On Wed, Apr 21, 2021 at 4:24 PM Martin Sebor <mse...@gmail.com> wrote:
>
> On 4/21/21 2:58 PM, H.J. Lu wrote:
> > 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
>
> Yes, that's the rather obscure error I think I commented on before
> and suggested should be improved. Based on r99744-3.c I don't think
> this has changed in the improved patch.
My goal is to fix the inline failures, not to improve the compiler error
message.
> >
> >> 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.
>
> Without an article the part "that functions use only general purpose
> registers" is unclear and/or grammatically incorrect. What functions?
> If the function the attribute is applied to, it needs an article, e.g.,
> "the function" or "a function", and singular. (Otherwise it could be
> read as talking about the functions called from the one with
> the attribute, or some other functions altogether).
>
> I tried to correct that above but, if you prefer, the following would
> be closer to your phrasing but more correct/accurate:
>
> The @code{general_regs_only} function attribute informs
> the compiler that the function uses only general purpose
> registers.
>
> I don't understand what the second sentence is trying to say, and
> without a better error message for the problem in r99744, I suspect
> few users will either. I am suggesting to explain in the text you
> are adding, under what conditions inlining might fail without
> the attribute, and what effect the attribute has on the function
> that prevents the inlining failure.
How about this?
@item general_regs_only
@cindex @code{general_regs_only} function attribute, x86
The @code{general_regs_only} function attribute informs the compiler
that the function uses only general purpose registers. When the
compiler inlines a function with the @code{always_inline} attribute,
target-specific compilation options may lead to inline failures.
The @code{general_regs_only} attribute, if applicable, can be used
together with the @code{always_inline} attribute to reduce inlining
failure.
> (If we can't explain what the effect is then I wonder why
> the attribute is being added at all instead of teaching GCC to
> always behave as if the attribute were there when its absence
> would otherwise lead to an error.)
>
Inlining an always_inline function into a function, which doesn't
support the ISA needed by the always_inline function, should fail.
But when inlining the always_inline function, the compiler doesn't
know if GPR instructions are sufficient for the always_inline function.
The general_regs_only informs the compiler that the function uses
only general purpose registers. There are no other ways for the
compiler to deduce such info at this stage.
--
H.J.
