[Patches] [ python-Patches-1408710 ] use computed goto's in ceval loop

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Patches item #1408710, was opened at 2006-01-18 03:21
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Category: Core (C code)
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Simon David Burton (simonb1)
Assigned to: Nobody/Anonymous (nobody)
Summary: use computed goto's in ceval loop

Initial Comment:

An experiment where the big switch in ceval.c
is replaced with a computed goto construct.
Uses #define's to make it optional.
Inspired by Mono's MINT interpreter code,
and Neil Norwitz's attempt to use a function
pointer table:

http://sourceforge.net/tracker/index.php?func=detail&aid=693638&group_id=5470&atid=305470

Result: about 1% slower on the pystone benchmark.

Also it seems to have broken the interpreter;
at least one test fails (test_StringIO).

Not sure if the other switch speedup
hacks (eg. PREDICT(op)) conflict 
at all with this patch (ie. make it
slower than it could be).

Mono actually uses a much larger opcode
set, with 2-byte opcodes, that includes
type info in each opcode. This means that
the actual case statements are much faster.
My initial thought about using computed goto's
(January 2003) was that the python opcode cases
were much fatter than mono's (often involving
a function call) and that the overhead of
branching on the opcode would be insignificant.
It seems that this is true indeed.

Patch is for python revision 42025.


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