static variables access and gp

static variables access and gp

[Tinga Shilo]

Hi,
I am implementing a kernel mechanism which is 
very performance oriented. Along my long critical
path,
there is a static variable that needs to be accessed
quite a few times. This variable is a structure which
is approximately 60 bytes big.
In there any way I can "convince" my kernel (compiled
with gcc) to access this variable using gp ?
Is gp usually used for this purpose in mips-linux ?
Can it be ?

A while ago I saw a small discussion here about usage
of gp for static variables, but it didn't provide
any definite answers.
TIA
T


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Re: static variables access and gp

[Jun Sun]

On Wed, Mar 05, 2003 at 11:30:17PM -0800, Tinga Shilo wrote:
> Hi,
> I am implementing a kernel mechanism which is 
> very performance oriented. Along my long critical
> path,
> there is a static variable that needs to be accessed
> quite a few times. This variable is a structure which
> is approximately 60 bytes big.
> In there any way I can "convince" my kernel (compiled
> with gcc) to access this variable using gp ?
> Is gp usually used for this purpose in mips-linux ?
> Can it be ?
>

No.  gp is used by kernel to hold current process in 2.4
and current thread in 2.5.  Don't mess with it unless
you are absolutely sure what you are doing.

Jun

Re: static variables access and gp

[Tinga Shilo]

Thx. Got it.
Any other solution to speed up the access to this
variable ? It's location never changes from kernel
init to shutdown if I haven't made that clear.

--- Jun Sun <jsun@mvista.com> wrote:
> On Wed, Mar 05, 2003 at 11:30:17PM -0800, Tinga
> Shilo wrote:
> > Hi,
> > I am implementing a kernel mechanism which is 
> > very performance oriented. Along my long critical
> > path,
> > there is a static variable that needs to be
> accessed
> > quite a few times. This variable is a structure
> which
> > is approximately 60 bytes big.
> > In there any way I can "convince" my kernel
> (compiled
> > with gcc) to access this variable using gp ?
> > Is gp usually used for this purpose in mips-linux
> ?
> > Can it be ?
> >
> 
> No.  gp is used by kernel to hold current process in
> 2.4
> and current thread in 2.5.  Don't mess with it
> unless
> you are absolutely sure what you are doing.
> 
> Jun


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Re: static variables access and gp

[Tinga Shilo]

Thx. Got it.
Any other solution to speed up the access to this
variable ? It's location never changes from kernel
init to shutdown if I haven't made that clear.

--- Jun Sun <jsun@mvista.com> wrote:
> On Wed, Mar 05, 2003 at 11:30:17PM -0800, Tinga
> Shilo wrote:
> > Hi,
> > I am implementing a kernel mechanism which is 
> > very performance oriented. Along my long critical
> > path,
> > there is a static variable that needs to be
> accessed
> > quite a few times. This variable is a structure
> which
> > is approximately 60 bytes big.
> > In there any way I can "convince" my kernel
> (compiled
> > with gcc) to access this variable using gp ?
> > Is gp usually used for this purpose in mips-linux
> ?
> > Can it be ?
> >
> 
> No.  gp is used by kernel to hold current process in
> 2.4
> and current thread in 2.5.  Don't mess with it
> unless
> you are absolutely sure what you are doing.
> 
> Jun


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Re: static variables access and gp

[Ralf Baechle]

On Wed, Mar 05, 2003 at 11:30:17PM -0800, Tinga Shilo wrote:

> I am implementing a kernel mechanism which is 
> very performance oriented. Along my long critical
> path,
> there is a static variable that needs to be accessed
> quite a few times. This variable is a structure which
> is approximately 60 bytes big.
> In there any way I can "convince" my kernel (compiled
> with gcc) to access this variable using gp ?
> Is gp usually used for this purpose in mips-linux ?
> Can it be ?
> 
> A while ago I saw a small discussion here about usage
> of gp for static variables, but it didn't provide
> any definite answers.

Use a pointer to the that structure.  Gcc will keep this pointer in a
registers wherever it considers that sensible.  Any reference to members
of the structure can then be made with just a single instruction.

$28 is used for the current pointer so only suitable for per-process
data.

  Ralf