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Fast log2 implementation.

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Big improvement in code size (2034 bytes!). Baseband FFT (idle) thread CPU cut in half.
This commit is contained in:
Jared Boone 2016-02-08 12:55:06 -08:00
parent cdd15fcf79
commit 5cd423bb20
1 changed files with 15 additions and 2 deletions
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17
firmware/common/utility.cpp
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@ -22,7 +22,6 @@
#include "utility.hpp" #include "utility.hpp"
#include <cstdint> #include <cstdint>
#include <cmath>
#if 0 #if 0
uint32_t gcd(const uint32_t u, const uint32_t v) { uint32_t gcd(const uint32_t u, const uint32_t v) {
@ -60,6 +59,20 @@ uint32_t gcd(const uint32_t u, const uint32_t v) {
} }
#endif #endif
static float fast_log2(const float val) {
// Thank you Stack Overflow!
// http://stackoverflow.com/questions/9411823/fast-log2float-x-implementation-c
union {
float val;
int32_t x;
} u = { val };
float log_2 = (((u.x >> 23) & 255) - 128);
u.x &= ~(255 << 23);
u.x += (127 << 23);
log_2 += ((-0.34484843f) * u.val + 2.02466578f) * u.val - 0.67487759f;
return log_2;
}
float complex16_mag_squared_to_dbv_norm(const float c16_mag_squared) { float complex16_mag_squared_to_dbv_norm(const float c16_mag_squared) {
constexpr float input_component_max = 32768; constexpr float input_component_max = 32768;
constexpr float mag2_max = (input_component_max * input_component_max) * 2; constexpr float mag2_max = (input_component_max * input_component_max) * 2;
@ -68,7 +81,7 @@ float complex16_mag_squared_to_dbv_norm(const float c16_mag_squared) {
constexpr float log2_log10_factor = std::log10(2.0f); constexpr float log2_log10_factor = std::log10(2.0f);
constexpr float log10_dbv_factor = 20.0f; constexpr float log10_dbv_factor = 20.0f;
constexpr float mag2_to_db_factor = log_mag2_mag_factor * log2_log10_factor * log10_dbv_factor; constexpr float mag2_to_db_factor = log_mag2_mag_factor * log2_log10_factor * log10_dbv_factor;
return (std::log2(c16_mag_squared) - mag2_log2_max) * mag2_to_db_factor; return (fast_log2(c16_mag_squared) - mag2_log2_max) * mag2_to_db_factor;
} }
/* GCD implementation derived from recursive implementation at /* GCD implementation derived from recursive implementation at