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Re: [Emacs-diffs] /srv/bzr/emacs/trunk r109327: Generalize INTERNAL_FIEL


From: Dmitry Antipov
Subject: Re: [Emacs-diffs] /srv/bzr/emacs/trunk r109327: Generalize INTERNAL_FIELD between buffers, keyboards and frames.
Date: Mon, 06 Aug 2012 15:44:02 +0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:14.0) Gecko/20120713 Thunderbird/14.0

On 08/06/2012 02:54 PM, Stefan Monnier wrote:

I want the C part of Emacs to look like C code.  So, if the
structure-field access does not require extra machinery (i.e. no read
barrier), I want to write it "foo->bar".

I (and I hope all others) too. But we do AREF and ASET for vectors,
thinking about that vector implementation may be changed so direct access
to 'contents' becomes invalid or nonsense. So, I think that xGET and xSET
for complex structures fits this approach and don't add too much obfuscation.

Moreover, C itself is not designed to provide GC, and GC-aware system is
"unusual" C program. So, I don't see a problem if such a program contains
some "unusual" constructs which aren't quite similar to a core language.
–ěn the contrary, this sets a good point for a newcomer: "look at these
strange things, they're implemented in a such unusual way because they're
really important, you should understand it before you write something".

If we want to make sure that the write-barrier is used everywhere where
it's needed, than we need to find a way to check this with some tool.
Such a tool might require some extra annotations in the source code, and
I'm willing to accept some such things, but I'd much rather avoid
"FVAR (foo, bar)" if at all possible.

If tools like coccinelle have trouble finding the right "foo->bar", then
maybe we can try to find some coding convention that helps coccinelle
find the right spots.  Or find some other tool that can do those checks
for us.

Development (and politics) is the art of the possible, and we have those
tools which are. I investigated this area, and I believe that coccinelle
is good enough to be used in our work; finally, I don't see the practical
reasons to wait until someone develops a wunderwaffe like GCC plugin
for automatic barrier insertion on any critical pointer stores.

Dmitry





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