Re: [PATCH] define WIDEST_HARDWARE_FP_SIZE on mips

[Geert Bosch <bosch@adacore.com>]

On Oct 10, 2007, at 08:25, Richard Sandiford wrote:
> Your reply seems to be about why it's useful to have a macro that
> provides the widest efficient floating-point type.  For avoidance
> of doubt, I can certainly appreciate that such a thing is useful.
> I'm just not sure why it's needed for correctness.  Olivier said:
...
> I was trying to understand why the runtime library raises a constraint
> error.  Is it simply that the library only supports software floating
> point for single and double precision (but supports hardware floating
> point for wider types)?  Is it not possible to construct a version  
> of the
> Ada test above that forces the use of the equivalent of C "long  
> double"?
> If so, would that fail in the same way, or don't we care?
As I'm travelling tomorrow, and don't have easy access to
a MIPS system right now, I won't have enough time
today to debug this issue in detail before next week.

There's one of two possibilities:
   - the GNAT run time doesn't handle 128-bit float properly
   - the 128-bit float doesn't have sane numerical semantics

As we haven't yet ported GNAT to a system with 128-bit hardware
floating point, it is very possible that some adaptations are
necessary. In fact, that's certainly the case.

>> From a MIPS perspective, it's also counterintuitive for
> WIDEST_HARDWARE_FP_SIZE to be 64 even for -msingle-float
> and -msoft-float targets (rather than 32 and 0 respectively).
> I can understand that 0 and 32 would be wrong given the way
> the macro is used in ada/targtyps.c, but it seems that Ada is
> using this macro for correctness -- and even to control an aspect
> of the API and ABI -- whereas libgcc2.c is using it for optimisation.

I definitely would be reasonable to set this to 32 and 0
for -msingle-float and -msoft-float targets. Yes, that would
mean we'd have to change targtypes.c, but that's fine.
Especially with -msingle-float this would seem a clear improvement.
> It seems on the fact of it (to a non-Ada expert, of course) that  
> given:
> get_target_long_double_size really ought to be using something called
> ADA_LONG_DOUBLE_TYPE_SIZE, given that it isn't always the same as
> LONG_DOUBLE_TYPE_SIZE.

Yes, there definitely is room for improvement here.
It's on my to do list.

   -Geert