gh-119786: move exception handling doc to InternalDocs

InternalDocs/index.md → InternalDocs/README.md

@@ -10,3 +10,7 @@ to hold for other implementations of the Python language.
 The core dev team attempts to keep this documentation up to date. If
 it is not, please report that through the
 [issue tracker](https://github.com/python/cpython/issues).
+
+
+[Exception Handling](exception_handling.md)
+

InternalDocs/exception_handling.md

@@ -0,0 +1,201 @@
+Description of exception handling
+---------------------------------
+
+Python uses a technique known as "zero-cost" exception handling, which
+minimizes the cost of supporting exceptions. In the common case (where
+no exception is raised) the cost is reduced to zero (or close to zero).
+The cost of raising an exception is increased, but not by much.
+
+The following code:
+
+```
+try:
+    g(0)
+except:
+    res = "fail"
+
+```
+
+compiles into intermediate code like the following:
+
+```
+                  RESUME                   0
+
+     1            SETUP_FINALLY            8 (to L1)
+
+     2            LOAD_NAME                0 (g)
+                  PUSH_NULL
+                  LOAD_CONST               0 (0)
+                  CALL                     1
+                  POP_TOP
+                  POP_BLOCK
+
+    --   L1:      PUSH_EXC_INFO
+
+     3            POP_TOP
+
+     4            LOAD_CONST               1 ('fail')
+                  STORE_NAME               1 (res)
+```
+
+`SETUP_FINALLY` and `POP_BLOCK` are pseudo-instructions. This means
+that they can appear in intermediate code but they are not bytecode
+instructions. `SETUP_FINALLY` specifies that henceforth, exceptions
+are handled by the code at label L1. The `POP_BLOCK` instruction
+reverses the effect of the last `SETUP` instruction, so that the
+active exception handler reverts to what it was before.
+
+`SETUP_FINALLY` and `POP_BLOCK` have no effect when no exceptions
+are raised. The idea of zero-cost exception handling is to replace
+these pseudo-instructions by metadata which is stored alongside the
+bytecode, and which is inspected only when an exception occurs.
+This metadata is the exception table, and it is stored in the code
+object's `co_exceptiontable` field.
+
+When the pseudo-instructions are translated into bytecode,
+`SETUP_FINALLY` and `POP_BLOCK` are removed, and the exception
+table is constructed, mapping each instruction to the exception
+handler that covers it, if any. Instructions which are not
+covered by any exception handler within the same code object's
+bytecode, do not appear in the exception table at all.
+
+For the code object in our example above, the table has a single
+entry specifying that all instructions that were between the
+`SETUP_FINALLY` and the `POP_BLOCK` are covered by the exception
+handler located at label `L1`.
+
+Handling Exceptions
+-------------------
+
+At runtime, when an exception occurs, the interpreter looks up
+the offset of the current instruction in the exception table. If
+it finds a handler, control flow transfers to it. Otherwise, the
+exception bubbles up to the caller, and the caller's frame is
+checked for a handler covering the `CALL` instruction. This
+repeats until a handler is found or the topmost frame is reached.
+If no handler is found, the program terminates. During unwinding,
+the traceback is constructed as each frame is added to it.
+
+Along with the location of an exception handler, each entry of the
+exception table also contains the stack depth of the `try` instruction
+and a boolean `lasti` value, which indicates whether the instruction
+offset of the raising instruction should be pushed to the stack.
+
+Handling an exception, once an exception table entry is found, consists
+of the following steps:
+
+ 1. pop values from the stack until it matches the stack depth for the handler.
+ 2. if `lasti` is true, then push the offset that the exception was raised at.
+ 3. push the exception to the stack.
+ 4. jump to the target offset and resume execution.
+
+
+Reraising Exceptions and `lasti`
+--------------------------------
+
+The purpose of pushing `lasti` to the stack is for cases where an exception
+needs to be re-raised, and be associated with the original instruction that
+raised it. This happens, for example, at the end of a `finally` block, when
+any in-flight exception needs to be propagated on. As the frame's instruction
+pointer now points into the finally block, a `RERAISE` instruction
+(with `oparg > 0`) sets it to the `lasti` value from the stack.
+
+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`
+    4. `stack-depth`
+    5. `push-lasti` (boolean)
+```
+
+All offsets and lengths are in code units, not bytes.
+
+We want the format to be compact, but quickly searchable.
+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.
+For it to be searchable quickly, we need to support binary search giving us log(n) performance in all cases.
+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`.
+
+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.
+
+So, we need to encode:
+```
+    `start` (up to 30 bits)
+    `size` (up to 30 bits)
+    `target` (up to 30 bits)
+    `depth` (up to ~8 bits)
+    `lasti` (1 bit)
+```
+
+We need a marker for the start of the entry, so the first byte of entry will have the most significant bit set.
+Since the most significant bit is reserved for marking the start of an entry, we have 7 bits per byte to encode offsets.
+Encoding uses a standard varint encoding, but with only 7 bits instead of the usual 8.
+The 8 bits of a byte 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.
+
+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 by first 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)
+    8   (size)
+    65  (Extend bit + 1)
+    36  (Remainder of target, 100 == (1<<6)+36)
+    6
+```
+
+for a total of five bytes.
+
+
+Script to parse the exception table
+-----------------------------------
+
+```
+def parse_varint(iterator):
+    b = next(iterator)
+    val = b & 63
+    while b&64:
+        val <<= 6
+        b = next(iterator)
+        val |= b&63
+    return val
+```
+```
+def parse_exception_table(code):
+    iterator = iter(code.co_exceptiontable)
+    try:
+        while True:
+            start = parse_varint(iterator)*2
+            length = parse_varint(iterator)*2
+            end = start + length - 2 # Present as inclusive, not exclusive
+            target = parse_varint(iterator)*2
+            dl = parse_varint(iterator)
+            depth = dl >> 1
+            lasti = bool(dl&1)
+            yield start, end, target, depth, lasti
+    except StopIteration:
+        return
+```