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bits: replace per-probe multiplies with one division in lc3_get_symbol #89

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bits: replace per-probe multiplies with one division in lc3_get_symbol #89

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16 changes: 12 additions & 4 deletions src/bits.h
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Original file line number Diff line number Diff line change
Expand Up @@ -289,12 +289,20 @@ LC3_HOT static inline unsigned lc3_get_symbol(

int s = 16;

/* Each probe of the binary search would chain a data-dependent
* multiply on the critical path. For the decoder's unsigned values
* (low < 2^24, and 0x40 <= range <= 0x3fff from the renormalization
* invariant ac->range >= 0x10000),
* low < range * symbols[s].low <=> low / range < symbols[s].low
* so the quotient, computed once, decides every probe instead. */

if (ac->low < range * symbols[s].low) {
unsigned q = ac->low / range;
s >>= 1;
s -= ac->low < range * symbols[s].low ? 4 : -4;
s -= ac->low < range * symbols[s].low ? 2 : -2;
s -= ac->low < range * symbols[s].low ? 1 : -1;
s -= ac->low < range * symbols[s].low;
s -= q < symbols[s].low ? 4 : -4;
s -= q < symbols[s].low ? 2 : -2;
s -= q < symbols[s].low ? 1 : -1;
s -= q < symbols[s].low;
}

ac->low -= range * symbols[s].low;
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