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igzip: optimize convert_dist_to_dist_sym to branchless

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convert_dist_to_dist_sym uses long if/else branch to get look back distance.
The distance calculation is well formed for each distance range, so it could
be optimized for a branchless version.

Change-Id: I4e1e5170f8b3238631f3048087f95acc53e4498e
Signed-off-by: Jun He <jun.he@arm.com>
This commit is contained in:
Jun He 2019-08-06 15:58:46 +08:00
parent e2997062fb
commit b721db98e5
1 changed files with 6 additions and 33 deletions
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39
igzip/huff_codes.c
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@ -728,39 +728,12 @@ void isal_update_histogram_base(uint8_t * start_stream, int length,
static uint32_t convert_dist_to_dist_sym(uint32_t dist)
{
assert(dist <= 32768 && dist > 0);
if (dist <= 2)
return dist - 1;
else if (dist <= 4)
return 0 + (dist - 1) / 1;
else if (dist <= 8)
return 2 + (dist - 1) / 2;
else if (dist <= 16)
return 4 + (dist - 1) / 4;
else if (dist <= 32)
return 6 + (dist - 1) / 8;
else if (dist <= 64)
return 8 + (dist - 1) / 16;
else if (dist <= 128)
return 10 + (dist - 1) / 32;
else if (dist <= 256)
return 12 + (dist - 1) / 64;
else if (dist <= 512)
return 14 + (dist - 1) / 128;
else if (dist <= 1024)
return 16 + (dist - 1) / 256;
else if (dist <= 2048)
return 18 + (dist - 1) / 512;
else if (dist <= 4096)
return 20 + (dist - 1) / 1024;
else if (dist <= 8192)
return 22 + (dist - 1) / 2048;
else if (dist <= 16384)
return 24 + (dist - 1) / 4096;
else if (dist <= 32768)
return 26 + (dist - 1) / 8192;
else
return ~0; /* ~0 is an invalid distance code */
if (dist <= 32768) {
uint32_t msb = dist > 4 ? bsr(dist - 1) - 2 : 0;
return (msb * 2) + ((dist - 1) >> msb);
} else {
return ~0;
}
}
/**