Re: va_start without the first argument?

From	BartC <bc@freeuk.com>
Newsgroups	comp.lang.c
Subject	Re: va_start without the first argument?
Date	2017-03-12 18:04 +0000
Organization	A noiseless patient Spider
Message-ID	<oa42ej$dms$1@dont-email.me> (permalink)
References	(5 earlier) <3fb4148c-8b5b-468c-8399-e5a5fc8575ee@googlegroups.com> <oa1gud$79a$1@dont-email.me> <oa3g4n$egd$1@dont-email.me> <oa3mfq$3m6$1@dont-email.me> <oa3vea$39f$1@dont-email.me>

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On 12/03/2017 17:13, David Brown wrote:
> On 12/03/17 15:40, BartC wrote:

>> I've done this and was hoping to use the same approach for compiling C
>> (for x64). But it won't work because there is now no distinction between
>> C functions which are compiled here, and C functions compiled elsewhere
>> by another compiler.
>
> Sure there is - if your compiler is the one compiling a function, and
> you know that all uses of the function are through code that you
> compile, then you can do what you want.  This is usually the case for
> static functions (unless you take their address and pass it outside the
> current translation unit).

Yeah, maybe. But when 99% of calls could be your own call convention, 
then it would be worth doing (and with the 1% using the ABI, the 
efficiency of those wouldn't matter). But if it's only static functions, 
and only those you can be sure can't be called by any other means ...

Even then, sometimes you want to take the address of a static function. 
Then when you call via a pointer, it's not going to be easy to determine 
if an arbitrary pointer points to a static function, or a global one, or 
can be either during the course of the program. For example, you pass a 
function pointer to a function; that function won't know its provenance.

Of course an implementation can choose to generate two copies of static 
functions, one of which is used only for direct internal calls. But this 
is way past the point where it would just be easier to use the official ABI.

>> (Not that this ABI makes things that much faster, if at all. If I take
>> my current project and compile with gcc -O0, then the 32-bit version is
>> 8% faster than the 64-bit, despite the latter passing parameters in
>> registers for most functions.)
>>
>
> Speed comparisons without optimisation are /always/ pointless.

No, it makes it easier to compare how long things take to execute 
(rather than how long it takes /not/ to execute them as often happens 
with gcc).

Anyway, if I use -O3 for both, then the 32-bit version is 12% faster! 
Despite having fewer and narrower registers.

> And there are a variety of things that can affect the difference in
> speed between 32-bit and 64-bit versions (even when you have allowed the
> compiler to generate decent code by using at least -O1).  Pointers in
> 64-bit mode are 64-bit wide - that means more cache is needed, meaning
> more cache misses, and more memory bandwidth is needed.  For some
> applications, the difference here can be significant.  And in Windows,
> the aforementioned idiotic requirement of allocating stack space for the
> arguments passed in registers means wasted time and wasted cache space
> on every function call in 64-bit mode.

The pointer one I know about (I think it cost me 5% loss of performance 
when I switched an old x64 compiler from 32- to 64-bit pointers).

But how much would allocating those 32 bytes of stack cost? Bear in mind 
that probably nothing will be written to them.

(It'll reduce cache capacity to a certain extent, but only as far as the 
maximum call-nesting depth, which usually isn't very much. If the 
maximum level is 30 calls deep, then we're talking 1KB in total.)

