Re: [parisc-linux] Does it lakes some cloberred r1 in __put_kernel_asm() 64bit?

["Michael S. Zick" <mszick@morethan.org>]

On Tue April 18 2006 15:35, Carlos O'Donell wrote:
> On 4/16/06, Joel Soete <soete.joel@tiscali.be> wrote:
> > Hello all,
> >
> > I tried to apply my 'beautify' asm template to some uaccess.h and figure out that
> > this hunk din't mentioned that "r1" was cloberred (iirc in fixup text) while
> > the 32bit one did:
> >
> > mmm, doesn't seems to change anything (I just check sys_parisc32.s) but may be elsewhere?
> 
> Please put on your thinking cap.
>
Please do not assume that was not step 1.

> 
> a) What does a clobber mean from the viewpoint of an __asm?
>
Why not from the syntax used?

Two possibilities:
1) Compiler can not distinguish the effects of assemble statements
on register usage.  It has to be told, as a general rule.

2) Compiler can distinguish the effects of assemble statements.
In this case, there is no reason for the clobber field.

Since there IS a clobber field in the syntax, then (2) must be
the intended assumption.

> b) Which insn clobber r1?
>
If (1) holds, it does not matter, the compiler already knows
when r1 will be trashed.

If (2) holds, the code author should know.
 
> Research both and return with the answer.
> Double marks if you solve...
> 
> c) Should any of the __put_kernel_asm's have r1 listed as a clobber?
> 
if (1) holds, it does not matter.

if (2) holds, then at least two more possibilities:

3) The compiler never carries a live value in r1 across an __asm block.
This is the same result as if r1 was listed as a clobber from the viewpoint
of preserving a value in r1 - there isn't one to preserve.

4) The compiler considers r1 a general purpose register.
In this case, the code author should not make an exception from the 
general rule implied by the supplied syntax of __asm blocks.

Q.E.D:
Safe coding practice over the lifetime of changing compiler versions
answers the question - r1 should appear in the clobber list of any
block that changes its value.

Either:
a) It makes no difference for some combinations of the above;
Or:
b) It is required.

> Cheers,
Your welcome,
Mike

> Carlos.
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> 
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