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Subject: Re: optimization to epicsTime
From: Andrew Johnson <anj@aps.anl.gov>
To: core-talk@aps.anl.gov
Date: Wed, 14 Nov 2012 15:33:51 -0500
Hi Jeff,

Looks fine to me, don't worry about committing minor changes like this 
directly to the 3.15 branch; it's the big changes that need reviewing.

- Andrew

On 2012-11-14 Hill, Jeff wrote:
> Dear All,
> 
> I inadvertently committed the attached change to the main trunk on a
>  checkout instead of a branch and therefore bypassed the normal review
>  process. If there is (any) opposition I will back out the change and make
>  a branch. I did run the epicsTime regression test, which didn't detect any
>  issues, before making this commit.
> 
> Jeff
> 
> Commit message:
> 
> After optimizing the epicsTime::addNanoSec function we now have a larger
>  code which uses only unsigned integer arithmetic. This is for the benefit
>  of embedded cpu's lacking a hardware floating point coprocessor at the
>  expense of some additional code to maintain. I measured the cpu load
>  improving from 40% free to 80% free after making this change, on a nios2
>  soft core processor.
> 
> Change:
> 
> == modified file src/libCom/osi/epicsTime.cpp
> --- src/libCom/osi/epicsTime.cpp	2012-05-03 17:19:34 +0000
> +++ src/libCom/osi/epicsTime.cpp	2012-11-14 19:51:30 +0000
> @@ -113,8 +113,48 @@
>  //
>  inline void epicsTime::addNanoSec (long nSecAdj)
>  {
> -    double secAdj = static_cast <double> (nSecAdj) / nSecPerSec;
> -    *this += secAdj;
> +    // After optimizing this function we now have a larger
> +    // code which uses only unsigned integer arithmetic.
> +    // This is for the benefit of embedded cpu's lacking
> +    // a hardware floating point coprocessor at the
> +    // expense of some additional code to maintain.
> +    // joh 14-11-2012
> +    if ( nSecAdj >= 0 ) {
> +        unsigned long nSecOffsetLong =
> +              static_cast < unsigned long > ( nSecAdj );
> +        while ( nSecOffsetLong >= nSecPerSec ) {
> +            this->secPastEpoch++; // overflow expected
> +            nSecOffsetLong -= nSecPerSec;
> +        }
> +        const epicsUInt32 nSecOffset =
> +            static_cast < epicsUInt32 > ( nSecOffsetLong );
> +        epicsUInt32 nSecPerSecRemaining = nSecPerSec - nSecOffset;
> +        if ( this->nSec >= nSecPerSecRemaining ) {
> +            this->secPastEpoch++; // overflow expected
> +            this->nSec -= nSecPerSecRemaining;
> +        }
> +        else {
> +            this->nSec += nSecOffset;
> +        }
> +    }
> +    else {
> +        unsigned long nSecOffsetLong =
> +            static_cast <unsigned long> ( -nSecAdj );
> +        while ( nSecOffsetLong >= nSecPerSec ) {
> +            this->secPastEpoch--; // underflow expected
> +            nSecOffsetLong -= nSecPerSec;
> +        }
> +        const epicsUInt32 nSecOffset =
> +            static_cast < epicsUInt32 > ( nSecOffsetLong );
> +        if ( this->nSec >= nSecOffset ) {
> +            this->nSec -= nSecOffset;
> +        }
> +        else {
> +            // borrow
> +            this->secPastEpoch--; // underflow expected
> +            this->nSec += nSecPerSec - nSecOffset;
> +        }
> +    }
>  }
> 
>  //
> 
> 
> Jeff
> ______________________________________________________
> Jeffrey O. Hill           Email        johill@lanl.gov
> LANL MS H820              Voice        505 665 1831
> Los Alamos NM 87545 USA   FAX          505 665 5107
> 
> Message content: TSPA
> 


- Andrew
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References:
optimization to epicsTime Hill, Jeff

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