Ulrich Weigand wrote:
> Richard Guenther wrote:
>> On Tue, Aug 2, 2011 at 3:23 PM, Ian Lance Taylor <i...@google.com> wrote:
>>> Richard Guenther <richard.guent...@gmail.com> writes:
>>>> I suggest to amend the documentation for local call-clobbered register
>>>> variables to say that the only valid sequence using them is from a
>>>> non-inlinable function that contains only direct initializations of the
>>>> register variables from constants or parameters.
>>> Let's just implement those requirements in the compiler itself.
>> Doesn't work for existing code, no? And if thinking new code then
>> I'd rather have explicit dependences (and a way to represent them).
>> Thus, for example
>>
>> asm ("scall" : : "asm("r0")" (10), ...)
>>
>> thus, why force new constraints when we already can figure out
>> local register vars by register name? Why not extend the constraint
>> syntax somehow to allow specifying the same effect?
Yes this would be exact equivalence of
register int var asm ("r0") = 10;
...
asm ("scall" : : "r" (var), ...)
> Maybe it would be possible to implement this while keeping the syntax
> of existing code by (re-)defining the semantics of register asm to
> basically say that:
>
> If a variable X is declared as register asm for register Y, and X
> is later on used as operand to an inline asm, the register allocator
> will choose register Y to hold that asm operand. (And this is the
> full specification of register asm semantics, nothing beyond this
> is guaranteed.)
Yes, that's reasonable. As I understand the docs, in code like
void foo ()
{
register int var asm ("r1") = 10;
asm (";; use r1");
}
there is nothing that connects var to the asm and assuming that
r1 holds 10 in the asm is a user error.
The only place where the asm attached to a variable needs to have
effect are the inline asm sequences that explicitly refer to
respective variables. If there is no inline asm referencing a
local register variable, there is on difference to a non-register
auto variable; there could even be a warning that in such a case
that
register int var asm ("r1") = 10;
is equivalent to
int var = 10;
> It seems this semantics could be implemented very early on, probably
> in the frontend itself. The frontend would mark the *asm* statement
> as using the specified register (there would be no special handling
> of the *variable* as such, after the frontend is done). The optimizers
> would then simply be required to pass the asm-statement register
> annotations though, much like today they pass constraints through.
> At the point where register allocation decisions are made, those
> register annotations would then be acted on.
>
> Bye,
> Ulrich
I wonder why it does not work like that in the current implementation.
Local register variable is just like using a similar constraint
(with the only difference that in general there is no such constraint,
otherwise the developer would use it). A pass like .asmcons could
take care of it just the same way it does for constraints and no
optimizer passed would have to bother if a variable is a local register
or not.
This would render local register variables even more functional
because no one needed to care if there were implicit library calls
or things like that.
Johann
