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[Python-Dev] Do more at compile time; less at runtime

MRAB python at mrabarnett.plus.com
Mon Dec 10 00:29:16 CET 2012 
On 2012-12-09 22:22, Mark Shannon wrote:
> Hi all,
>
> The current CPython bytecode interpreter is rather more complex than it
> needs to be. A number of bytecodes could be eliminated and a few more
> simplified by moving the work involved in handling compound statements
> (loops, try-blocks, etc) from the interpreter to the compiler.
>
> This simplest example of this is the while loop...
> while cond:
>      body
>
> This currently compiled as
>
> start:
>      if not cond goto end
>      body
>      goto start
> end:
>
> but it could be compiled as
>
> goto test:
> start:
>       body
>       if cond goto start
>
> which eliminates one instruction per iteration.
>
> A more complex example is a return in a try-finally block.
>
> try:
>       part1
>       if cond:
>           return X
>       part2
> finally:
>       part3
>
> Currently, handling the return is complex and involves "pseudo
> exceptions", but if part3 were duplicated by the compiler, then the
> RETURN bytecode could just perform a simple return.
> The code above would be compiled thus...
>
>       PUSH_BLOCK try
>       part1
>       if not X goto endif
>       push X
>       POP_BLOCK
>       part3           <<< duplicated
>       RETURN_VALUE
> endif:
>       part2
>       POP_BLOCK
>       part3           <<< duplicated
>
> The changes I am proposing are:
>
[snip]
Is it necessary to duplicate part3? Is it possible to put it into a
subroutine if it's long? (And I do mean a simple cheap subroutine.)
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