On Mon, Aug 24, 2020 at 6:17 PM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> On Mon, Aug 24, 2020 at 7:55 AM Uros Bizjak <ubiz...@gmail.com> wrote:
> >
> > On Mon, Aug 24, 2020 at 3:23 PM H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> > > > Speaking of pragmas, these should be added outside cpuid.h, like:
> > > >
> > > > #pragma GCC push_options
> > > > #pragma GCC target("general-regs-only")
> > > >
> > > > #include <cpuid.h>
> > > >
> > > > void cpuid_check ()
> > > > ...
> > > >
> > > > #pragma GCC pop_options
> > > >
> > > > >footnote
> > > >
> > > > Nowadays, -march=native is mostly used outside generic target
> > > > compilations, so for relevant avx512 targets, we still generate spills
> > > > to mask regs. In future, we can review the setting of the tuning flag
> > > > for a generic target in the same way as with SSE2 inter-reg moves.
> > > >
> > >
> > > Florian raised an issue that we need to limit <cpuid.h> to the basic ISAs.
> > > <cpuid.h> should be handled similarly to other intrinsic header files.
> > > That is <cpuid.h> should use
> > >
> > > #pragma GCC push_options
> > > #ifdef __x86_64__
> > > #pragma GCC target("arch=x86-64")
> > > #else
> > > #pragma GCC target("arch=i386")
> > > ...
> > > #pragma GCC pop_options
> > >
> > > Here is a patch. OK for master?
> >
> > -ENOPATCH
> >
> > However, how will this affect inlining? Every single function in
> > cpuid.h is defined as static __inline, and due to target flags
> > mismatch, it won't be inlined anymore. These inline functions are used
> > in some bit testing functions, and to keep them inlined, these should
> > also use the same options to avoid non-basic ISAs. This is the reason
> > cpuid.h should be #included after pragma, together with bit testing
> > functions, as shown above.
> >
>
> How about target("baseline-isas-only")? All CPUID functions are
> inlined.
No, I don't think this is a good idea. Now consider the situation that
caller functions are compiled with e.g. -mgeneral-regs-only. Due to
#pragmas, CPUID functions are compiled with a superset ISAs, so they
again won't be inlined. ISAs of caller functions and CPUID should
match, the best way is to include <cpuid.h> after the #pragma. And
IMO, general-regs-only target #pragma is an excellent setting for
both: cpuid.h and caller bit testing functions.
So, if we care about inlining, decorating cpuid.h with target pragmas
is a bad idea.
Uros.
