Why is -msoft-float used in Kernel compilation?

Why is -msoft-float used in Kernel compilation?

[Gavriel State]

I can understand using -msoft-float if CONFIG__8xx is on,
but why use it when compiling for 60x or 7x0 CPUs?

-Gav

--
Gavriel State
Engineering Project Leader - Linux Apps
Corel Corp
gavriels@corel.com



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Re: Why is -msoft-float used in Kernel compilation?

[Michael Meissner]

On Sat, Feb 13, 1999 at 09:37:46AM -0500, Gavriel State wrote:
> 
> I can understand using -msoft-float if CONFIG__8xx is on,
> but why use it when compiling for 60x or 7x0 CPUs?

I would imagine to prevent the compiler from using the FPU to move 64-bit
aligned items via fp registers.

-- 
Michael Meissner, Cygnus Solutions (Massachusetts office)
4th floor, 955 Massachusetts Avenue, Cambridge, MA 02139, USA
meissner@cygnus.com,	617-354-5416 (office),	617-354-7161 (fax)

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Re: Why is -msoft-float used in Kernel compilation?

[Michel Lanners]

On  13 Feb, this message from Gavriel State echoed through cyberspace:
> 
> I can understand using -msoft-float if CONFIG__8xx is on,
> but why use it when compiling for 60x or 7x0 CPUs?

I've heard something about a 'no floiting point in kernel' rule, and
I've tried compiling without -msoft-float.... it does boot, but it's an
ugly sight... fp warning messages all over!

I have, however, never seen a clear explanation of why that rule
exists...

Any guru around?

Michel

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Re: Why is -msoft-float used in Kernel compilation?

[Troy Benjegerdes]

On Sun, 14 Feb 1999, Michel Lanners wrote:

> 
> On  13 Feb, this message from Gavriel State echoed through cyberspace:
> > 
> > I can understand using -msoft-float if CONFIG__8xx is on,
> > but why use it when compiling for 60x or 7x0 CPUs?
> 
> I've heard something about a 'no floiting point in kernel' rule, and
> I've tried compiling without -msoft-float.... it does boot, but it's an
> ugly sight... fp warning messages all over!
> 
> I have, however, never seen a clear explanation of why that rule
> exists...

I'm not sure, but I believe it's an optimization to allow the kernel to
not have to save floating point registers on every context switch.

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| This message composed with 100% free software.    http://www.gnu.org   |
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Re: Why is -msoft-float used in Kernel compilation?

[Gary Thomas]

On 15-Feb-99 Troy Benjegerdes wrote:
> 
> On Sun, 14 Feb 1999, Michel Lanners wrote:
> 
>> 
>> On  13 Feb, this message from Gavriel State echoed through cyberspace:
>> > 
>> > I can understand using -msoft-float if CONFIG__8xx is on,
>> > but why use it when compiling for 60x or 7x0 CPUs?
>> 
>> I've heard something about a 'no floiting point in kernel' rule, and
>> I've tried compiling without -msoft-float.... it does boot, but it's an
>> ugly sight... fp warning messages all over!
>> 
>> I have, however, never seen a clear explanation of why that rule
>> exists...
> 
> I'm not sure, but I believe it's an optimization to allow the kernel to
> not have to save floating point registers on every context switch.
> 

More importantly it is to keep GCC from trying to use FP registers to
do structure copies.  Without this option, GCC will try to use the FP
registers to perform high[er] performance copying of structures.  This
would mean that the kernel would have to save/restore some number of
FP registers on every context change, not just the switch from user
to kernel mode, because these registers are considered totally volatile
by the compiler.

------------------------------------------------------------------------
Gary Thomas                              |
email: gdt@linuxppc.org                  | "Fine wine is a necessity of
   ... opinions expressed here are mine  |        life for me"
       and no one else would claim them! |
                                         |      Thomas Jefferson
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