[avr-libc-dev] [bug #14616] definition of sei() in interrupts.h is faulty

[anonymous]

Follow-up Comment #2, bug #14616 (project avr-libc):

This addition will worsen quality of a code. Look an example:

int acc;
int volatile val;

void foo1 (void)
{
    unsigned char i;
    for (i = 10; i; i--) {
        asm volatile ("cli");
        acc += val;
        asm volatile ("sei");
    }
}

void foo2 (void)
{
    unsigned char i;
    for (i = 10; i; i--) {
        asm volatile ("cli");
        acc += val;
        asm volatile ("sei" ::: "memory");
    }
}

In function 'foo1' the compiler has born reading (and record) a variable
'acc' for frameworks of a cycle:
        ...
        lds r18,acc
        lds r19,(acc)+1
.L6:
/* #APP */
        cli
/* #NOAPP */
        lds r24,val
        lds r25,(val)+1
        add r18,r24
        adc r19,r25
/* #APP */
        sei
/* #NOAPP */
        subi r20,lo8(-(-1))
        brne .L6
        sts (acc)+1,r19
        sts acc,r18

In function 'foo2' reading/writing of a variable 'acc' is carried out on
everyone itteration:
        ...
.L15:
/* #APP */
        cli
/* #NOAPP */
        lds r24,acc
        lds r25,(acc)+1
        lds r18,val
        lds r19,(val)+1
        add r24,r18
        adc r25,r19
        sts (acc)+1,r25
        sts acc,r24
/* #APP */
        sei
/* #NOAPP */
        subi r20,lo8(-(-1))
        brne .L15
        ...

May be, if necessary it is better to add 'volatile' in
definition of a concrete variable, than to endow speed always?

Dmitry.


    _______________________________________________________

Reply to this item at:

  <http://savannah.nongnu.org/bugs/?func=detailitem&item_id=14616>

_______________________________________________
  Message sent via/by Savannah
  http://savannah.nongnu.org/