--- Andrew Pinski wrote:
> On May 24, 2006, at 2:54 AM, Etienne Lorrain wrote:
> > Another calling convention could be to not only return the "return
> > value" in %eax (or %edx:%eax for long long returns) but also its
> > comparisson to zero in the flags, so that you get:
> > call foo
> > jg .Lwarning
> > jnz .Lerror
>
> And you think this will help? It will at most 1-10 cycles depending
> on the processor.
The same can be said of register passing argument, at least for ia32
passing a parameter in %edx/%eax instead of the stack saves one or
two loads so few cycles - and by increasing the register pressure you
often get a slower and (a lot) bigger function (what I have seen).
But in some cases, a function attribute placed in the right position
(usually very small functions) can help.
> And if you have a call in the hot loop, you are screwed
> anyways because you will have to deal with the overhead of the call.
I am not sure of what you are refering to, but there is plenty of
places where you are screwed - for instance the stack readjustment is
the better done by "mov %ebp,%esp" instead of "add $16,%esp".
> So it will end up being about even.
I was thinking of very small functions, kind of one instruction,
something like:
asm (" atomic_dec: \n lock dec (%eax) \n ret ");
extern unsigned atomic_dec (unsigned *counter) __attribute__((return_flags));
void fct (void) { while (atomic_dec()) wait(); }
But it was an intermediate solution to the problem of passing two return
values to a function - I am not sure it worth the time to implement.
Etienne.
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