RFC Power PC G3 optimized sqrtf function.

[Conn Clark]

Steve Munroe writes: 

> 
> libc-alpha-owner@sourceware.org wrote on 12/14/2006 01:05:57 PM: 
> 
>> Hi everybody, 
>>
>> This is my 1st post and attempt at contributing to glibc 
>>
> Thanks Conn. To get started, submittions to libc are normally in the form
> of a patch with a changelog header. Please review
> http://www.gnu.org/prep/standards/standards.html section 6.8. 
> 
>>   I have written a sqrtf function that is much faster on a PowerPC G3
> than
>> the original one used. It uses the frsqrte instruction and Newton

<SNIP> 

>> will not work on a  601 processor. 
>>
> Next, I assume you intend to add this to the powerpc-cpu add-on using
> --with-cpu=g3 configuration? 
> 

Correct 

> In this case we need to place your e_sqrtf.S file an appropriate directory
> so that it does not impact PowerPCs that do have fsqrt. For example: 
> 
> ./powerpc-cpu/sysdeps/powerpc/powerpc32/g3/fpu/e_sqrtf.S 
> 

Okay, I'll do that.

> You will also need an Implies file in the sysdeps/unix/sysv/linux tree to
> make sure your new directory is early enough in the search order to
> override the e_sqrtf in libc trunc. 
> 
> For example: 
> 
> ./powerpc-cpu/sysdeps/unix/sysv/linux/powerpc/powerpc32/g3/fpu/Implies 
> 
> would contain: 
> 
> powerpc/powerpc32/g3/fpu 
> 
> If you want g4 to default to the g3 implementation, create
> powerpc/powerpc32/g4/fpu directories with Implies files referencing the
> powerpc/powerpc32/g3/fpu directories. Similarly for 603, 604, ... 
> 
> See the powerpc-cpu README for more details. 
> 
> You patch should reflect this directory detail. 
> 
>>   The limiting factor on ieee conformance is the frsqrte instruction must
> 
>> produce a result that is within 1/59th of the correct value. A timing
> test
>> on all valid values using the current glibc function takes about 26
> minutes
>> on a iMac g3 400MHz machine. With my implementation it takes about 21
>> minutes. 
>>
> 
> Not sure what you are getting at here. The PowerPC Arch 2.0x (V1.x also)
> states that frsqrte is "correct to one part in 32". Does you algorithm
> require better precision then the Arch provides?

Yes 

> The Arch does say that
> results may vary between implementations. So does G3/G4 frsqrte provide
> better then 1/32 precision?

Yes they do. All implementations 


>> Please read the header for more details and give me some feedback. 
>>
>> P.S. Do I need to file copyright assignment papers for this? 
>>

> Yes you do.

Is there a link on how to do this?

> Steven J. Munroe
> Linux on Power Toolchain Architect
> IBM Corporation, Linux Technology Center 
> 

Thank you, 

Conn 

 ---------------------------------------
Conn Clark 

Electronic Systems Technology
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