Re: [Qemu-devel] [PATCH 13/19] Add VSX ISA2.06 Multiply Add Instructions

[Richard Henderson <rth@twiddle.net>]

On 10/25/2013 09:25 AM, Tom Musta wrote:
> 
> I tried recoding xsmaddadp using float64_muladd.  The problem that I hit is the
> boundary case where the intermediate product and the summand are infinities of
> the opposite sign.  This is the case handled by the first "if" in the code
> snippet above.  PowerPC has a dedicated FPSCR bit for this type of condition
> (VXISI) as well as a general invalid operation bit (VX).  As far as I can tell,
> the softfloat code only has the equivalent of the VX bit.   Thus the
> implementation
> that I proposed is a more accurate representation of the Power ISA.
> 
> The VSX code was modeled after the existing fmadd FPU instruction.  I suspect
> the author of that code wrote it this way for similar reasons.
> 
> I am inclined to keep my proposed implementation, which is consistent with
> the existing PowerPC code.
> 
> Thoughts?

Hmm.  I won't object to your current implementation, since it does produce
correct results.

I believe that a better implementation could use float*_muladd, and check the
result for float_flag_invalid.  If set, compute the intermediate product so you
can figure out the VXISI setting.  But we'd expect that to be an unlikely path.


r~