Re: PROPOSAL: Extend inline asm syntax with size spec

Richard Biener
2018-10-10T10:29:10Z

On Wed, 10 Oct 2018, Segher Boessenkool wrote:

> On Wed, Oct 10, 2018 at 09:22:40AM +0200, Ingo Molnar wrote:
> > * Richard Biener <rguenther@suse.de> wrote:
> > > Can kernel folks give this a second and third thought please so we
> > > don't implement sth that in the end won't satisfy you guys?
> >
> > So this basically passes '0 size' to the inliner, which should be better
> > than passing in the explicit size, as we'd inevitably get it wrong
> > in cases.
>
> The code immediately after this makes it size 1, even for things like
> asm(""), I suppose this works better for the inliner. But that's a detail
> (and it might change); the description says "consider this asm as minimum
> length and cost for inlining decisions", which works for either 0 or 1.

It was made 1 as otherwise the inliner happily explodes on

void foo () { asm(""); foo(); }

> > I also like 'size 0' for the reason that we tend to write assembly code
> > and mark it 'inline' if we really think it matters to performance,
> > so making it more likely to be inlined when used within another inline
> > function is a plus as well.
>
> You can think of it as meaning "we want this asm inlined always", and then
> whether that actually happens depends on if the function around it is
> inlined or not.

Richard.