Re: Help optimize c++ code

Ron <>
Tue, 7 Jul 2009 16:47:50 CST
On Jul 7, 3:18 pm, MickeyP <> wrote:

Thank you everyone for the suggestions, much appreciated. No this was
not a homework assignment, I am a software developer. I used the high
performance timer in the Windows environment ( x86 architecture ),
vs2005 c++ compiler. Without unrolling the loop, I tried various
combinations of the suggestions. The following code proved to be the
fastest method which was plenty fast for my purpose, 0.5962
millisecond average.

static const uint32_t alphaMask( g_alphaMask );
static const uint32_t threshold( 4U << g_alphaShift );

while( numPixels-- )
     const unsigned int alpha( *ptrPixel & alphaMask );

     if( alpha >= threshold )
        *ptrPixel = ( *ptrPixel & ~g_alphaMask ) | ( alpha - threshold );


Be aware that the code above behaves differently than your original
when the alpha is 1, 2, or 3 (after the shift). In the original code
you set the alpha to zero in those cases. In the new code you leave
it unchanged.

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