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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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10 changes: 10 additions & 0 deletions src/bits.h
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Original file line number Diff line number Diff line change
Expand Up @@ -291,10 +291,20 @@ LC3_HOT static inline unsigned lc3_get_symbol(

if (ac->low < range * symbols[s].low) {
s >>= 1;
#if LC3_FAST_UDIV
/* low < range * L <=> low / range < L : one division
* instead of a dependent multiply at every probe */
unsigned q = ac->low / range;
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;
#else
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;
#endif
}

ac->low -= range * symbols[s].low;
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15 changes: 15 additions & 0 deletions src/common.h
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Expand Up @@ -74,6 +74,21 @@
#endif /* __clang__ */


/**
* Fast hardware unsigned division
* Set on cores where a runtime 32-bit division beats a few dependent
* multiplies (x86-64, AArch64). Unset for MCUs and audio DSPs.
*/

#ifndef LC3_FAST_UDIV
#if defined(__x86_64__) || defined(__aarch64__) || defined(_M_X64) || defined(_M_ARM64)
#define LC3_FAST_UDIV 1
#else
#define LC3_FAST_UDIV 0
#endif
#endif


/**
* Macros
* MIN/MAX Minimum and maximum between 2 values
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