BSD Dissatisfied with gcc... why? [LWN.net]

BSD Dissatisfied with gcc... why?

Posted Nov 19, 2008 14:36 UTC (Wed) by dgm (subscriber, #49227)
In reply to: BSD Dissatisfied with gcc... why? by etienne_lorrain@yahoo.fr
Parent article: pcc seeks contributions to reach 1.0 milestone

> the basic use of volatile (register mapped) integers

Wrong. I think you were thinking of the "register" keyword.
"Volatile" means that the contents of a variable can be changed externally at any time. Think, for instance, of a memory mapped hardware register. Basically it instructs the compiler to avoid certain optimizations based on knowledge of the previous value, and forces it to read it every time.

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BSD Dissatisfied with gcc... why?

Posted Nov 19, 2008 20:57 UTC (Wed) by nix (subscriber, #2304) [Link]

In particular, 'volatile' is still useful. 'register' is, basically, not,
with the single rare exception of global register variables (a GCC
extension).

BSD Dissatisfied with gcc... why?

Posted Nov 20, 2008 10:34 UTC (Thu) by etienne_lorrain@yahoo.fr (guest, #38022) [Link]

> > the basic use of volatile (register mapped) integers
> Wrong. I think you were thinking of the "register" keyword.

No, I was and am still talking of "volatile".
The problem is that the C++ standard treats all attributes the same way, and gives examples with "const".
I do understand that to overwrite the "const" attribute, I need to do a dirty cast to non-const, but the basic use of volatile is to copy them into standard variables, at a controled place of the software:

volatile unsigned ethernet_status;
/* -> address of ethernet_status defined in the linker file */
int fct(void) {
unsigned status = ethernet_status; // single read of "ethernet_status"
return ((status & 1) || ((status & 2) ^ (status & 4)));
}

Now tell me how to put ethernet_status and fct() into a class and compile without warning and without casting ethernet_status to non-volatile...

But the worst for me is still considering a volatile structure of bitfields as a structure of volatile bitfields, even if I see no problem to consider a constant structure of bitfields as a structure of constant bitfields...
For instance:
volatile struct color_s { char red, green, blue, transparent; } volcolor;
int fct (void) {
struct color_s color = volcolor;
return color.red == color.blue;
}
Because volcolor is considered as a structure of volatile fields, volcolor is read twice (two byte access) when optimising.