-- 
bartc

Thread

va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-10 11:57 -0800
  Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-10 12:04 -0800
  Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-10 12:32 -0800
    Re: va_start without the first argument? "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2017-03-10 15:51 -0500
    Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-10 14:36 -0800
      Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-10 14:53 -0800
        Re: va_start without the first argument? Robert Wessel <robertwessel2@yahoo.com> - 2017-03-10 17:14 -0600
          Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-10 15:37 -0800
            Re: va_start without the first argument? Robert Wessel <robertwessel2@yahoo.com> - 2017-03-10 23:12 -0600
              Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 10:29 -0700
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 11:31 -0700
                Re: va_start without the first argument? scott@slp53.sl.home (Scott Lurndal) - 2017-03-13 18:40 +0000
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 12:31 -0700
            Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-11 19:53 +0100
              Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-11 19:05 +0000
                Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-11 11:56 -0800
              Re: va_start without the first argument? antispam@math.uni.wroc.pl - 2017-03-12 05:41 +0000
              Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-12 13:52 +0100
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-12 14:40 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-12 18:13 +0100
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-12 18:04 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-12 21:04 +0100
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-12 21:02 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-12 22:40 +0100
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-12 15:51 -0700
                Re: va_start without the first argument? Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-03-12 16:52 -0700
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-13 00:56 +0100
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-13 09:17 -0700
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-13 17:54 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-13 22:12 +0100
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-13 22:48 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-14 13:46 +0100
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-14 13:37 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-14 15:24 +0100
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-14 08:25 -0700
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-14 17:02 +0100
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-14 09:45 -0700
                Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-14 09:15 -0700
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-14 10:02 -0700
                Re: va_start without the first argument? jt@toerring.de (Jens Thoms Toerring) - 2017-03-14 20:32 +0000
                Re: va_start without the first argument? raltbos@xs4all.nl (Richard Bos) - 2017-03-14 07:44 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-14 13:38 +0100
                Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-14 08:57 -0700
                Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-13 11:21 -0700
                Re: va_start without the first argument? scott@slp53.sl.home (Scott Lurndal) - 2017-03-13 18:33 +0000
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-13 22:08 +0100
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-13 15:23 -0700
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-14 13:58 +0100
                Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-14 09:18 -0700
                Re: va_start without the first argument? scott@slp53.sl.home (Scott Lurndal) - 2017-03-14 16:52 +0000
                Re: va_start without the first argument? Keith Thompson <kst-u@mib.org> - 2017-03-14 11:34 -0700
                Re: va_start without the first argument? scott@slp53.sl.home (Scott Lurndal) - 2017-03-14 18:54 +0000
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-14 13:30 -0700
                Re: va_start without the first argument? Ike Naar <ike@iceland.freeshell.org> - 2017-03-15 21:55 +0000
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-15 15:16 -0700
                Re: va_start without the first argument? Philip Lantz <prl@canterey.us> - 2017-03-23 21:04 -0700
                Re: va_start without the first argument? David Brown <david.brown@hesbynett.no> - 2017-03-14 21:48 +0100
                Re: va_start without the first argument? scott@slp53.sl.home (Scott Lurndal) - 2017-03-14 14:01 +0000
                Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-14 07:42 -0700
                Re: va_start without the first argument? scott@slp53.sl.home (Scott Lurndal) - 2017-03-14 12:38 +0000
                Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-13 01:11 +0100
                Re: va_start without the first argument? Ian Collins <ian-news@hotmail.com> - 2017-03-13 15:43 +1300
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-13 10:36 +0000
                Re: va_start without the first argument? Ian Collins <ian-news@hotmail.com> - 2017-03-14 07:33 +1300
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-13 19:00 +0000
                Re: va_start without the first argument? Ian Collins <ian-news@hotmail.com> - 2017-03-14 08:05 +1300
                Re: va_start without the first argument? BartC <bc@freeuk.com> - 2017-03-13 23:10 +0000
                Re: va_start without the first argument? Ian Collins <ian-news@hotmail.com> - 2017-03-14 17:38 +1300
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-14 05:48 -0700
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 12:12 -0700
                Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-12 19:39 +0100
  Re: va_start without the first argument? jt@toerring.de (Jens Thoms Toerring) - 2017-03-10 20:49 +0000
    Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-10 14:41 -0800
      Re: va_start without the first argument? John Bode <jfbode1029@gmail.com> - 2017-03-10 16:05 -0800
        Re: va_start without the first argument? jacob navia <jacob@jacob.remcomp.fr> - 2017-03-11 21:47 +0100
          Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-11 13:07 -0800
            Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-11 22:24 +0100
              Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 05:44 -0700
                Re: va_start without the first argument? Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-13 08:59 -0400
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 06:17 -0700
                Re: va_start without the first argument? Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-13 13:27 -0400
                Re: va_start without the first argument? Jerry Stuckle <jstucklex@attglobal.net> - 2017-03-13 13:30 -0400
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 11:00 -0700
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 10:34 -0700
          Re: va_start without the first argument? antispam@math.uni.wroc.pl - 2017-03-12 06:00 +0000
          Re: va_start without the first argument? gordonb.gas5o@burditt.org (Gordon Burditt) - 2017-03-17 09:46 -0500
            Re: va_start without the first argument? supercat@casperkitty.com - 2017-03-17 09:07 -0700
        Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 05:39 -0700
          Re: va_start without the first argument? John Bode <jfbode1029@gmail.com> - 2017-03-13 11:40 -0700
            Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 12:01 -0700
              Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-13 23:17 +0100
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 16:15 -0700
                Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-14 00:26 +0100
                Re: va_start without the first argument? kikiins@gmail.com - 2017-03-13 16:51 -0700
                Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-14 01:12 +0100
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 18:24 -0700
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-13 18:57 -0700
                Re: va_start without the first argument? raltbos@xs4all.nl (Richard Bos) - 2017-03-14 07:59 +0000
                Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-14 06:45 -0700
                Re: va_start without the first argument? Robert Wessel <robertwessel2@yahoo.com> - 2017-03-13 18:20 -0500
                Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-14 00:31 +0100
    Re: va_start without the first argument? raltbos@xs4all.nl (Richard Bos) - 2017-03-13 16:51 +0000
  Re: va_start without the first argument? jacobnavia <jacob@jacob.remcomp.fr> - 2017-03-11 19:29 +0100
  Re: va_start without the first argument? Siri Cruise <chine.bleu@yahoo.com> - 2017-03-11 12:12 -0800
  Re: va_start without the first argument? luser droog <luser.droog@gmail.com> - 2017-03-13 05:37 -0700
    Re: va_start without the first argument? luser droog <luser.droog@gmail.com> - 2017-03-15 11:45 -0700
      Re: va_start without the first argument? Thiago Adams <thiago.adams@gmail.com> - 2017-03-15 13:39 -0700

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