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adler32: add comment for the SIMD code 

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
This commit is contained in:
Michael Niedermayer 2012-02-04 20:27:02 +01:00
parent 26585d2a7f
commit d3f912c1f0
1 changed files with 5 additions and 0 deletions
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5
libavutil/adler32.c
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@ -57,6 +57,11 @@ unsigned long av_adler32_update(unsigned long adler, const uint8_t * buf,
len2 -= 8; len2 -= 8;
buf+=8; buf+=8;
} }
//We combine the 8 interleaved adler32 checksums without overflows
//Decreasing the number of iterations would allow below code to be
//simplified but would likely be slower due to the fewer iterations
//of the inner loop
s1 += ((a1+b1)*0x1000100010001)>>48; s1 += ((a1+b1)*0x1000100010001)>>48;
s2 += ((((a2&0xFFFF0000FFFF)+(b2&0xFFFF0000FFFF)+((a2>>16)&0xFFFF0000FFFF)+((b2>>16)&0xFFFF0000FFFF))*0x800000008)>>32) s2 += ((((a2&0xFFFF0000FFFF)+(b2&0xFFFF0000FFFF)+((a2>>16)&0xFFFF0000FFFF)+((b2>>16)&0xFFFF0000FFFF))*0x800000008)>>32)
#if HAVE_BIGENDIAN #if HAVE_BIGENDIAN