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Re: Compile Assembly and C files together

From David Brown <david.brown@hesbynett.no>
Newsgroups comp.lang.c
Subject Re: Compile Assembly and C files together
Date 2018-03-07 17:54 +0100
Organization A noiseless patient Spider
Message-ID <p7p5fp$gee$1@dont-email.me> (permalink)
References (6 earlier) <hrznC.15256$5c2.6550@fx09.am4> <slrnp9tmm0.eu5.grahn+nntp@frailea.sa.invalid> <p7ohnt$po$1@dont-email.me> <slrnpa03eb.eu5.grahn+nntp@frailea.sa.invalid> <uGUnC.244480$3K4.25330@fx12.am4>

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On 07/03/18 17:45, bartc wrote:
> On 07/03/2018 16:05, Jorgen Grahn wrote:
>> On Wed, 2018-03-07, David Brown wrote:
>>> On 06/03/18 19:15, Jorgen Grahn wrote:
>>>> On Tue, 2018-03-06, bartc wrote:
>> ...
>>>>> The important thing though is not to let gcc optimise the code.
>>>>
>>>> Why?  The optimization level cannot change the calling conventions, or
>>>> you'd need a copy of e.g. the standard library for each optimization
>>>> setting.
>>>>
>>>
>>> Perhaps he just wants to make poorer quality code from the compiler, so
>>> that he can pretend the hand-made assembly is better?  Maybe he doesn't
>>> want to read the documentation, and learn how to integrate assembly with
>>> the compiler generation?
>>
>> Sounds likely; I mostly wanted to oppose the idea that calling
>> conventions change with optimization.  (Not sure if that was his idea,
>> but it was easy to get that impression.)
> 

Some people make additional assumptions about code generation beyond 
what you get in the ABI - such as assuming when and how data is written 
to memory, or which registers are used for which purpose.  It is, of 
course, a terrible idea - but some people do it.

> They can do.

No, they can't.

> If a compiler knows that B() will only ever be called from 
> A(), then it can generate whatever code is best, no matter what the API 
> says. That includes inlining B().
> 

If B() is declared with external linkage, the compiler will generate 
code that allows it to be used externally using the ABI for the target 
(and compiler).  It might /also/ generate inlined versions, or clones 
with different calling conventions, or clones with partial inlining, or 
other optimisation.


> And in my example, the normal entry/exit code needed to conform to the 
> ABI, and which was present with -O0, was eliminated completely with -O3, 
> to just a jump. It shows that when you have a chain of calls, some 
> overheads can be removed.

Of course - normally you don't want more overhead and entry/exit code 
than is actually necessary.

> 
> But in my example, you need those overheads in place as they may be 
> needed when you put real code into the function via the .s file; the 
> compiler doesn't know that. It assumes the code submitted to it (as .c) 
> is the code that is to be run.

No, the compiler is perfectly happy with code written in any supported 
language, including assembly - it just needs to follow the rules of the ABI.

Your assembly code - even if it is in an external .s file - does not 
need any more entry/exit code than required by the ABI.

> 
> This is why you want more conservative asm code as a start point.
> 

Perhaps /you/ want it, but it is certainly not required.  At best it 
might give you a higher chance of working than pure guesswork, but it is 
no substitution for actually learning what you are doing.

