Re: user-mode port 0.44-2.4.7

[Davide Libenzi <davidel@xmailserver.org>]

On 24-Jul-2001 Alexander Viro wrote:
> 
> 
> On Tue, 24 Jul 2001, Davide Libenzi wrote:
> 
>> I would not call, to pretend the compiler to issue memory loads every time it access
>> a variable, a nontrivial way.
>> It sounds pretty clear to me.
> 
> You know, one of the nice things about C is that unless you abuse
> preprocessor, reading code doesn't require doing far lookups. Most
> of it can be read and understood with very little context. "Do it
> once when you declare a variable" goes against that and that's
> not a good thing.

Look, you're not going to request any kind of black magic over that variable.
You're simply telling the compiler the way it has to ( not ) optimize the code.
This is IMHO a declaration time issue.
Looking at this code :

while (jiffies < ...) {
        ...
}

the "natural" behaviour that a reader expects is that the "content" of the memory
pointed by  jiffied  is loaded and compared.
That content, not the content of a register loaded 100 asm instructions before the load.
If you like this code more :

for (;;) {
        barrier();
        if (jiffies >= ...)
                break;
        ...
}

It's clear that a declaration like :

__locked_access__ struct pio {
        int a, b, c;
};


for (;;) {
        ++a;
        if (a > b && c < a)
        ...
}

sounds a "Bad Thing"(tm) even to me.




- Davide