Floating point usage

Floating point usage

[Pavel Roskin]

Hello!

I tried to compile the current grub on Fedora 9 for PowerPC, but it
failed due to floating point usage in ls.  Sorry, I don't have the exact
message handy.

Anyway, there are several references to "float" and "double" in the
code, and I think it's wrong.  GRUB should be able to work on systems
with no floating point support and on systems where the floating point
device needs to be activated.  Even Linux kernel avoids using floating
point.

If everyone is fine, I'll try to remove all floating point stuff from
the code.

-- 
Regards,
Pavel Roskin

Re: Floating point usage

[Jan Kleinsorge]

Pavel Roskin wrote:
> Hello!
> 
> I tried to compile the current grub on Fedora 9 for PowerPC, but it
> failed due to floating point usage in ls.  Sorry, I don't have the exact
> message handy.
> 
> Anyway, there are several references to "float" and "double" in the
> code, and I think it's wrong.  GRUB should be able to work on systems
> with no floating point support and on systems where the floating point
> device needs to be activated.  Even Linux kernel avoids using floating
> point.
> 
> If everyone is fine, I'll try to remove all floating point stuff from
> the code.
> 
 Hi Pavel,

there is no need for hard-float support on the target machine. The
libgcc provides all the software-emulation routines. Given that GRUB is
linked against it. Which is likely as it is a necessity when compiling
with gcc. Maybe your compilation is simply broken because of that one
missing. Could you please post the error message?


Jan

Re: Floating point usage

[Stefan Reinauer]

Jan Kleinsorge wrote:
> Pavel Roskin wrote:
>   
>> Hello!
>>
>> I tried to compile the current grub on Fedora 9 for PowerPC, but it
>> failed due to floating point usage in ls.  Sorry, I don't have the exact
>> message handy.
>>
>> Anyway, there are several references to "float" and "double" in the
>> code, and I think it's wrong.  GRUB should be able to work on systems
>> with no floating point support and on systems where the floating point
>> device needs to be activated.  Even Linux kernel avoids using floating
>> point.
>>
>> If everyone is fine, I'll try to remove all floating point stuff from
>> the code.
>>
>>     
>  Hi Pavel,
>
> there is no need for hard-float support on the target machine. The
> libgcc provides all the software-emulation routines. Given that GRUB is
> linked against it. Which is likely as it is a necessity when compiling
> with gcc. Maybe your compilation is simply broken because of that one
> missing. Could you please post the error message?
Nonetheless, using Floating Point operations in bootloaders and firmware  is highly discouraged. If Floating Point is required for a bootloader (except, maybe, possibly, at all, for encryption) the bootloader is definitely broken, either in implementation or in design.

Also keep in mind, most operating systems expect the FPU to be in power on state.

Stefan


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Re: Floating point usage

[Colin D Bennett]

On Thu, 05 Jun 2008 17:23:00 +0200
Stefan Reinauer <stepan@coresystems.de> wrote:

> Jan Kleinsorge wrote:
> > Pavel Roskin wrote:
> >   
> >> Hello!
> >>
> >> I tried to compile the current grub on Fedora 9 for PowerPC, but it
> >> failed due to floating point usage in ls.  Sorry, I don't have the
> >> exact message handy.
> >>
> >> Anyway, there are several references to "float" and "double" in the
> >> code, and I think it's wrong.  GRUB should be able to work on
> >> systems with no floating point support and on systems where the
> >> floating point device needs to be activated.  Even Linux kernel
> >> avoids using floating point.
> >>
> >> If everyone is fine, I'll try to remove all floating point stuff
> >> from the code.
> >>
> >>     
> >  Hi Pavel,
> >
> > there is no need for hard-float support on the target machine. The
> > libgcc provides all the software-emulation routines. Given that
> > GRUB is linked against it. Which is likely as it is a necessity
> > when compiling with gcc. Maybe your compilation is simply broken
> > because of that one missing. Could you please post the error
> > message?
> Nonetheless, using Floating Point operations in bootloaders and
> firmware  is highly discouraged. If Floating Point is required for a
> bootloader (except, maybe, possibly, at all, for encryption) the
> bootloader is definitely broken, either in implementation or in
> design.
> 
> Also keep in mind, most operating systems expect the FPU to be in
> power on state.
> 
> Stefan

