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Re: PERL_RC_STACK branch: first cut

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Dave Mitchell
February 28, 2023 10:26
Re: PERL_RC_STACK branch: first cut
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On Mon, Feb 27, 2023 at 07:39:34AM +0000, Vadim V Konovalov via perl5-porters wrote:
> - what's the benefit? Does it fix bug or introduce nice new feature?
> Payment is high (in terms of speed 20%) so I guess the benefit is huge
> :)

A simple example of the class of bug it fixes is:

    my @a = (1,2,3);
    sub f { @a = (); print "(@_)\n" };
    f(@a, 4);

which currently prints undefined or even garbage values, as it is
accessing SVs which have been freed or even reallocated.

The 20% loss in speed in PERL_RC_STACK builds is *temporary*, and will
never appear in a production perl.

It's present because, as a *transition* measure, most ops are wrapped. This
means to make a function such as pp_const() work on a reference-counted
stack, I just changed its definition from

    PP(pp_const) { ... }


    PP_wrapped(pp_const, 0, 0) { ... }

On PERL_RC_STACK builds, this declares Perl_pp_const() as a small
stub function which calls a function called pp_wrap(), which adjusts the
reference count of the arguments on the stack, calls the real pp_const()
function, then adjusts the counts again on return. For a simple function
like pp_const() which just pushes an SV pointer on the stack, that is a
huge extra overhead. Phase two of this project is to go through all the
hot and/or simple PP functions and unwrap them. For something simple like
pp_const(), that's little more than a 1 line change from


Once that has been done, I expect (but can't be certain) that nearly all
of that 20% will be reclaimed.

Then there will be further optimisations to be made. For example, in PP
functions, virtually every need to mortalise a newly-created SV can be
eliminated, now that the argument stack will automatically keep the SV
alive. At the moment in non void context scope exits, there is a
complicated dance to free any temporaries created by the last statement,
while not freeing any TEMP return values, while creating TEMP copies of
those return args. In a PERL_RC_STACK environment, all that code can be
ripped out.
> - out of curiosity, is this a TPF grant project?

I am being funded via a TPF general perl maintenance grant rather than
one for this particular project.

A major Starfleet emergency breaks out near the Enterprise, but
fortunately some other ships in the area are able to deal with it to
everyone's satisfaction.
    -- Things That Never Happen in "Star Trek" #13

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