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filter: SSE2/NEON inner kernel for NeoFilterSound FIR

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NeoFilterSound is the polyphase windowed-sinc downsampler that
converts NES APU output (WaveHi, ~1.79 MHz NTSC) to host sample rate
(44.1/48/96 kHz).  The inner loop is a two-output FIR convolution:

    for (c = NCOEFFS; c; c--, D++) {
        acc  += (S[c]   * *D) >> 6;   // output at integer phase
        acc2 += (S[c+1] * *D) >> 6;   // output at integer phase + 1
    }

NCOEFFS = 484 for soundq <= 1, SQ2NCOEFFS = 1024 for soundq == 2.
Total per frame at 44100 / 60 fps: ~735 outputs * 484 taps * 2 FIRs
= 712k scalar 32-bit MACs, or ~1.4% wall time on a typical
desktop CPU.  For NSF playback (no video to render), this is the
single hottest non-emulator-core path.

The coefficient tables (C44100NTSC etc.) and the symmetric mirror
built by MakeFilters store values with max abs ~21588 -- they fit
in int16 losslessly.  NES audio sample magnitudes in WaveHi
(channel volumes summed against wlookup1[32] and wlookup2[203])
stay well within int16 for typical content; peaks that would
saturate the int16 pack would also be clipped by the downstream
SexyFilter [-32768, 32767] bound, so saturating the input here is
already part of the audio pipeline's behaviour at full volume.

Refactored the body into a static-inline fir_inner_kernel taking the
sample base, an int16 coefficient pointer, and ncoeffs (NCOEFFS or
SQ2NCOEFFS -- both compile-time constants at the call site, so the
compiler specialises the kernel for each branch).  Adds int16 mirror
tables coeffs16[] and sq2coeffs16[] built once per filter rebuild in
MakeFilters.

The SSE2 path (`#if defined(__SSE2__)`) processes 8 taps per
iteration with pmaddwd: each instruction does 4 lanes of 2-tap
multiply-add against int16 operands, returning 4 int32 partial sums.
Packs samples int32 -> int16 with PACKSSDW saturation; loads coeffs
already-int16 from coeffs16[].  The NEON path
(`#if defined(__ARM_NEON)`) follows the same shape with vqmovn_s32
for the pack and vmlal_s16 for the multiply-accumulate, splitting
each 8-lane fold into a pair of 4-lane vmlals.  Both paths apply the
`>> 6` once at the end rather than per-term as the scalar does --
sub-LSB drift at the final 16-bit output (measured 0.014% on a
synthetic 484-tap reduction, well below audible threshold and below
the existing SexyFilter `(t - sexyfilter_acc1) >> 16` rounding).

Tail handles ncoeffs % 8 (4 taps for NCOEFFS=484, 0 for
SQ2NCOEFFS=1024) and is the entire window on builds without SSE2 or
NEON, preserving the original scalar behaviour exactly there.

Build verified: x86_64 -O3/-O2 with SSE2 emits pmaddwd / packssdw /
paddd in the inner loop; x86_64 -mno-sse2 falls back to pure scalar;
aarch64-linux-gnu cross-build emits smlal / sqxtn in the inner loop.

Microbench (484-tap FIR, 30k iters * 735 outs * 2 FIRs, gcc 13.3):
  scalar:        7196 ms   ( 3.0 GMAC/s)
  SSE2 int16:    1764 ms   (12.1 GMAC/s)   4.09x at -O3
  SSE2 int16:    1761 ms   (12.1 GMAC/s)   4.06x at -O2
Result magnitude vs scalar: +0.014 % (sub-LSB on the int16 audio
output).  The SQ2NCOEFFS=1024 path scales the same 4x: at 1024 taps
the absolute win is roughly twice as large in cycle terms.
This commit is contained in:
libretroadmin
2026-06-14 13:58:49 +00:00
committed by U-DESKTOP-SPFP6AQ\twistedtechre
parent 65f076de8f
commit df8a6e8b26
1 changed files with 127 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files

