Re: code snippet

From	Ian Collins <ian-news@hotmail.com>
Newsgroups	comp.lang.c
Subject	Re: code snippet
Date	2012-01-04 15:37 +1300
Message-ID	<9mhsc5FpeqU5@mid.individual.net> (permalink)
References	(14 earlier) <9453445.440.1324566701765.JavaMail.geo-discussion-forums@prez5> <slrnjf6ocv.1rti.usenet-nospam@guild.seebs.net> <jcvrjq$feg$1@dont-email.me> <4F035609.2E6947C8@bytecraft.com> <20120103113739.535@kylheku.com>

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On 01/ 4/12 08:58 AM, Kaz Kylheku wrote:
> On 2012-01-03, Walter Banks<walter@bytecraft.com>  wrote:
>>
>> First there is very few things that can be done in asm that cannot be done in C.
>
> For instance, take garbage collection.  Suppose that in some function there is
> a statement "x = NULL", where x is local and there are no more uses of x in the
> remaining block.  An optimizing C compiler could simply not bother with this
> assignment since x has no next use.  But what if there is garbage collection?
> The variable x could be the last reference to a large amount of memory, whose
> reclamation will be prevented if the assignment is optimized away.

Only for as long as x remains in scope.

> The C compiler does not know that x has a kind of next use: it can be visited
> "behind the scenes" by the garbage collector!  C compilers are GC-unfriendly in
> numerous ways, one of which is this generation of semantic garbage: keeping
> values in memory or registers beyond the point at which they have become
> garbage in the source semantics.  In assembly we don't have this problem;
> since we know what we put into every memory location and register.

Again, for how long?  If a value is no longer used, the optimiser will 
reuse the register.  If a block if dynamic memory is no longer used, why 
keep the pointer to it in scope?  It sounds like you are trying to 
compensate form poor design, at least in the context of gc.

> We would like to be able to extend the C compiler with new rules such as
> "whenever a pointer variable has no next use, generate code to overwrite it
> with null", but the language doesn't give us a way to direct the translation in
> such ways.

I still don't see why.  If the pointer is in the current scope, the work 
is probably unnecessary.  If not, the work may be a very expensive way 
to fix a bad design.

