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Re: Malloc Query

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On 05/23/11 05:57 PM, Uncle Steve wrote:
> On Mon, May 23, 2011 at 11:22:57AM +1200, Ian Collins wrote:
>> On 05/23/11 11:18 AM, Uncle Steve wrote:
>>> On Sun, May 22, 2011 at 1u1:18:36PM +0100, Ben Bacarisse wrote:
>>>> Uncle Steve<stevet810@gmail.com>   writes:
>>>>
>>>>> On Sun, May 22, 2011 at 02:49:51AM -0700, luser- -droog wrote:
>>>>>>
>>>>>> I've been using integer offsets instead of pointers a lot lately and
>>>>>> I think you might be better off using the array formula explicitly:
>>>>>>
>>>>>>   #define arena_obj_addr(x, n) ((void *) (x->arena + n * x->obj_size))
>>>>
>>>> Everyone is taking chances here.  The x and n should be in parentheses
>>>> so that the macro expands as expected.  This is one reason why functions
>>>> are usually preferred.
>>>
>>> For a long time I've thought macros were preferable to functions for
>>> the reason that a function call has to save registers, stuff it's
>>> arguments on the
>>> stack, and so on.  Macros should just fold into the program text, but
>>> today inline functions are just as fast as macros.  It has become a
>>> matter of preference.  gcc even has the ability to use register
>>> calling conventions for function calls in some circumstances.  The
>>> distinction between a macro and a function call is becoming moot.
>>
>> No it isn't.  All the disadvantages of function like macros still exist,
>> but the single (possible) disadvantage of function calls has gone.
>
> Not so fast, Mr. Collins.  Macros are useful in numerous situations
> where function calls would be inefficient.

The only real use for macros is passing __FILE__ and __LINE__ to debug logs!

> For my purposes, it is
> sometimes nice to write a macro which uses conditionals to select a
> particular code path through the macro.  If the conditional can be
> resolved at compile-time, the dead code branches in the macro are
> pruned, saving space.  A similarly structured function call won't be
> optimized in this fashion unless (maybe) it is inlined.

Why not?  Whether the function is called or in line, the conditional 
compiles are still there.

> Not enough?  Is it smart to rely on the compiler to optimize the crap
> out of everything, or is it better to write efficient code at the
> outset?

Inlining is a very simple optimisation.  You also overlook the opposite 
optimisation - not inlining.  There are situations where a smaller 
executable performs better (or even fit the target device!).  The 
compiler should be able to judge where it is appropriate to inline. 
Compilers targeted at embedded devices often have options to tune these 
heuristics.  Decent compilers for hosted environments offer profile 
feedback to further tune optimisations.

In short, the more choice you give the compiler, the better job it can do.

> There are a number of C compilers out there that are not gcc,
> and which may or may not generate equivalent code for otherwise
> similar macros/functions.  I'm O.K. with peppering my code with
> well-designed macros and ignoring functions for simple common
> procedures, but then I'm biased that way from habit.

I guess working with C++ for many years has biased me the other way. 
Aggressive inlining has long been a feature of C++ compilers.  It used 
to be a unique feature, but those days are long past.

