On Wed, Apr 21, 2021 at 10:09 AM Martin Sebor <mse...@gmail.com> wrote:
>
> On 4/14/21 4:39 PM, H.J. Lu wrote:
> > commit 87c753ac241f25d222d46ba1ac66ceba89d6a200
> > Author: H.J. Lu <hjl.to...@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.
