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

[Jeff Law <law@redhat.com>]

On 09/13/2016 02:41 AM, Jakub Jelinek wrote:
> On Mon, Sep 12, 2016 at 04:19:32PM +0000, Tamar Christina wrote:
>> 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.
>
> Is it generally preferable to use integer operations for this instead
> of floating point operations?  I mean various targets have quite high costs
> of moving data in between the general purpose and floating point register
> file, often it has to go through memory etc.
Bit testing/twiddling is obviously a trade-off for a non-addressable 
object.  I don't think there's any reasonable way to always generate the 
most efficient code as it's going to depend on (for example) register 
allocation behavior.

So what we're stuck doing is relying on the target costing bits to guide 
this kind of thing.

jeff