Re: [PATCH] Optimise the fpclassify builtin to perform integer operations when possible

[Michael Meissner <meissner@linux.vnet.ibm.com>]

On Mon, Sep 12, 2016 at 04:19:32PM +0000, Tamar Christina wrote:
> Hi All,
> 
> This patch adds an optimized route to the fpclassify builtin
> for floating point numbers which are similar to IEEE-754 in format.
> 
> The goal is to make it faster by:
> 1. Trying to determine the most common case first
>    (e.g. the float is a Normal number) and then the
>    rest. The amount of code generated at -O2 are
>    about the same +/- 1 instruction, but the code
>    is much better.
> 2. Using integer operation in the optimized path.
> 
> At a high level, the optimized path uses integer operations
> to perform the following:
> 
>   if (exponent bits aren't all set or unset)
>      return Normal;
>   else if (no bits are set on the number after masking out
> 	   sign bits then)
>      return Zero;
>   else if (exponent has no bits set)
>      return Subnormal;
>   else if (mantissa has no bits set)
>      return Infinite;
>   else
>      return NaN;

I haven't looked at fpclassify.  I assume we can define a backend insn to do
the right thing?  One of the things we've noticed over the years with the
PowerPC is that it can be rather expensive to move things from the floating
point/vector unit to the integer registers and vice versa.  This is
particularly true if you having to do the transfer via the memory unit via
stores and loads of different sizes.

-- 
Michael Meissner, IBM
IBM, M/S 2506R, 550 King Street, Littleton, MA 01460-6245, USA
email: meissner@linux.vnet.ibm.com, phone: +1 (978) 899-4797