exclusive)
3. `target`
Expand All @@ -108,7 +108,7 @@ For it to be searchable quickly, we need to support binary search giving us log(
Binary search typically assumes fixed size entries, but that is not necessary, as long as we can identify the start of an entry.

It is worth noting that the size (end-start) is always smaller than the end, so we encode the entries as:
`start, size, target, depth, push-lasti`
`start, size, target, depth, push-lasti`.

Also, sizes are limited to 2**30 as the code length cannot exceed 2**31 and each code unit takes 2 bytes.
It also happens that depth is generally quite small.
Expand All @@ -128,17 +128,21 @@ The 8 bits of a byte are (msb left) SXdddddd where S is the start bit. X is the
In addition, we combine `depth` and `lasti` into a single value, `((depth<<1)+lasti)`, before encoding.

For example, the exception entry:
```
`start`: 20
`end`: 28
`target`: 100
`depth`: 3
`lasti`: False
```

is encoded first by converting to the more compact four value form:
```
`start`: 20
`size`: 8
`target`: 100
`depth<<1+lasti`: 6
```

which is then encoded as:
148 (MSB + 20 for start)
Expand Down
0