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rtl_sdr_fm: SIMD optimize USB byte→int16 widening in callback

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This commit is contained in:
arancormonk
2025-09-03 20:20:43 -04:00
parent 6fc327c3b6
commit 0cb38d2c25
1 changed files with 49 additions and 3 deletions
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52
src/rtl_sdr_fm.cpp
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@@ -33,6 +33,14 @@
#include <rtl-sdr.h>
#include "dsd.h"
/* Optional SIMD intrinsics for USB byte->int16 widening */
#if defined(__SSE2__)
#include <emmintrin.h>
#endif
#if defined(__ARM_NEON) || defined(__ARM_NEON__)
#include <arm_neon.h>
#endif
#define DEFAULT_SAMPLE_RATE 48000
#define DEFAULT_BUF_LENGTH (1 * 16384)
#define MAXIMUM_OVERSAMPLE 16
@@ -74,6 +82,45 @@ static int atan_lut_coef = 8;
static pthread_once_t atan_lut_once = PTHREAD_ONCE_INIT;
static pthread_mutex_t atan_lut_mutex = PTHREAD_MUTEX_INITIALIZER;
/* =====================
USB widening helper (u8 -> s16 centered at 127) with SIMD and scalar fallback
===================== */
static inline void widen_u8_to_s16_bias127(const unsigned char *src, int16_t *dst, uint32_t len)
{
uint32_t i = 0;
#if defined(__SSE2__)
/* Process 16 bytes per iteration */
const __m128i bias = _mm_set1_epi16(127);
const __m128i zero = _mm_setzero_si128();
for (; i + 16 <= len; i += 16) {
__m128i b = _mm_loadu_si128((const __m128i*)(src + i));
__m128i lo = _mm_unpacklo_epi8(b, zero); /* widen to 16-bit unsigned */
__m128i hi = _mm_unpackhi_epi8(b, zero);
lo = _mm_sub_epi16(lo, bias); /* center at 127 */
hi = _mm_sub_epi16(hi, bias);
_mm_storeu_si128((__m128i*)(dst + i), lo);
_mm_storeu_si128((__m128i*)(dst + i + 8), hi);
}
#elif defined(__ARM_NEON) || defined(__ARM_NEON__)
/* Process 16 bytes per iteration */
const uint8x8_t bias8 = vdup_n_u8(127);
for (; i + 16 <= len; i += 16) {
uint8x16_t v = vld1q_u8(src + i);
uint8x8_t v_lo = vget_low_u8(v);
uint8x8_t v_hi = vget_high_u8(v);
/* signed widen subtract: (uint8 - 127) -> int16 */
int16x8_t lo = vsubl_u8(v_lo, bias8);
int16x8_t hi = vsubl_u8(v_hi, bias8);
vst1q_s16(dst + i, lo);
vst1q_s16(dst + i + 8, hi);
}
#endif
/* Scalar tail or whole buffer on non-SIMD builds */
for (; i < len; i++) {
dst[i] = (int16_t)src[i] - 127;
}
}
/* =====================
Saturating helpers
===================== */
@@ -1098,9 +1145,8 @@ static void rtlsdr_callback(unsigned char *buf, uint32_t len, void *ctx)
/* Write directly into the current write buffer */
int wb = d->write_buf_index.load();
int16_t *dst = d->input_buffers[wb];
for (i=0; i<(int)len; i++) {
dst[i] = (int16_t)buf[i] - 127;
}
/* SIMD-accelerated widening (u8 -> s16 centered at 127) with scalar fallback */
widen_u8_to_s16_bias127(buf, dst, len);
d->input_len[wb].store(len);
/* Flip buffers atomically: the buffer we just wrote becomes ready */
d->ready_buf_index.store(wb);