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[Python-Dev] General concerns about C API changes


On Tue, Nov 13, 2018 at 7:06 PM Raymond Hettinger <
raymond.hettinger at gmail.com> wrote:

> Overall, I support the efforts to improve the C API, but over the last few
> weeks have become worried.  I don't want to hold up progress with fear,
> uncertainty, and doubt.  Yet, I would like to be more comfortable that
> we're all aware of what is occurring and what are the potential benefits
> and risks.
>
> * Inline functions are great.  They provide true local variables, better
> separation of concerns, are far less kludgy than text based macro
> substitution, and will typically generate the same code as the equivalent
> macro.  This is good tech when used with in a single source file where it
> has predictable results.
>
> However, I'm not at all confident about moving these into header files
> which are included in multiple target .c files which need be compiled into
> separate .o files and linked to other existing libraries.
>
> With a macro, I know for sure that the substitution is taking place.  This
> happens at all levels of optimization and in a debug mode.  The effects are
> 100% predictable and have a well-established track record in our mature
> battle-tested code base.  With cross module function calls, I'm less
> confident about what is happening, partly because compilers are free to
> ignore inline directives and partly because the semantics of inlining are
> less clear when the crossing module boundaries.
>
> * Other categories of changes that we make tend to have only a shallow
> reach.  However, these C API changes will likely touch every C extension
> that has ever been written, some of which is highly tuned but not actively
> re-examined.  If any mistakes are make, they will likely be pervasive.
> Accordingly, caution is warranted.
>
> My expectation was that the changes would be conducted in experimental
> branches. But extensive changes are already being made (or about to be
> made) on the 3.8 master. If a year from now, we decide that the changes
> were destabilizing or that the promised benefits didn't materialize, they
> will be difficult to undo because there are so many of them and because
> they will be interleaved with other changes.
>
> The original motivation was to achieve a 2x speedup in return for
> significantly churning the C API. However, the current rearranging of the
> include files and macro-to-inline-function changes only give us churn.  At
> the very best, they will be performance neutral.  At worst, formerly cheap
> macro calls will become expensive in places that we haven't thought to run
> timings on.  Given that compilers don't have to honor an inline directive,
> we can't really know for sure -- perhaps today it works out fine, and
> perhaps tomorrow the compilers opt for a different behavior.
>
> Maybe everything that is going on is fine.  Maybe it's not. I am not
> expert enough to know for sure, but we should be careful before
> green-lighting such an extensive series of changes directly to master.
> Reasonable questions to ask are: 1) What are the risks to third party
> modules, 2) Do we really know that the macro-to-inline-function
> transformations are semantically neutral. 3) If there is no performance
> benefit (none has been seen so far, nor is any promised in the pending
> PRs), is it worth it?
>
> We do know that PyPy folks have had their share of issues with the C API,
> but I'm not sure that we can make any of this go away without changing the
> foundations of the whole ecosystem.  It is inconvenient for a full GC
> environment to interact with the API for a reference counted environment --
> I don't think we can make this challenge go away without giving up
> reference counting.  It is inconvenient for a system that manifests objects
> on demand to interact with an API that assumes that objects have identity
> and never more once they are created -- I don't think we can make this go
> away either.  It is inconvenient to a system that uses unboxed data to
> interact with our API where everything is an object that includes a type
> pointer and reference count -- We have provided an API for boxing and
> boxing, but the trip back-and-forth is inconveniently expensive -- I don't
> think we can make that go away either because too much of the ecosystem
> depends on that API.  There are some things that ca
>  n be mitigated such as challenges with borrowed references but that
> doesn't seem to have been the focus on any of the PRs.
>
> In short, I'm somewhat concerned about the extensive changes that are
> occurring.  I do know they will touch substantially every C module in the
> entire ecosystem.  I don't know whether they are safe or whether they will
> give any real benefit.
>
> FWIW, none of this is a criticism of the work being done.  Someone needs
> to think deeply about the C API or else progress will never be made.  That
> said, it is a high risk project with many PRs going directly into master,
> so it does warrant having buy in that the churn isn't destabilizing and
> will actually produce a benefit that is worth it.
>
>
> Raymond
>
>
While I haven't looked at *all* of the existing changes, glancing at an
example one, the XINCREF and XDECREF macro -> static inline .h function
change from
https://github.com/python/cpython/commit/541497e6197268517b0d492856027774c43e0949,
I don't have any concerns.  Because I am confident that gcc and clang will
behave well with this type of change.

>From my point of view: A static inline function is a much nicer modern code
style than a C preprocessor macro.

I expect the largest visible impact may be that a profiler may now
attribute CPU cycles takes by these code snippets to the function from the
.h file rather than directly to the functions the macro expanded in in the
past due to additional debug symbol info attribution. Just more data.
Consider that a win.

I do not believe any compiler will generate poor code from this on any
meaningful system and compiler configuration (-O0 or -Og code seems
irrelevant, nothing ships that way).  Regardless, pablogsal appears to be
doing the homework to double check which seems like a good plan before we
release 3.8. :)

-gps
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