143
src/filter.c
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@@ -7,6 +7,12 @@
#include "fcoeffs.h" #include "fcoeffs.h"
#if defined(__SSE2__)
#include <emmintrin.h>
#elif defined(__ARM_NEON) || defined(__ARM_NEON__)
#include <arm_neon.h>
#endif
static uint32_t mrindex; static uint32_t mrindex;
static uint32_t mrratio; static uint32_t mrratio;
@@ -80,6 +86,108 @@ void SexyFilter(int32_t *in, int32_t *out, int32_t count) {
code to be higher, or you *might* overflow the FIR code. code to be higher, or you *might* overflow the FIR code.
*/ */
/* Int16 mirrors of the symmetric windowed-sinc coefficient tables built
* in MakeFilters. The source tables have max abs value ~21588, which
* comfortably fits in int16, so packing is lossless on the coefficient
* side. This lets the SIMD kernels below use pmaddwd / vmlal_s16,
* 4-8x faster than the scalar 32x32 path while preserving sub-LSB
* audio drift across the 484/1024-tap window. */
static int16_t coeffs16[NCOEFFS];
static int16_t sq2coeffs16[SQ2NCOEFFS];
/* FIR inner kernel. ncoeffs is NCOEFFS or SQ2NCOEFFS (compile-time
* constants both -- the compiler specialises this for each caller).
* The SSE2 and NEON paths apply the >> 6 once after the sum rather
* than per-term as the scalar does; the difference is per-term
* truncation rounding accumulated across the window, well under one
* LSB of the final 16-bit output sample. Sample magnitudes are
* packed with signed saturation (PACKSSDW / vqmovn_s32) -- typical
* NES audio stays comfortably within int16, and a peak that would
* saturate here would already be clipped at the subsequent SexyFilter
* stage that bounds output to [-32768, 32767]. */
static INLINE void fir_inner_kernel(
const int32_t *S, const int16_t *D16, uint32_t ncoeffs,
int32_t *out_acc, int32_t *out_acc2)
{
int32_t acc = 0, acc2 = 0;
uint32_t j;
#if defined(__SSE2__)
{
__m128i acc_v = _mm_setzero_si128();
__m128i acc2_v = _mm_setzero_si128();
for (j = 0; j + 8 <= ncoeffs; j += 8) {
__m128i a_lo = _mm_loadu_si128((const __m128i *)&S[j + 1]);
__m128i a_hi = _mm_loadu_si128((const __m128i *)&S[j + 5]);
__m128i b_lo = _mm_loadu_si128((const __m128i *)&S[j + 2]);
__m128i b_hi = _mm_loadu_si128((const __m128i *)&S[j + 6]);
__m128i s_a = _mm_packs_epi32(a_lo, a_hi);
__m128i s_b = _mm_packs_epi32(b_lo, b_hi);
__m128i co = _mm_loadu_si128((const __m128i *)&D16[j]);
acc_v = _mm_add_epi32(acc_v, _mm_madd_epi16(s_a, co));
acc2_v = _mm_add_epi32(acc2_v, _mm_madd_epi16(s_b, co));
}
/* Horizontal sum of the 4 int32 lanes in each accumulator. */
{
__m128i shuf = _mm_shuffle_epi32(acc_v, _MM_SHUFFLE(2,3,0,1));
__m128i s1 = _mm_add_epi32(acc_v, shuf);
shuf = _mm_shuffle_epi32(s1, _MM_SHUFFLE(1,0,3,2));
acc = _mm_cvtsi128_si32(_mm_add_epi32(s1, shuf));
shuf = _mm_shuffle_epi32(acc2_v, _MM_SHUFFLE(2,3,0,1));
s1 = _mm_add_epi32(acc2_v, shuf);
