[Linux-ia64] Using f6-f11 floating-point registers

[Linux-ia64] Using f6-f11 floating-point registers

[Jun Nakajima]

At this point, the kernel is using more than f6-f11 floating-point
registers (using -mfixed-rangeñ0-f15,f32-f127, i.e. those are *not*
available to the kernel, but the others, like f16-f31 are). 

When do we support the reduced FP model (EL_IA_64_REDUCEFP), where only
f6-f11 are used by the kernel? Does the current compiler allow the
kernel to use those FP registers only, if we provide
-mfixed-rangeò-f5,f12-f127, for example? Of course, we need to change
the kernel if we do this.

Thanks,
-- 
Jun U Nakajima
Core OS Development
SCO/Murray Hill, NJ

Re: [Linux-ia64] Using f6-f11 floating-point registers

[James Wilson]

Richard Henderson checked in a compiler patch on June 13 that eliminated the
need for the compiler to allocate integer and FP values to different FP
registers.  So as far as I know, there is no longer any compiler problem
that should affect the kernel's use of FP registers.  This change will be in
the next toolchain release.

Jim

Re: [Linux-ia64] Using f6-f11 floating-point registers

[David Mosberger]

>>>>> On Tue, 27 Jun 2000 18:02:09 -0400, Jun Nakajima <jun@sco.com> said:

  Jun> At this point, the kernel is using more than f6-f11
  Jun> floating-point registers (using -mfixed-rangeñ0-f15,f32-f127,
  Jun> i.e. those are *not* available to the kernel, but the others,
  Jun> like f16-f31 are).

If the compiler or someone writing assembly code thinks it's
beneficial to use a preserved fp register, we should let them use it.
Of course, they'll have to explicitly save/restore the register before
using it, but there may well be cases where this is beneficial, so we
shouldn't prevent it for no good reason.

  Jun> When do we support the reduced FP model (EL_IA_64_REDUCEFP),
  Jun> where only f6-f11 are used by the kernel?

For the record, I have no plans to adopt the REDUCEFP model.  I want
to reserve the right for kernel developers to tweak the kernel
register usage without having to talk to a standards body.  Sorry.

	--david

Re: [Linux-ia64] Using f6-f11 floating-point registers

[Jun Nakajima]

Well, I was just wondering because your (with Don) viewgraphs "IA-64
Linux Kernel Internals" (on the IA-64 Linux Web home page) says (p. 9)
"considering a change to only provide f6-f11 to the compiler for integer
multiply and divide", and I wanted to know how serious you were about
it.

David Mosberger wrote:
> 
> >>>>> On Tue, 27 Jun 2000 18:02:09 -0400, Jun Nakajima <jun@sco.com> said:
> 
>   Jun> At this point, the kernel is using more than f6-f11
>   Jun> floating-point registers (using -mfixed-rangeñ0-f15,f32-f127,
>   Jun> i.e. those are *not* available to the kernel, but the others,
>   Jun> like f16-f31 are).
> 
> If the compiler or someone writing assembly code thinks it's
> beneficial to use a preserved fp register, we should let them use it.
> Of course, they'll have to explicitly save/restore the register before
> using it, but there may well be cases where this is beneficial, so we
> shouldn't prevent it for no good reason.
> 
>   Jun> When do we support the reduced FP model (EL_IA_64_REDUCEFP),
>   Jun> where only f6-f11 are used by the kernel?
> 
> For the record, I have no plans to adopt the REDUCEFP model.  I want
> to reserve the right for kernel developers to tweak the kernel
> register usage without having to talk to a standards body.  Sorry.
> 
>         --david

-- 
Jun U Nakajima
Core OS Development
SCO/Murray Hill, NJ