* Re: [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature.
[not found] <E1XYzSf-00075J-Dr@vcs.savannah.gnu.org>
@ 2014-09-30 17:55 ` Stefan Monnier
2014-09-30 19:15 ` Paul Eggert
0 siblings, 1 reply; 6+ messages in thread
From: Stefan Monnier @ 2014-09-30 17:55 UTC (permalink / raw)
To: Dmitry Antipov; +Cc: emacs-devel
> + In C, this is implemented as a special macros which expands to
> +a @code{Lisp_Object} with block-scoped semantics and lifetime (see
> +the code around @code{USE_STACK_LISP_OBJECTS} in @file{lisp.h}). This
> +means that these objects are not managed by the garbage collector;
> +instead, they are allocated like local variables in C and automatically
> +freed when an execution reaches an end of the corresponding scope. Thus,
> +allocation and freeing are faster than using garbage collector. But
> +remember that passing them out of their scope results in undefined
> +behavior. Think twice before using this feature and carefully debug
> +your code with @code{GC_CHECK_MARKED_OBJECTS} (see @file{alloc.c}).
I'm an old-style programmer, so I don't know C99 and I hence don't know
what is the "corresponding scope" of those new thingies.
See my other message about these macros. I'd much prefer
declaration-level macros, which would come with clear scoping.
Stefan
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature.
2014-09-30 17:55 ` [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature Stefan Monnier
@ 2014-09-30 19:15 ` Paul Eggert
2014-09-30 19:29 ` Eli Zaretskii
2014-09-30 20:20 ` Stefan Monnier
0 siblings, 2 replies; 6+ messages in thread
From: Paul Eggert @ 2014-09-30 19:15 UTC (permalink / raw)
To: emacs-devel
On 09/30/2014 10:55 AM, Stefan Monnier wrote:
> I don't know C99 and I hence don't know
> what is the "corresponding scope" of those new thingies.
I gave a shot at improving that part of the documentation in trunk bzr
117990.
> See my other message about these macros.
Sorry, I don't remember which message that was?
> I'd much prefer
> declaration-level macros, which would come with clear scoping.
Although I can see the attraction of their clearer scoping, requiring a
name for every temporary can make code considerably harder to read. For
example, we'd have to replace this:
caller = concat3 (SCOPED_STRING (" <"), caller, SCOPED_STRING (">"));
with something like this:
SCOPED_STRING (space_lessthan, " <");
SCOPED_STRING (greaterthan, ">");
caller = concat3 (space_lessthan, caller, greaterthan);
Regardless of whether we use declaration-style macros, there is one
thing I'd like to change: the macro names. These macros are not about
*scope*; they are about *lifetime*. How about the prefix "auto_" (from
the C keyword 'auto') rather than "scope_"? Or maybe "block_" because
it's block lifetime? ("auto_" is shorter....)
Also, I capitalized SCOPED_STRING on the theory that it is often not
implemented as a function. On second thought since it can be (and
sometimes is) implemented as a function I'm thinking we should make it
lower-case, as scoped_cons etc. are. (Correcting its prefix of course.)
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature.
2014-09-30 19:15 ` Paul Eggert
@ 2014-09-30 19:29 ` Eli Zaretskii
2014-09-30 19:49 ` David Kastrup
2014-09-30 20:20 ` Stefan Monnier
1 sibling, 1 reply; 6+ messages in thread
From: Eli Zaretskii @ 2014-09-30 19:29 UTC (permalink / raw)
To: Paul Eggert; +Cc: emacs-devel
> Date: Tue, 30 Sep 2014 12:15:34 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
>
> Regardless of whether we use declaration-style macros, there is one
> thing I'd like to change: the macro names.
Yes, the names are not helpful.
> These macros are not about *scope*; they are about *lifetime*. How
> about the prefix "auto_" (from the C keyword 'auto') rather than
> "scope_"? Or maybe "block_" because it's block lifetime? ("auto_"
> is shorter....)
