Re: [PATCH v4 2/2] x86: Add general_regs_only function attribute

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, Apr 21, 2021 at 10:09 AM Martin Sebor <msebor@gmail.com> wrote:
>
> On 4/14/21 4:39 PM, H.J. Lu wrote:
> > commit 87c753ac241f25d222d46ba1ac66ceba89d6a200
> > Author: H.J. Lu <hjl.tools@gmail.com>
> > Date:   Fri Aug 21 09:42:49 2020 -0700
> >
> >      x86: Add target("general-regs-only") function attribute
> >
> > is incomplete since it is impossible to call integer intrinsics from
> > a function with general-regs-only target attribute.
> >
> > 1. Add general_regs_only function attribute to inform the compiler that
> > functions use only general purpose registers.  When making inlining
> > decisions on such functions, non-GPR compiler options are excluded.
> > 2. Add general_regs_only attribute to x86 intrinsics which use only
> > general purpose registers.
> >
> ...
> > --- a/gcc/doc/extend.texi
> > +++ b/gcc/doc/extend.texi
> > @@ -7066,6 +7066,11 @@ On x86 targets, the @code{fentry_section} attribute sets the name
> >   of the section to record function entry instrumentation calls in when
> >   enabled with @option{-pg -mrecord-mcount}
> >
> > +@item general_regs_only
> > +@cindex @code{general_regs_only} function attribute, x86
> > +The @code{general_regs_only} attribute on functions is used to
> > +inform the compiler that functions use only general purpose registers.
>
> I'll just reiterate basically the same comment as before: it's not
> clear from the very brief description above what the requirements
> are for using the attribute.  I'm guessing it can be applied to
> any function (inline or otherwise) but only has any effect when
> the function is actually inlined and otherwise doesn't constrain
> what the function can do.  (Whatever the constraints are, I think
> the manual should spell them out, and likewise for its effects.)

That is correct.

> Similarly it's not clear what should be expected when the function
> does use some other register.  Ideally, I think GCC would check and
> issue a nice error message whether or not the function is inlined
> or called.  I suspect that might only be possible for inline
> functions that are actually called and for which the back end must
> emit code.

This is what GCC does today:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99744

> Other than that, I'd suggest to improve the phrasing a bit:
>
>    The @code{general_regs_only} function attribute indicates that
>    the function uses only general purpose registers... [text
>    explaining constraints and errors follows].
>
> Martin

How about this

@item general_regs_only
@cindex @code{general_regs_only} function attribute, x86
The @code{general_regs_only} attribute on functions is used to inform
the compiler that functions use only general purpose registers.  It
can be used together with the @code{always_inline} attribute to avoid
inlining failure when there is a mismatch in compiler vector options.

Thanks.

-- 
H.J.