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Thread

Compile Assembly and C files together Ilias Abrams <iliasabrams@gmail.com> - 2018-03-05 08:23 -0800
  Re: Compile Assembly and C files together "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-03-06 19:20 +0800
    Re: Compile Assembly and C files together druck <news@druck.org.uk> - 2018-03-06 13:53 +0000
      Re: Compile Assembly and C files together Rob Morley <nospam@ntlworld.com> - 2018-03-06 14:59 +0000
        Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-06 16:35 +0100
          Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-06 07:57 -0800
            Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-06 16:35 +0000
              Re: Compile Assembly and C files together Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-03-06 18:15 +0000
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-06 18:51 +0000
                Re: Compile Assembly and C files together Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-03-06 19:22 +0000
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-06 19:30 +0000
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-06 12:31 -0800
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-06 23:05 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 12:17 +0100
                Re: Compile Assembly and C files together Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-03-07 16:05 +0000
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-07 16:45 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 17:54 +0100
                Re: Compile Assembly and C files together Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-03-07 16:59 +0000
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-07 17:13 +0000
              Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 12:26 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-07 14:25 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 15:57 +0100
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-07 15:24 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 17:56 +0100
            Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 11:37 +0100
              Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-07 13:11 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-07 15:47 +0100
                Re: Compile Assembly and C files together Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2018-03-07 07:31 -0800
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-07 21:00 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-08 12:33 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-08 13:05 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-08 15:14 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-08 15:23 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-08 23:22 +0100
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-08 14:40 -0800
                Re: Compile Assembly and C files together Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-03-08 23:59 +0000
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-08 16:54 -0800
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-09 10:37 +0100
                Re: Compile Assembly and C files together Ben Bacarisse <ben.usenet@bsb.me.uk> - 2018-03-09 10:47 +0000
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-09 09:19 -0800
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-09 10:23 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-09 12:28 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-09 16:43 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-09 17:14 +0000
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-09 09:32 -0800
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-08 15:26 +0000
                Re: Compile Assembly and C files together Jorgen Grahn <grahn+nntp@snipabacken.se> - 2018-03-08 19:23 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-08 23:27 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-08 16:17 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-08 23:47 +0100
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-09 11:59 +0000
              Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-07 09:13 -0800
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-07 17:42 +0000
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-07 10:55 -0800
      Re: Compile Assembly and C files together "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-03-06 23:01 +0800
    Re: Compile Assembly and C files together ray carter <ray@zianet.com> - 2018-03-06 15:44 +0000
      Re: Compile Assembly and C files together "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-03-07 00:07 +0800
        Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-07 22:39 +0200
          Re: Compile Assembly and C files together "Mr. Man-wai Chang" <toylet.toylet@gmail.com> - 2018-03-08 18:18 +0800
            Re: Compile Assembly and C files together Martin Gregorie <martin@mydomain.invalid> - 2018-03-08 12:37 +0000
              Re: Compile Assembly and C files together Martin Gregorie <martin@mydomain.invalid> - 2018-03-08 13:13 +0000
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-08 13:34 +0000
                Re: Compile Assembly and C files together Martin Gregorie <martin@mydomain.invalid> - 2018-03-08 15:27 +0000
                Re: Compile Assembly and C files together mm0fmf <none@invalid.com> - 2018-03-08 17:33 +0000
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-09 09:06 +0000
                Re: Compile Assembly and C files together Richard Kettlewell <invalid@invalid.invalid> - 2018-03-09 09:17 +0000
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-09 09:30 +0000
                Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-09 15:56 +0200
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-09 14:13 +0000
                Re: Compile Assembly and C files together Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2018-03-09 11:14 -0500
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-09 17:44 +0000
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-09 17:16 +0100
                Re: Compile Assembly and C files together Robert Wessel <robertwessel2@yahoo.com> - 2018-03-09 11:19 -0600
                Re: Compile Assembly and C files together bartc <bc@freeuk.com> - 2018-03-09 18:11 +0000
                Re: Compile Assembly and C files together Robert Wessel <robertwessel2@yahoo.com> - 2018-03-09 13:41 -0600
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-09 10:12 -0800
                Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-09 19:20 +0100
                Re: Compile Assembly and C files together supercat@casperkitty.com - 2018-03-09 11:45 -0800
                Re: Compile Assembly and C files together Robert Wessel <robertwessel2@yahoo.com> - 2018-03-09 13:48 -0600
                Re: Compile Assembly and C files together Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2018-03-09 13:16 -0500
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-09 18:29 +0000
                Re: Compile Assembly and C files together Jack Fearnley <jack@alcor.concordia.ca> - 2018-03-09 19:07 +0000
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-09 19:13 +0000
                Re: Compile Assembly and C files together Michael J. Mahon <mjmahon@aol.com> - 2018-03-10 13:16 -0600
                Re: Compile Assembly and C files together Martin Gregorie <martin@mydomain.invalid> - 2018-03-11 19:39 +0000
                Re: Compile Assembly and C files together Michael J. Mahon <mjmahon@aol.com> - 2018-03-12 01:16 -0500
                Re: Compile Assembly and C files together Martin Gregorie <martin@mydomain.invalid> - 2018-03-12 20:49 +0000
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-12 16:11 +0000
                Re: Compile Assembly and C files together Michael J. Mahon <mjmahon@aol.com> - 2018-03-12 14:58 -0500
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-12 20:38 +0000
                Re: Compile Assembly and C files together scott@slp53.sl.home (Scott Lurndal) - 2018-03-12 16:06 +0000
                Re: Compile Assembly and C files together Michael J. Mahon <mjmahon@aol.com> - 2018-03-12 14:58 -0500
                Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-09 21:27 +0200
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-09 17:40 +0000
                Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-09 21:30 +0200
                Re: Compile Assembly and C files together Richard Damon <Richard@Damon-Family.org> - 2018-03-09 13:16 -0500
                Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-09 21:33 +0200
                Re: Compile Assembly and C files together mm0fmf <none@invalid.com> - 2018-03-09 17:40 +0000
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-09 17:47 +0000
                Re: Compile Assembly and C files together mm0fmf <none@invalid.com> - 2018-03-09 18:05 +0000
                Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-09 09:05 +0000
                Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-09 16:01 +0200
            Re: Compile Assembly and C files together Tauno Voipio <tauno.voipio@notused.fi.invalid> - 2018-03-08 17:25 +0200
    Re: Compile Assembly and C files together The Natural Philosopher <tnp@invalid.invalid> - 2018-03-07 07:36 +0000
  Re: Compile Assembly and C files together Richard Kettlewell <invalid@invalid.invalid> - 2018-03-06 15:29 +0000
    Re: Compile Assembly and C files together David Brown <david.brown@hesbynett.no> - 2018-03-06 16:38 +0100

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