[New-bugs-announce] [issue44990] Change layout of frames back to specials-locals-stack (from locals-specials-stack)

[Mark Shannon]

New submission from Mark Shannon <mark at hotpy.org>:

The two plausible layouts from evaluation stack frames are described here:

https://github.com/faster-cpython/ideas/issues/31#issuecomment-844263795

We opted for layout A, although it is a bit more complex to manage and slightly more expensive in terms of pointers. The reason for this was that it theoretically allows zero-copying Python-to-Python calls.

I now believe this was the wrong decision and we should have chosen layout B.

B is cheaper. It needs 2 pointers, not 3, meaning that there is another register available for use in the interpreter.
Also the linkage area doesn't need the nlocalsplus field.

The benefit of zero-copy calls is much smaller than I thought:
* Any calls from a generator functions do not benefit
* An additional check is needed to make sure that both frames are in the same stack chunk
* Any jitted code will keep stack values in registers, so stores will still be needed in either case.
* The average number of arguments copied is low (typically 2 or 3).

Even in the ideal case (interpreter, no generator, same stack chunk) changing to layout B
will cost 2/3 memory moves (independent of each other), but will gain us extra code for checking chunks, and one move (moving nlocalsplus). So at best we only save 1/2 moves.

In other cases layout B is better.

One final improvement to layout B: saving the stackdepth as an offset from locals[0] not from stack[0] further speeds frame handling.

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assignee: Mark.Shannon
messages: 400202
nosy: Mark.Shannon, pablogsal
priority: normal
severity: normal
status: open
title: Change layout of frames back to specials-locals-stack (from locals-specials-stack)

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue44990>
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