Or maybe "alloca_*", as in alloca_string, alloca_list1, etc.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature.
2014-09-30 19:29 ` Eli Zaretskii
@ 2014-09-30 19:49 ` David Kastrup
0 siblings, 0 replies; 6+ messages in thread
From: David Kastrup @ 2014-09-30 19:49 UTC (permalink / raw)
To: emacs-devel
Eli Zaretskii <eliz@gnu.org> writes:
>> Date: Tue, 30 Sep 2014 12:15:34 -0700
>> From: Paul Eggert <eggert@cs.ucla.edu>
>>
>> Regardless of whether we use declaration-style macros, there is one
>> thing I'd like to change: the macro names.
>
> Yes, the names are not helpful.
>
>> These macros are not about *scope*; they are about *lifetime*. How
>> about the prefix "auto_" (from the C keyword 'auto') rather than
>> "scope_"? Or maybe "block_" because it's block lifetime? ("auto_"
>> is shorter....)
>
> Or maybe "alloca_*", as in alloca_string, alloca_list1, etc.
alloca is "to end of function". Language-supported dynamic allocations
are "to end of block". That's not quite the same.
--
David Kastrup
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature.
2014-09-30 19:15 ` Paul Eggert
2014-09-30 19:29 ` Eli Zaretskii
@ 2014-09-30 20:20 ` Stefan Monnier
2014-10-01 3:31 ` Paul Eggert
1 sibling, 1 reply; 6+ messages in thread
From: Stefan Monnier @ 2014-09-30 20:20 UTC (permalink / raw)
To: Paul Eggert; +Cc: emacs-devel
> For example, we'd have to replace this:
> caller = concat3 (SCOPED_STRING (" <"), caller, SCOPED_STRING (">"));
> with something like this:
> SCOPED_STRING (space_lessthan, " <");
> SCOPED_STRING (greaterthan, ">");
> caller = concat3 (space_lessthan, caller, greaterthan);
Well, the second looks much better to me. Its semantics is much clearer.
The extra verbosity helps the programmer think a bit harder about what
she's doing and whether it is really safe to use stack allocation.
> Regardless of whether we use declaration-style macros, there is one thing
> I'd like to change: the macro names. These macros are not about *scope*;
> they are about *lifetime*. How about the prefix "auto_" (from the C keyword
> 'auto') rather than "scope_"? Or maybe "block_" because it's block
> lifetime? ("auto_" is shorter....)
Maybe it's too obvious, but how 'bout STACK_ALLOC_STRING?
Bikeshedding et al.
> Also, I capitalized SCOPED_STRING on the theory that it is often not
> implemented as a function. On second thought since it can be (and sometimes
> is) implemented as a function I'm thinking we should make it lower-case, as
> scoped_cons etc. are. (Correcting its prefix of course.)
That's another advantage of the declaration version: there's no arguing
whether it should be capitalized or not, since it can't be implemented
as a function.
Stefan
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature.
2014-09-30 20:20 ` Stefan Monnier
@ 2014-10-01 3:31 ` Paul Eggert
0 siblings, 0 replies; 6+ messages in thread
From: Paul Eggert @ 2014-10-01 3:31 UTC (permalink / raw)
To: Stefan Monnier; +Cc: emacs-devel
Stefan Monnier wrote:
> Well, the second looks much better to me. Its semantics is much clearer.
OK, I gave that a shot in trunk bzr 117994. I used the prefix 'AUTO_', as it's
shorter and more-specific than 'STACK_ALLOC_'. It turned out better than I
expected, and thanks for the suggestion.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2014-09-30 17:55 ` [Emacs-diffs] trunk r117987: * internals.texi (Stack-allocated Objects): Describe this feature Stefan Monnier
2014-09-30 19:15 ` Paul Eggert
2014-09-30 19:29 ` Eli Zaretskii
2014-09-30 19:49 ` David Kastrup
2014-09-30 20:20 ` Stefan Monnier
2014-10-01 3:31 ` Paul Eggert
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