Re: [PATCH 2/2] asm inline

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Sun, Nov 11, 2018 at 02:33:34PM -0700, Martin Sebor wrote:
> On 10/30/2018 11:30 AM, Segher Boessenkool wrote:
> >The Linux kernel people want a feature that makes GCC pretend some
> >inline assembler code is tiny (while it would think it is huge), so
> >that such code will be inlined essentially always instead of
> >essentially never.
> >
> >This patch lets you say "asm inline" instead of just "asm", with the
> >result that that inline assembler is always counted as minimum cost
> >for inlining.  It implements this for C and C++.
> 
> Rather than overloading the inline keyword I think it would be
> more in keeping both with the design of existing features to
> control inlining in GCC and with the way the languages are
> evolving to allow the always_inline (and perhaps also noinline)
> attribute to apply to asm statements.

We already "overloaded" the volatile and goto keywords here (it's not
overloading, the contexts are completely disjoint).

always_inline is a function attribute, and you want to make it a statement
attribute (which we do not currently support except for empty statements!)
as well.

> The inline keyword is commonly understood to be just a hint to
> the compiler.

That is exactly what it is here.  It hints the compiler that this asm
is cheap, whatever it may look like.

> The attributes, on the other hand, are recognized
> as binding requests to inline (if possible) or avoid inlining,
> respectively.

And that is not what this does (that would be hard to do, in fact).

> >+You can use @code{asm inline} instead of @code{asm} to have the assembler
> >+code counted as mimimum size for inlining purposes; @pxref{Size of an 
> >asm}.


Segher