-- 
Ian Collins

Thread

code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-18 20:51 -0500
  Re: code snippet Geoff <geoff@invalid.invalid> - 2011-12-18 18:29 -0800
    Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-18 21:51 -0500
      Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-18 22:24 -0500
        Re: code snippet Geoff <geoff@invalid.invalid> - 2011-12-18 19:53 -0800
          Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-18 23:32 -0500
            Re: code snippet Geoff <geoff@invalid.invalid> - 2011-12-18 20:57 -0800
              Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-19 00:01 -0500
                Re: code snippet Geoff <geoff@invalid.invalid> - 2011-12-18 21:13 -0800
                Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-19 16:42 -0500
      Re: code snippet Keith Thompson <kst-u@mib.org> - 2011-12-18 20:14 -0800
        Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-18 23:27 -0500
      Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-19 12:33 +0000
        Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-19 16:33 -0500
          Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-20 00:20 +0000
            Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-19 19:46 -0500
              Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-20 01:14 +0000
                Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-20 15:51 -0500
                Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-21 14:11 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-21 07:03 -0800
                Re: code snippet Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-21 21:41 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-21 16:35 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 01:03 +0000
                Re: code snippet Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-22 03:19 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 07:11 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 16:59 +0000
                Re: code snippet "io_x" <a@b.c.invalid> - 2011-12-22 18:38 +0100
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 10:40 -0800
                Re: code snippet Markus Wichmann <nullplan@gmx.net> - 2011-12-22 19:12 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-28 03:50 -0800
                Re: code snippet "io_x" <a@b.c.invalid> - 2011-12-28 17:06 +0100
                Re: code snippet "io_x" <a@b.c.invalid> - 2012-01-01 20:04 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-01 12:38 -0800
                Re: code snippet "io_x" <a@b.c.invalid> - 2012-01-02 10:14 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-03 04:56 -0800
                Re: code snippet "io_x" <a@b.c.invalid> - 2012-01-03 18:26 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-04 00:09 -0800
                Re: code snippet Ian Collins <ian-news@hotmail.com> - 2012-01-04 21:53 +1300
                Re: code snippet "io_x" <a@b.c.invalid> - 2012-01-04 16:14 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-01 12:28 -0800
                Re: code snippet "io_x" <a@b.c.invalid> - 2012-01-02 10:18 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-03 04:56 -0800
                Re: code snippet "BartC" <bc@freeuk.com> - 2011-12-22 18:05 +0000
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 18:14 +0000
                Re: code snippet superpollo <superpollo@tznvy.pbz> - 2011-12-22 20:00 +0100
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 19:08 +0000
                Re: code snippet superpollo <superpollo@tznvy.pbz> - 2011-12-22 22:43 +0100
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 21:57 +0000
                Re: code snippet BruceS <bruces42@hotmail.com> - 2011-12-23 16:26 -0800
                Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-22 19:21 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 11:33 -0800
                Re: code snippet "io_x" <a@b.c.invalid> - 2011-12-23 09:10 +0100
                Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-23 14:43 +0000
                Re: code snippet Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-23 17:14 +0000
                Re: code snippet Keith Thompson <kst-u@mib.org> - 2011-12-23 12:10 -0800
                Re: code snippet Walter Banks <walter@bytecraft.com> - 2012-01-03 14:24 -0500
                Re: code snippet Kaz Kylheku <kaz@kylheku.com> - 2012-01-03 19:58 +0000
                Re: code snippet Walter Banks <walter@bytecraft.com> - 2012-01-03 16:15 -0500
                Re: code snippet blp@cs.stanford.edu (Ben Pfaff) - 2012-01-03 13:26 -0800
                Re: code snippet Kaz Kylheku <kaz@kylheku.com> - 2012-01-03 22:04 +0000
                Re: code snippet Ian Collins <ian-news@hotmail.com> - 2012-01-04 15:44 +1300
                Re: code snippet "BartC" <bc@freeuk.com> - 2012-01-04 23:34 +0000
                Re: code snippet James Harris <james.harris.1@googlemail.com> - 2012-01-04 15:57 -0800
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-05 23:34 -0800
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-06 00:40 -0800
                Re: code snippet Kleuske <kleuske@nowhere.net> - 2012-01-06 10:33 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-06 04:30 -0800
                Re: code snippet Ian Collins <ian-news@hotmail.com> - 2012-01-04 15:37 +1300
                Re: code snippet Kaz Kylheku <kaz@kylheku.com> - 2012-01-04 03:37 +0000
                Re: code snippet Ian Collins <ian-news@hotmail.com> - 2012-01-04 20:23 +1300
                Re: code snippet jacob navia <jacob@spamsink.net> - 2011-12-22 18:26 +0100
                Re: code snippet Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-22 19:34 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 11:57 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 20:18 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 12:52 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 20:37 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 13:27 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 21:57 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 14:09 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 21:57 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-23 04:01 -0800
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-22 14:24 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-22 21:57 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-01 23:06 -0800
                Re: code snippet Hans Vlems <hvlems@freenet.de> - 2011-12-22 23:35 -0800
                Re: code snippet "BartC" <bc@freeuk.com> - 2011-12-23 12:26 +0000
                Re: code snippet jacob navia <jacob@spamsink.net> - 2011-12-23 18:32 +0100
                Re: code snippet Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-28 04:04 -0800
                Re: code snippet "io_x" <a@b.c.invalid> - 2011-12-23 09:10 +0100
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-25 11:13 -0800
                Re: code snippet Seebs <usenet-nospam@seebs.net> - 2011-12-25 19:31 +0000
                Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-26 21:53 +0000
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-26 16:10 -0800
                Re: code snippet Keith Thompson <kst-u@mib.org> - 2011-12-26 18:11 -0800
                Re: code snippet 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-26 23:56 -0800
                Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-20 15:57 -0500
                Re: code snippet Rui Maciel <rui.maciel@gmail.com> - 2011-12-21 12:39 +0000
                Re: code snippet Bill Reid <hormelfree@gmail.com> - 2011-12-22 15:29 -0800
          Re: code snippet Geoff <geoff@invalid.invalid> - 2011-12-19 19:19 -0800
            Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-20 13:51 -0500
              Re: code snippet Geoff <geoff@invalid.invalid> - 2011-12-20 11:08 -0800
  Re: code snippet Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-19 11:37 +0000
    Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-19 16:38 -0500
      Re: code snippet "Morris Keesan" <mkeesan@post.harvard.edu> - 2011-12-19 17:02 -0500
    Re: code snippet "Bill Cunningham" <nospam@nspam.invalid> - 2011-12-19 17:21 -0500
      Re: code snippet John Gordon <gordon@panix.com> - 2011-12-19 22:35 +0000

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