The bootloader is "broken" if it uses floating point arithmetic?  I
think that while GRUB should not REQUIRE an FPU, it could use software
floating point if there was a good reason to do so (and it would not
be broken as long is it was done portably). Though I only have minimal
knowledge of encryption algorithms, I don't think that floating point
math is often used.

I think that avoiding floating point math as Pavel Roskin suggested is
good, though I don't agree that Stefan's assertion that any use of
floating point math is necessarily "broken" -- it might be better to
say that we'd prefer not to pull in softfloat libraries (necessary
since many targets have no FPU); therefore it is desirable to avoid
floating point math.

Colin

Re: Floating point usage

[Vincent Pelletier]

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Le Thursday 05 June 2008 17:12:26 Jan Kleinsorge, vous avez écrit :
> Given that GRUB is linked against it. Which is likely as it is a necessity
> when compiling with gcc.

...except if -nostdlib is given to gcc, which is the case in grub2. For the 
bootloader part at least, grub-emu is another question.
And there is an except to this except: If -lgcc is specified it will be used, 
and it is for the following archs, as far as I can see:
 - powerpc-ieee1275
 - i386-linuxbios
 - i386-ieee1275

So half the supported archs don't need libgcc, I think it's enough to try 
avoiding that dependency.

-- 
Vincent Pelletier

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Re: Floating point usage

[Robert Millan]

On Thu, Jun 05, 2008 at 10:01:38PM +0200, Vincent Pelletier wrote:
> And there is an except to this except: If -lgcc is specified it will be used, 
> and it is for the following archs, as far as I can see:
>  - powerpc-ieee1275
>  - i386-linuxbios
>  - i386-ieee1275

Why would i386-linuxbios and i386-ieee1275 need libgcc but i386-pc don't?
IIRC the tendency is that CISC platforms tend not to need libgcc for floating
point but RISC ones do.  Did I make a mistake on these two ports?

-- 
Robert Millan

<GPLv2> I know my rights; I want my phone call!
<DRM> What good is a phone call… if you are unable to speak?
(as seen on /.)

Re: Floating point usage

[Stefan Reinauer]

Robert Millan wrote:
> Why would i386-linuxbios and i386-ieee1275 need libgcc but i386-pc don't?
> IIRC the tendency is that CISC platforms tend not to need libgcc for floating
> point but RISC ones do.  Did I make a mistake on these two ports?
>   
Those platforms require libgcc even for double precision integer
divisions. It's a bit nasty.


-- 
coresystems GmbH • Brahmsstr. 16 • D-79104 Freiburg i. Br.
      Tel.: +49 761 7668825 • Fax: +49 761 7664613
Email: info@coresystems.de  • http://www.coresystems.de/
Registergericht: Amtsgericht Freiburg • HRB 7656
Geschäftsführer: Stefan Reinauer • Ust-IdNr.: DE245674866

Re: Floating point usage

[Vesa Jääskeläinen]

Pavel Roskin wrote:
> Hello!
> 
> I tried to compile the current grub on Fedora 9 for PowerPC, but it
> failed due to floating point usage in ls.  Sorry, I don't have the exact
> message handy.
> 
> Anyway, there are several references to "float" and "double" in the
> code, and I think it's wrong.  GRUB should be able to work on systems
> with no floating point support and on systems where the floating point
> device needs to be activated.  Even Linux kernel avoids using floating
> point.
> 
> If everyone is fine, I'll try to remove all floating point stuff from
> the code.

Its only for printing units in human readable format. So its only small 
arithmetics to get it going nicely with integers. While you are there, 
can you modify it to use more proper units... MiB , KiB,...