-- 
Ian Collins

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Malloc Query Pushpa <pushpa@nospam.invalid> - 2011-05-17 19:39 +0000
  Re: Malloc Query blp@cs.stanford.edu (Ben Pfaff) - 2011-05-17 12:55 -0700
    Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-18 12:05 +0100
      Re: Malloc Query Pushpa <pushpa@nospam.invalid> - 2011-05-18 17:11 +0000
        Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-18 22:42 +0100
        Re: Malloc Query Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-18 21:10 -0400
      Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-18 19:34 +0100
        Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-18 17:10 -0400
          Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-19 00:26 +0100
            Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-18 21:25 -0400
              Re: Malloc Query James Kuyper <jameskuyper@verizon.net> - 2011-05-18 21:38 -0400
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-19 14:11 -0400
                Re: Malloc Query James Kuyper <jameskuyper@verizon.net> - 2011-05-19 22:55 -0400
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-20 12:14 -0400
              Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-19 03:22 +0100
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-19 14:20 -0400
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-19 23:51 +0100
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-19 20:01 -0400
                Re: Malloc Query luser- -droog <mijoryx@yahoo.com> - 2011-05-22 02:49 -0700
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-22 10:46 -0400
                Re: Malloc Query Shao Miller <sha0.miller@gmail.com> - 2011-05-22 12:37 -0500
                Re: Malloc Query Shao Miller <sha0.miller@gmail.com> - 2011-05-22 13:02 -0500
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-22 14:12 -0400
                Re: Malloc Query Shao Miller <sha0.miller@gmail.com> - 2011-05-23 03:06 -0500
                Re: Malloc Query luser- -droog <mijoryx@yahoo.com> - 2011-05-23 00:42 -0700
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 11:10 -0400
                Re: Malloc Query Keith Thompson <kst-u@mib.org> - 2011-05-23 09:06 -0700
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-22 23:18 +0100
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-22 19:18 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-23 11:22 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 01:57 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-23 19:24 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 14:20 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-24 07:41 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 16:24 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-24 09:20 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 18:05 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-24 10:37 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 19:34 -0400
                Re: Malloc Query "robertwessel2@yahoo.com" <robertwessel2@yahoo.com> - 2011-05-23 17:48 -0700
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-24 12:56 +1200
                Re: Malloc Query James Kuyper <jameskuyper@verizon.net> - 2011-05-24 07:19 -0400
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-24 11:32 -0400
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-23 01:04 +0100
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 02:36 -0400
                Re: Malloc Query luser- -droog <mijoryx@yahoo.com> - 2011-05-23 00:39 -0700
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-23 14:44 +0100
                Re: Malloc Query Seebs <usenet-nospam@seebs.net> - 2011-05-23 15:16 +0000
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-23 11:41 -0400
                Re: Malloc Query David Thompson <dave.thompson2@verizon.net> - 2011-06-01 16:33 -0400
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-19 18:34 -0400
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-20 02:36 +0100
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-20 12:26 -0400
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-20 23:32 +0100
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-20 21:17 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-21 14:30 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-21 11:13 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-22 08:50 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-21 20:03 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-22 13:23 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-22 10:01 -0400
                Re: Malloc Query "io_x" <a@b.c.invalid> - 2011-05-26 08:12 +0200
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-26 19:16 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-26 15:03 -0400
                Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-27 09:16 +1200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-26 17:55 -0400
                Re: Malloc Query "io_x" <a@b.c.invalid> - 2011-05-27 05:47 +0200
                Re: Malloc Query Uncle Steve <stevet810@gmail.com> - 2011-05-26 15:02 -0400
        Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-18 22:31 +0100
          Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-18 23:37 +0100
            Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-19 01:50 +0100
              Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-19 02:50 +0100
                Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-19 12:14 +0100
                Re: Malloc Query Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-05-19 15:10 +0100
                Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-19 15:48 +0100
      Re: Malloc Query cri@tiac.net (Richard Harter) - 2011-05-18 19:31 +0000
      Re: Malloc Query Ike Naar <ike@iceland.freeshell.org> - 2011-05-18 21:43 +0000
        Re: Malloc Query "BartC" <bc@freeuk.com> - 2011-05-18 22:58 +0100
  Re: Malloc Query Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-05-17 14:01 -0600
  Re: Malloc Query Columbus sailed the ocean China Blue <chine.bleu@yahoo.com> - 2011-05-17 13:21 -0700
  Re: Malloc Query BGB <cr88192@hotmail.com> - 2011-05-17 14:08 -0700
    Re: Malloc Query Michael Press <rubrum@pacbell.net> - 2011-05-17 19:49 -0700
  Re: Malloc Query Lauri Alanko <la@iki.fi> - 2011-05-18 00:50 +0000
  Re: Malloc Query Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-17 21:21 -0400
    Re: Malloc Query "io_x" <a@b.c.invalid> - 2011-05-18 19:31 +0200
    Re: Malloc Query Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-05-18 21:36 +0000
      Re: Malloc Query Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-18 21:26 -0400
    Re: Malloc Query Tim Rentsch <txr@alumni.caltech.edu> - 2011-05-19 02:07 -0700
      Re: Malloc Query Ian Collins <ian-news@hotmail.com> - 2011-05-19 21:13 +1200
  Re: Malloc Query Nobody <nobody@nowhere.com> - 2011-05-18 07:08 +0100
  Re: Malloc Query David Resnick <lndresnick@gmail.com> - 2011-05-19 07:51 -0700
    Re: Malloc Query Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2011-05-19 09:12 -0600

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