Re: [Qemu-devel] [PATCH 15/19] Add VSX xmax/xmin Instructions

[Tom Musta]

On 10/24/2013 3:45 PM, Richard Henderson wrote:
> On 10/24/2013 09:26 AM, Tom Musta wrote:
> > Because of the Power ISA definitions of maximum and minimum
> > on various boundary cases, the standard softfloat comparison
> > routines (e.g. float64_lt) do not work as well as one might
> > think.  Therefore specific routines for comparing 64 and 32
> > bit floating point numbers are implemented in the PowerPC
> > helper code.
>
> Really?  All I see in the document is ">fp", used both here
> in the minmax insn and in the cmp insn.
>
> If the softfloat compare isn't good enough for minmax, how
> can it be good enough for cmp?

Example:

The ISA is very explicit that max(-0.0, +0.0) = +0.0.

But the comparison operations (and instructions) both consider
-0.0 == +0.0.  Because of this, I do not see how it is possible
to implement max using float*_eq, float*_lt and float*_le.

See, for example, table 58 (Actions for xsmaxdp) on p. 369 of the
V2.06 ISA.