On Tue, Aug 2, 2011 at 3:23 PM, Ian Lance Taylor <i...@google.com> wrote:
> Richard Guenther <richard.guent...@gmail.com> writes:
>
>> Or go one step further and deprecate local register variables alltogether
>> (they IMHO don't make much sense, and rather the targets should provide
>> a way to properly constrain asm inputs and outputs).
>
> No, local register variables are documented as working and many programs
> rely on them. They are a straightforward way to get an asm argument in
> a specific register, and I don't see any reason to break that.
Well, maybe they look like so. But in reality there is _no_ connection
from the register setup to the actual asm. Which is the problem the
compiler faces here (apart from the libcall issue). If there should be
an implicit dependence of all asms to all local register var setters
and users then this isn't implemented on gimple (or rather it works
by chance there as we treat register vars as memory and do not
disambiguate anything across asms (yet)).
>> 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?
Richard.
> Ian
>
