Re: [PATCH] ARM half-precision floating point, 3/8 (target hooks)

[Sandra Loosemore <sandra at codesourcery dot com>]

Paolo Bonzini wrote:
> Sandra Loosemore wrote:
> > This patch adds the target hooks that are required to get the C and
> > C++ language-level support for HFmode, specifically including the
> > implicit promotion and restrictions on function argument/return types.
> > The included documentation patch should be sufficient to explain the
> > details.
>
> Is this necessary at all?  Can we define HFmode to be a storage-only
> type at the language level?

It would be nice if we could do that.  I imagine that now that half-precision is 
in the IEEE standard, it's going to be picked up by other targets and may make 
it into future versions of the C/C++ language standards as well (although maybe 
not with the exact semantics that I've implemented here).  I thought that the 
language-level issues were quite straightforward, both conceptually and 
implementation-wise.

OTOH, it was a lot of work to retro-fit the ARM back end to know about HFmode, 
and most of it was not even related to the 2 new instructions.  (The constant 
pool handling gave us fits, for instance.)  I clearly do not have either the 
time or target-specific knowledge to retrofit all the other back ends, too, and 
it's not realistic to think somebody else is going to do all that work.  ;-)  So 
if we did add HFmode knowledge to the compiler front end, we'd still need some 
sort of target hook to control whether it's supported at all by the back end.

Anyway, I could rework this part of the implementation if there's consensus that 
it's the right thing to do, that the semantics as currently implemented for ARM 
are useful in general, etc.  I'd have to give it some thought to come up with a 
new design for the front end/back end interface, though -- so I'd prefer to get 
some guidance that it's really the right direction before diving into it.

-Sandra