Options for using hardware implementation of remainder and square root on i32

[Kwok, Yipkei]

Hi Tim,

Thank you very much for your promptly response. Please see below.

I really appreciate your help. Thanks again!

Regards,
Yipkei

Tim Prince wrote:
> Kwok, Yipkei wrote:
>> Although the Intel x86 processors are capable of computing remainder 
>> and square root operations in hardware, both g++ and icpc (Intel 
>> compiler) realize these operations in software [1].
>>
>> Question 1: Is there any specific result why they do it this way 
>> (using software implementation instead of hardware)?
> The usual way to implement sqrt, when optimizing, would be to execute 
> the sqrt instruction, check the processor flags, and retry with the 
> library function if any flag is set which would indicate a requirement 
> for errno processing or exception handling.
> 
> I don't know the answer as to whether C99 remainder functions are 
> commonly available in compilers when -std=c99 is set, and what g++ and 
> icpc may do to permit their use as an extension beyond C++ standard.  
> Assuming that they have to be implemented with x87 code, while one would 
> normally be using SSE code generation options, the performance 
> implications become such that the code size vs performance tradeoff 
> dictates a library function call.
> When you have performance or accuracy critical code requiring 
> remaindering, you probably have to study the facilities of the target 
> platform.

I will give c99 is try. Thank you very much for suggesting it.

>>
>> Question 2: Is there any compiler option that force these operations 
>> to be done in hardware?
>>
> In order to support Fortran, gcc has a specific flag to turn off the 
> sqrt retry, thus breaking errno and exception handling.  Maybe g++ 
> -ffast-math or icpc -fp-model fast=2 might include such an option.  
> Those are big hammers which evidently aren't suitable for general use.

Does it mean that no executable of any Fortran program compiled with 
gfortran is able to execute the sqrt hardware instruction as, from your 
description above, it takes the sqrt retry flag for errno processing and 
exception handling?

>> I need these answers in order compare differences, if there is any, in 
>> terms of performance and results.
> If you want strictly correct results, according to IEEE754, you must 
> consider whether you want your operations to be widened, e.g. according 
> to x87 precision setting, and, with icpc, you must set -prec-sqrt.  This 
> is not a question of whether you have an in-line sqrt instruction, but 
> will destroy your conclusions if you ignore the issues. There is a clear 
> performance hit in supporting errno and exception handling, which you 
> normally expect to incur in C++.
> You have the option of writing in-line asm code, if that is what you 
> want to evaluate.
> 
> I imagine it's difficult to get people excited nowadays about something 
> specific to 32-bit compilers for a specific CPU architecture, when there 
> are no longer any 32-bit CPUs in production.
> 

I begin this adventure with the 32-bit systems and I will continue on 
the 64-bit systems. If anyone addresses this issue on the 64-bit 
systems, I would surely love to hear that.

> 
> 

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Yipkei Kwok
Ph.D. Student
Research Assistant
HiPerSys Lab
Department of Computer Science
The University of Texas at El Paso
Phone: 915 747 6433 (O)
E-mail: ykwok2 at miners dot utep dot edu
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