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Re: new __builtin_choose_type (patch)
On Tue, 2001-10-02 at 21:54, Geoff Keating wrote:
> Aldy Hernandez <firstname.lastname@example.org> writes:
> > btw, i can't get my builtin to return a given object (irregardless of
> > it's type) because the way builtins work seems to force me to choose a
> > return value type. i don't want a specific value type. i want
> > something generic. see code documentation below.
> What does va_arg do?
__builtin_next_arg returns a pointer to the next argument, obviously
only working on stack arguments.
if we were to follow a similar approach (have the builtin return a void
pointer), we could do:
float f1, f2, f3;
f2 = *((float *)__builtin_choose_type (f1, floatt, f3, etc)
/* f1 and floatt are of the same type, so eval and return f3 */
in theory, the builtin would return the address of "f3" as a void*. we
could then cast that to the appropriate type. However, this will only
work with lvalues (since they're the only ones with addresses :-)). and
will definitely not work with "f3" being a function call.
perhaps, the whole builtin mechanism needs to be hacked for it to return
IMO, this builtin return type needs to be calculated before
expand_builtin(). we could put in a hook when we're calling the builtin
to calculate and set the appropriate return type. this way the rest of
the FE can do proper type checking.
ok, so it sounds a bit complicated, but it's pretty clear in my head
:). i'll come up with a patch to demonstrate later today. but if
anyone has a better idea on how to implement anonymous return types in
builtins, feel free to chime in.
Aldy Hernandez E-mail: email@example.com
Professional Gypsy on a Motorcycle
Red Hat, Inc.