Re: Inline functions and locals

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c++
Subject	Re: Inline functions and locals
Date	2022-01-14 18:47 +0100
Organization	A noiseless patient Spider
Message-ID	<srsd0a$o56$1@dont-email.me> (permalink)
References	<srmvu4$oie$1@gioia.aioe.org> <srqkvk$3vi$1@dont-email.me> <srrmd6$1bo0$1@gioia.aioe.org> <srru5t$939$1@dont-email.me> <srs873$ijn$1@dont-email.me>

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On 14/01/2022 17:26, James Kuyper wrote:
> On 1/14/22 8:34 AM, James Kuyper wrote:
> ...
>> standard's wording above makes "inline" no more than a hint. An
>> implementation is free to perform inline substitution of any function
>> call, whether or not the function is declared 'inline', so long as the
>> result produces the same observable effects as calling a separate
>> function.
> 
> I wanted to point out that the opposite transformation is also
> permitted: and implementation can take code out of the function where it
> is written, and replace it with a call to a separate function created by
> the implementation containing that code.
> 
> Why would an implementation do this? For the opposite of the reason it
> would inline some code. In general, inlining trades off an increase in
> the execution speed against increased code size. There are important
> exceptions: the inlined code could be smaller than the code that would
> needed to implement the function call. Even if it isn't, inline
> substitution could open up opportunities for optimization involving
> interactions between the code inside the inlined function, and code
> surrounding the function call. However, when neither of those cases
> apply, each call to a function that gets inlined increases the size of
> the generated code.
> 
> When I first mentioned this possibility, I had no idea whether any
> existing compiler would do this, but I thought it unlikely. I was
> corresponding surprised by a response that identified a particular
> compiler that actually did it. As I might have anticipated, that
> compiler was cross-compiling for a memory-starved embedded system.
> Unfortunately, that was a decade ago, and I no longer remember any details.
> 

If you are interested in such transformations, gcc does them (given the
right flags, appropriate code, and perhaps additional information such
as LTO, profile-guided optimisation or "hot" and "cold" function
attributes).  In particular, it can pull out rarely used parts of code
into a separate function and re-use it from more than one place, or put
it in a different section to improve cache usage, or in connection with
function cloning and constant propagation.  (Imagine a function that
takes a boolean parameter.  In some cases, you get better results if you
have two versions of the function generated, each with a fixed value for
that parameter, and the choice of the function to use is made at the
call site.)  Functions can also be partially inlined.

How often the compiler does such transformations, and how effective they
are in practice, I have no idea.

Thread

Inline functions and locals Muttley@dastardlyhq.com - 2022-01-12 16:34 +0000
  Re: Inline functions and locals Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-12 17:38 +0100
  Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-12 12:47 -0500
  Re: Inline functions and locals Juha Nieminen <nospam@thanks.invalid> - 2022-01-13 07:00 +0000
    Re: Inline functions and locals "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-01-13 10:45 +0100
      Re: Inline functions and locals Juha Nieminen <nospam@thanks.invalid> - 2022-01-17 12:55 +0000
        Re: Inline functions and locals David Brown <david.brown@hesbynett.no> - 2022-01-17 15:14 +0100
        Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-17 10:24 -0800
          Re: Inline functions and locals David Brown <david.brown@hesbynett.no> - 2022-01-17 19:46 +0100
    Re: Inline functions and locals Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-13 11:08 +0100
      Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-13 19:30 -0800
  Re: Inline functions and locals Paavo Helde <eesnimi@osa.pri.ee> - 2022-01-13 12:46 +0200
    Re: Inline functions and locals Muttley@dastardlyhq.com - 2022-01-13 15:38 +0000
  Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-13 17:51 -0800
    Re: Inline functions and locals Muttley@dastardlyhq.com - 2022-01-14 11:22 +0000
      Re: Inline functions and locals Öö Tiib <ootiib@hot.ee> - 2022-01-14 04:02 -0800
        Re: Inline functions and locals Manfred <invalid@invalid.add> - 2022-01-14 17:30 +0100
          Re: Inline functions and locals Muttley@dastardlyhq.com - 2022-01-14 16:49 +0000
        Re: Inline functions and locals Muttley@dastardlyhq.com - 2022-01-14 16:40 +0000
          Re: Inline functions and locals Öö Tiib <ootiib@hot.ee> - 2022-01-14 11:28 -0800
          Re: Inline functions and locals "Alf P. Steinbach" <alf.p.steinbach@gmail.com> - 2022-01-14 20:29 +0100
      Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-14 08:34 -0500
        Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-14 11:26 -0500
          Re: Inline functions and locals David Brown <david.brown@hesbynett.no> - 2022-01-14 18:47 +0100
      Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-14 08:43 -0800
        Re: Inline functions and locals Juha Nieminen <nospam@thanks.invalid> - 2022-01-17 05:38 +0000
          Re: Inline functions and locals David Brown <david.brown@hesbynett.no> - 2022-01-17 08:12 +0100
            Re: Inline functions and locals "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-01-16 23:51 -0800
              Re: Inline functions and locals "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2022-01-16 23:55 -0800
              Re: Inline functions and locals David Brown <david.brown@hesbynett.no> - 2022-01-17 11:35 +0100
            Re: Inline functions and locals Juha Nieminen <nospam@thanks.invalid> - 2022-01-17 09:50 +0000
              Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-17 14:39 -0500
                Re: Inline functions and locals Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-17 15:34 -0800
                Re: Inline functions and locals "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-01-17 15:50 -0800
                Re: Inline functions and locals Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-04-25 03:42 -0700
                Re: Inline functions and locals "james...@alumni.caltech.edu" <jameskuyper@alumni.caltech.edu> - 2022-04-25 08:48 -0700
          Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-16 23:53 -0800
            Re: Inline functions and locals Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-17 15:29 -0800
    Re: Inline functions and locals Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-17 12:40 +0100
      Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-17 10:13 -0800
        Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-17 16:15 -0500
          Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-17 16:28 -0800
            Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-17 22:55 -0500
              Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-18 11:32 -0800
                Re: Inline functions and locals James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-18 19:03 -0500
                Re: Inline functions and locals Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-18 17:22 -0800
        Re: Inline functions and locals Bonita Montero <Bonita.Montero@gmail.com> - 2022-01-18 09:20 +0100

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