register

[orium69]

hi everyone, i'm wondering if there is a way to have sure that a
variable is allocated in the cache, after its declaration with
"register"? Tks!

[Gordon Burditt]

>hi everyone, i'm wondering if there is a way to have sure that a

variable is allocated in the cache, after its declaration with
"register"? Tks!

On all processors I have ever heard of with a cache, actual CPU
registers are never part of the cache. Actual CPU registers
are normally much faster than the cache.

You can never be sure that the keyword 'register' will affect
the code at all. And if you use it enough, you *WILL* run out
of actual CPU registers. If you could force a variable into a
CPU register, it might slow down the code because the register
might be better used as an invisible temporary. The compiler
is often smarter than you are.

Gordon L. Burditt

[Jordan Abel]

On 2005-11-09, Gordon Burditt <go***********@ burditt.org> wrote:

hi everyone, i'm wondering if there is a way to have sure that a
variable is allocated in the cache, after its declaration with
"register"? Tks!

On all processors I have ever heard of with a cache, actual CPU
registers are never part of the cache. Actual CPU registers
are normally much faster than the cache.

You can never be sure that the keyword 'register' will affect
the code at all. And if you use it enough, you *WILL* run out
of actual CPU registers. If you could force a variable into a
CPU register, it might slow down the code because the register
might be better used as an invisible temporary. The compiler
is often smarter than you are.

The compiler also isn't required to listen to you. All "register" means
is that the compiler is free to not give the variable an address. This
could otherwise be determined by code analysis, i'm sure, but a keyword
is simpler.

[Kenny McCormack]

In article <11************ *********@z14g2 000cwz.googlegr oups.com>,
<or*****@gmail. com> wrote:

hi everyone, i'm wondering if there is a way to have sure that a
variable is allocated in the cache, after its declaration with
"register"? Tks!

The answers you will get will all boil down to "You can't know and you
shouldn't want to know."

However, the bottom line is: Invoke the compiler with an option that causes
it to output assembly (e.g., "gcc -S") and then look at the generated
assembly. You should be able to figure out what it did with your
"register" directive.

[Mark McIntyre]

On Wed, 09 Nov 2005 20:10:12 GMT, in comp.lang.c ,
ga*****@yin.int eraccess.com (Kenny McCormack) wrote:

In article <11************ *********@z14g2 000cwz.googlegr oups.com>,
<or*****@gmail. com> wrote:

hi everyone, i'm wondering if there is a way to have sure that a
variable is allocated in the cache, after its declaration with
"register"? Tks!
The answers you will get will all boil down to "You can't know and you
shouldn't want to know."

However, the bottom line is: Invoke the compiler with an option that causes
it to output assembly (e.g., "gcc -S") and then look at the generated
assembly. You should be able to figure out what it did with your
"register" directive.

This won't tell you a thing about how the CPU will use the cache at
runtime. Never mind that if its in a register, it cannot be in a
cache...

In fact, this is a classic example of why answering offtopic questions
here is a bad idea.
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

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[Alex Fraser]

"Jordan Abel" <jm****@purdue. edu> wrote in message
news:sl******** ***********@ran dom.yi.org...
[snip]

The compiler also isn't required to listen to you. All "register" means
is that the compiler is free to not give the variable an address.
I would say all it means is that you cannot take the address of the object.
What you wrote follows from that fact.
This could otherwise be determined by code analysis, i'm sure, but a
keyword is simpler.

These days, such analysis (and then some) is certainly practical, but
presumably it wasn't always that way, and that is why the keyword exists.

Alex

[Keith Thompson]

Jordan Abel <jm****@purdue. edu> writes:

On 2005-11-09, Gordon Burditt <go***********@ burditt.org> wrote:

hi everyone, i'm wondering if there is a way to have sure that a
variable is allocated in the cache, after its declaration with
"register" ? Tks!

On all processors I have ever heard of with a cache, actual CPU
registers are never part of the cache. Actual CPU registers
are normally much faster than the cache.

You can never be sure that the keyword 'register' will affect
the code at all. And if you use it enough, you *WILL* run out
of actual CPU registers. If you could force a variable into a
CPU register, it might slow down the code because the register
might be better used as an invisible temporary. The compiler
is often smarter than you are.

The compiler also isn't required to listen to you. All "register" means
is that the compiler is free to not give the variable an address. This
could otherwise be determined by code analysis, i'm sure, but a keyword
is simpler.

Here's what the standard says, C99 6.7.1p4:

A declaration of an identifier for an object with storage-class
specifier register suggests that access to the object be as fast
as possible. The extent to which such suggestions are effective is
implementation-defined.

with a footnote:

The implementation may treat any register declaration simply as an
auto declaration. However, whether or not addressable storage is
actually used, the address of any part of an object declared with
storage-class specifier register cannot be computed, either
explicitly (by use of the unary & operator as discussed in
6.5.3.2) or implicitly (by converting an array name to a pointer
as discussed in 6.3.2.1). Thus, the only operator that can be
applied to an array declared with storage-class specifier register
is sizeof.

6.5.4.2 says you can't apply unary "&" to a register-qualified object.
6.3.2.1 says that the implicit conversion of an array expression to a
pointer to its first element invokes undefined behavior if the
designated array object has register storage class. (I'm not sure why
it's undefined behavior rather than a constraint violation.)n

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Kenny McCormack]

In article <mb************ *************** *****@4ax.com>,
Mark McIntyre <ma**********@s pamcop.net> wrote:
....

This won't tell you a thing about how the CPU will use the cache at
runtime. Never mind that if its in a register, it cannot be in a
cache...
You're being needlessly pedantic - and, might I add, definitely not PC.
I think we can assume that the OP meant "CPU register" when he said
"cache". Remember, these dorks use terminology very loosely and generally
just repeat words they've overheard.
In fact, this is a classic example of why answering offtopic questions
here is a bad idea.

Sez you.

[Keith Thompson]

ga*****@yin.int eraccess.com (Kenny McCormack) writes:

In article <mb************ *************** *****@4ax.com>,
Mark McIntyre <ma**********@s pamcop.net> wrote:
...

This won't tell you a thing about how the CPU will use the cache at
runtime. Never mind that if its in a register, it cannot be in a
cache...

You're being needlessly pedantic - and, might I add, definitely not PC.
I think we can assume that the OP meant "CPU register" when he said
"cache".

Perhaps, but I don't think we *need* to assume any such thing. It's
likely that the OP didn't understand what registers and cache really
are. If he's been following this thread, he should now have a much
better understanding of the terms.

(The term "cache" as a generic term is applicable to registers, but in
this context it more commonly refers to something else.)

I suppose we could have guessed what he meant by the term "cache" and
pretended his usage was correct. Then we could have fooled him into
thinking that he was getting a real answer to his question, something
like "Just use the register keyword, it guarantees that the variable
will be stored in cache."

But most of us are more interested in passing on knowledge than in
playing stupid games like that.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[pete]

Keith Thompson wrote:

ga*****@yin.int eraccess.com (Kenny McCormack) writes: But most of us are more interested in passing on knowledge than in
playing stupid games like that.

But Kenny isn't.

http://www.codecomments.com/Unix_Pro...age665290.html

<OT>
Poor Kenny. The people in comp.lang.c aren't amenable to redefining
the purpose of the newsgroup to suit him. One wonders why he continues
to hang around there ;-)
</OT>
I stick around, read, and post, for the same reasons I do in
any other joke newsgroup - because it is so much fun.

--
pete