Using temporary register Variable

[Shankar Agarwal]

I was trying to use a variable whose value is temporary and i don't want
it to go to the memory for performance reasons. Is there a way to specify
the variable like that so that any assignment to the variable does not go
to memory. I tried the type register but it does not seem to work.
Thanks
Shankar

[Mark A. Odell]

Shankar Agarwal <sh******@stanf ord.edu> wrote in
news:Pi******** *************** **************@ cardinal5.Stanf ord.EDU:

I was trying to use a variable whose value is temporary and i don't want
it to go to the memory for performance reasons. Is there a way to
specify the variable like that so that any assignment to the variable
does not go to memory. I tried the type register but it does not seem to
work.

'register' is just a hint, easily ignored by the compiler. Alas, this is
your only standard method to get into a real register. Assembler is your
friend if things are this critical.

--
- Mark ->
--

[Tristan Miller]

Greetings.

In article
<Pi************ *************** **********@card inal5.Stanford. EDU>,
Shankar Agarwal wrote:

I was trying to use a variable whose value is temporary and i don't
want it to go to the memory for performance reasons. Is there a way to
specify the variable like that so that any assignment to the variable
does not go to memory.

No, not in standard C. As you've discovered, the "register" specifier
is no more than a hint to the compiler, which it's free to ignore.
(And as most modern compilers are far better at optimizing code than
the average human, they usually do ignore it.) Your compiler may
provide an extension to force register allocation. Is the code really
that performance-critical, though? Optimization should always be done
at the algorithmic level first; worrying about little things like
register variables is rarely worth one's effort.

Regards,
Tristan

--
_
_V.-o Tristan Miller [en,(fr,de,ia)] >< Space is limited
/ |`-' -=-=-=-=-=-=-=-=-=-=-=-=-=-=-= <> In a haiku, so it's hard
(7_\\ http://www.nothingisreal.com/ >< To finish what you

[Malcolm]

"Shankar Agarwal" <sh******@stanf ord.edu> wrote in message

I tried the type register but it does not seem to work.

"register" is only a hint. You can try changing compilers, you can try
changing the type of the variable (is it an int?), you can try simplifying
the function it is in and removing other variables, though of course this
might not be possible.
Virtually every C compiler comes with an inline assembler, and you probably
want to resort to this.

[Joe Wright]

Malcolm wrote:

"Shankar Agarwal" <sh******@stanf ord.edu> wrote in message

I tried the type register but it does not seem to work.

"register" is only a hint. You can try changing compilers, you can try
changing the type of the variable (is it an int?), you can try simplifying
the function it is in and removing other variables, though of course this
might not be possible.
Virtually every C compiler comes with an inline assembler, and you probably
want to resort to this.

Ah.. but then it wouldn't be C, would it? Portable, etc.
--
Joe Wright http://www.jw-wright.com
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[Malcolm]

"Joe Wright" <jo********@ear thlink.net> wrote in message

Virtually every C compiler comes with an inline assembler, and you
probably want to resort to this.

Ah.. but then it wouldn't be C, would it? Portable, etc.

What he can do is write two versions of the function, one in C and one using
the assembler. Then use #ifdef [something defined by the compiler which
tells you what it is] to conditionally compile the code.

This isn't a perfect solution, since there's always the risk that a
maintainer might alter one version of the function and not another, breaking
the program. However it's probably the best that can be done.

[August Derleth]

Joe Wright <jo********@ear thlink.net> wrote in
news:3F******** ***@earthlink.n et on Sat 15 Nov 2003 11:52:02a:

Malcolm wrote:

"Shankar Agarwal" <sh******@stanf ord.edu> wrote in message

>
> I tried the type register but it does not seem to work.
>

"register" is only a hint. You can try changing compilers, you can try
changing the type of the variable (is it an int?), you can try
simplifying the function it is in and removing other variables, though
of course this might not be possible.
Virtually every C compiler comes with an inline assembler, and you
probably want to resort to this.

Ah.. but then it wouldn't be C, would it? Portable, etc.

It's not off-topic to say that you might need to use a non-portable method
to accomplish something, but the person should have the sense not to come
back here to discuss the nonportable aspects of the code (except maybe to
verify that they are, in fact, nonportable).

[Dan Pop]

In <Pi************ *************** **********@card inal5.Stanford. EDU> Shankar Agarwal <sh******@stanf ord.edu> writes:

I was trying to use a variable whose value is temporary and i don't want
it to go to the memory for performance reasons. Is there a way to specify
the variable like that so that any assignment to the variable does not go
to memory. I tried the type register but it does not seem to work.

If your compiler decided to ignore your request for keeping the variable
in a register, it is usually because it has better uses for the available
registers and satisfying your request would result in slower code. Do you
have a good reason for wanting to slow down the generatede code?

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Malcolm]

"Dan Pop" <Da*****@cern.c h> wrote in message

If your compiler decided to ignore your request for keeping the
variable in a register, it is usually because it has better uses for the
available registers and satisfying your request would result in slower
code. Do you have a good reason for wanting to slow down the
generatede code?

It's not always as easy as that.

Consider this function.

void child_benefit(S TUDENT *st, int N)
{
int i;
for(i=0;i<N;i++ )
{
for(j=0;j<st[i].nkids;j++)
{
st[i].benefit += 100;
}
}
}

Now as 21st century Westerners we know that N is likely to be large (lots of
students at a univerity) and that 99% of the time the nkids member will be
zero (most students have no children).
Therefore if there is a choice between keeping i in a register and keeping a
register hardwired to 100, we should keep i in the register.
Change the function to "professors " instead of "students" and "number of
lectures" instead of "number of children" and the dynamics chage again -
most universities only have a handful of professors who give quite a lot of
lectures in a year.
The compiler has no way of knowing this, and sometimes the human programmer
does know best.

[Chris Dollin]

Malcolm wrote:

It's not always as easy as that.

Consider this function.

void child_benefit(S TUDENT *st, int N)
{
int i;
for(i=0;i<N;i++ )
{
for(j=0;j<st[i].nkids;j++)
{
st[i].benefit += 100;
}
}
}

Now as 21st century Westerners we know that N is likely to be large (lots
of students at a univerity) and that 99% of the time the nkids member will
be zero (most students have no children).
Therefore if there is a choice between keeping i in a register and keeping
a register hardwired to 100, we should keep i in the register.

Just in passing, it seems to me that in this function we can keep all
of N, st, i, and 100 [if we need to] in registers. So it's not a
discriminator.

[Yeah, yeah, I'm stuck on this x86 box, but my heart [1] belongs to the
ARM.]

[1] That part of it that doesn't belong to Renaissance, Reiner Knizia,
Patricia McKillip, and Karnataka.

--
Chris "four chambers is Not Enough!" Dollin
C FAQs at: http://www.faqs.org/faqs/by-newsgrou...mp.lang.c.html
C welcome: http://www.angelfire.com/ms3/bchambl...me_to_clc.html