RTABI half-precision conversion functions (ping)

[Paul Brook]

> Hi,
> 
> The patch that Andrew Stubbs sent upstream here:
> 
> http://gcc.gnu.org/ml/gcc-patches/2011-04/msg02130.html
> 
> seems to have become stalled after Ramana's question here:
> 
> http://gcc.gnu.org/ml/gcc-patches/2011-05/msg00291.html
> 
> This was discussed with ARM personnel (i.e. Lee Smith) at the time the
> patch was created. AFAICT, our reasoning went like this (Paul, please
> correct me if I'm just making stuff up): we wish to use aliases for the
> old and new names for the helpers. The existing
> __gnu_{f2h,h2f}_{ieee,alternative} helpers take unsigned short
> arguments, and therefore assume (on function entry) that the values
> contained in those arguments are zero-extended (although, due to the
> vagaries of libcall handling in GCC, that may not actually be true --
> at the time, this was a latent bug, and may in fact still be). The
> __aeabi_* helpers are defined to pass/return unadorned "short" values,
> intending to convey that only the low-order 16 bits of half-float
> values contain meaningful data.

Yes, that matches my understanding.

> But, that means EABI-conformant callers are also perfectly entitled to
> sign-extend half-float values before calling our helper functions
> (although GCC itself won't do that). Using "unsigned int" and taking
> care to only examine the low-order bits of the value in the helper
> function itself serves to fix the latent bug, is compatible with
> existing code, allows us to be conformant with the eabi, and allows use
> of aliases to make the __gnu and __aeabi functions the same.

As long as LTO never sees this mismatch we should be fine :-)  AFAIK we don't 
curently have any way of expressing the actual ABI.
 
> The patch no longer applied as-is, so I've updated it (attached,
> re-tested). Note that there are no longer any target-independent changes
> (though I'm not certain that the symbol versions are still correct).
> 
> OK to apply?

I think this deserves a comment in the source.  Otherwise it's liable to get 
"fixed" in the future :-) Something allong the lines of 
"While the EABI describes the arguments to the half-float helper routines as 
'short', it does not require that they be extended to full register width.  
The normal ABI requres that the caller sign/zero extend short values to 32 
bit.  We use unsigned int arguments to prevent the gcc making assumptions 
about the high half of the register."

Paul