Re: Alpha tas() patch

Brent Verner <brent(at)rcfile(dot)org> writes:
> This is a revised patch that I sent earlier to allow building
> pg-7.1 with gcc as well as DEC's cc. I've had good results with this
> applied. Could some other Alpha users try this out. Even better, could
> an Alpha asm guru look over the asm that I'm using (instead of the
> original asm in the file).

tas() is not supposed to contain a loop. It can succeed or fail, but
it should not wait.

The code now in s_lock.h does seem rather gratuitously obscure about
the instructions it uses to load constants. I'd suggest

static __inline__ int
tas(volatile slock_t *lock)
{
register slock_t _res;

__asm__(" ldq $0, %0 \n\
bne $0, 2f \n\
ldq_l $0, %0 \n\
bne $0, 2f \n\
mov 1, $0 \n\
stq_c $0, %0 \n\
beq $0, 2f \n\
mov 0, %1 \n\
mb \n\
jmp $31, 3f \n\
2: mov 1, %1 \n\
3: nop ": "=m"(*lock), "=r"(_res): :"0");

return (int) _res;
}

As you say, the first two instructions don't seem to be really
necessary. I suppose the idea is to avoid setting the processor
lock address register unless there's a pretty good chance of
acquiring the lock ... but why bother? Does LDQ_L take a long
time to execute? Seems like avoiding the extra two instructions
would be a win most of the time.

regards, tom lane