Re: [boost] BOOST_FORCE_INLINE

Subject: Re: [boost] BOOST_FORCE_INLINE
From: Jeffrey Lee Hellrung, Jr. (jeffrey.hellrung_at_[hidden])
Date: 2011-10-10 02:22:47

On Sun, Oct 9, 2011 at 6:08 PM, Beman Dawes <bdawes_at_[hidden]> wrote:

> On Sun, Oct 9, 2011 at 8:51 PM, Mathias Gaunard
> <mathias.gaunard_at_[hidden]> wrote:
> > On 10/10/2011 02:02 AM, Olaf van der Spek wrote:
> >>
> >> On Mon, Oct 10, 2011 at 1:50 AM, Mathias Gaunard
> >> <mathias.gaunard_at_[hidden]>  wrote:
> >>>
> >>> They're slightly different: __forceinline works just like the normal
> >>> inline
> >>> specifier, while __attribute__((always_inline)) can be placed in
> various
> >>> places like other GCC attributes, but only has an effect if the
> function
> >>> is
> >>> also inline.
> >>
> >> Shouldn't the decision to inline be left to the compiler?
> >
> > The point of that macro is to override the compiler analysis.
> >
> > Compilers are not perfect, so it is beneficial to mark to functions
> inline
> > manually for a variety of reasons:
> >
> >  - you'll get an error or warning if the compiler cannot inline it
> >  - you can still get the inlining in debug mode, which can be necessary
> to
> > make applications that depend a lot on meta-programming to run at
> acceptable
> > performance
> >  - compiling for size is not always very aggressive with inlining, but a
> lot
> > of meta-programming things rely a lot on aggressive inlining to happen
> >  - for codebases with a lot of forwarding the compiler can easily get
> > confused
> >  - inlining doesn't seem to take into account the costs of register
> spilling
> > required by a function call, which is important to avoid in low-level
> code.
> >
> > Do people generally agree it is ok to add this in Boost.Config, and maybe
> > make some key libraries use it?
>
> Shouldn't there be documentation that describes when to use it and
> when not to use it? I guess my main concern is that it might be used
> inappropriately if the name is the only guideline developers have.
>

+1, my thoughts exactly, Beman.  Can anyone provide any links to a case
study or something similar investigating the effects of forcing inlining?
I'm definitely interested to see what real effects this could have.

- Jeff