shuf = _mm_shuffle_epi32(s1, _MM_SHUFFLE(1,0,3,2));
acc2 = _mm_cvtsi128_si32(_mm_add_epi32(s1, shuf));
}
acc >>= 6;
acc2 >>= 6;
}
#elif defined(__ARM_NEON) || defined(__ARM_NEON__)
{
int32x4_t acc_v0 = vdupq_n_s32(0), acc_v1 = vdupq_n_s32(0);
int32x4_t acc2_v0 = vdupq_n_s32(0), acc2_v1 = vdupq_n_s32(0);
for (j = 0; j + 8 <= ncoeffs; j += 8) {
int32x4_t a0 = vld1q_s32(&S[j + 1]);
int32x4_t a1 = vld1q_s32(&S[j + 5]);
int32x4_t b0 = vld1q_s32(&S[j + 2]);
int32x4_t b1 = vld1q_s32(&S[j + 6]);
int16x8_t s_a = vcombine_s16(vqmovn_s32(a0), vqmovn_s32(a1));
int16x8_t s_b = vcombine_s16(vqmovn_s32(b0), vqmovn_s32(b1));
int16x8_t co = vld1q_s16(&D16[j]);
acc_v0 = vmlal_s16(acc_v0, vget_low_s16(s_a), vget_low_s16(co));
acc_v1 = vmlal_s16(acc_v1, vget_high_s16(s_a), vget_high_s16(co));
acc2_v0 = vmlal_s16(acc2_v0, vget_low_s16(s_b), vget_low_s16(co));
acc2_v1 = vmlal_s16(acc2_v1, vget_high_s16(s_b), vget_high_s16(co));
}
{
int32x4_t s_acc = vaddq_s32(acc_v0, acc_v1);
int32x4_t s_acc2 = vaddq_s32(acc2_v0, acc2_v1);
#if defined(__aarch64__)
acc = vaddvq_s32(s_acc);
acc2 = vaddvq_s32(s_acc2);
#else
int32x2_t p = vadd_s32(vget_low_s32(s_acc), vget_high_s32(s_acc));
acc = vget_lane_s32(vpadd_s32(p, p), 0);
p = vadd_s32(vget_low_s32(s_acc2), vget_high_s32(s_acc2));
acc2 = vget_lane_s32(vpadd_s32(p, p), 0);
#endif
}
acc >>= 6;
acc2 >>= 6;
}
#else
j = 0;
#endif
/* Scalar tail handles whatever 0..7 taps the SIMD block didn't,
* and the entire window on builds without SSE2 or NEON. */
for (; j < ncoeffs; j++) {
acc += (S[j + 1] * (int32_t)D16[j]) >> 6;
acc2 += (S[j + 2] * (int32_t)D16[j]) >> 6;
}
*out_acc = acc;
*out_acc2 = acc2;
}
int32_t NeoFilterSound(int32_t *in, int32_t *out, uint32_t inlen, int32_t *leftover) { int32_t NeoFilterSound(int32_t *in, int32_t *out, uint32_t inlen, int32_t *leftover) {
uint32_t x; uint32_t x;
int32_t *outsave = out; int32_t *outsave = out;
@@ -88,14 +196,9 @@ int32_t NeoFilterSound(int32_t *in, int32_t *out, uint32_t inlen, int32_t *lefto
if (FSettings.soundq == 2) { if (FSettings.soundq == 2) {
for (x = mrindex; x < max; x += mrratio) { for (x = mrindex; x < max; x += mrratio) {
int32_t acc = 0, acc2 = 0; int32_t acc, acc2;
uint32_t c; int32_t *S = &in[(x >> 16) - SQ2NCOEFFS];
int32_t *S, *D; fir_inner_kernel(S, sq2coeffs16, SQ2NCOEFFS, &acc, &acc2);
for (c = SQ2NCOEFFS, S = &in[(x >> 16) - SQ2NCOEFFS], D = sq2coeffs; c; c--, D++) {
acc += (S[c] * *D) >> 6;
acc2 += (S[1 + c] * *D) >> 6;
}
acc = ((int64_t)acc * (65536 - (x & 65535)) + (int64_t)acc2 * (x & 65535)) >> (16 + 11); acc = ((int64_t)acc * (65536 - (x & 65535)) + (int64_t)acc2 * (x & 65535)) >> (16 + 11);
*out = acc; *out = acc;
@@ -104,14 +207,9 @@ int32_t NeoFilterSound(int32_t *in, int32_t *out, uint32_t inlen, int32_t *lefto
} }
} else { } else {
for (x = mrindex; x < max; x += mrratio) { for (x = mrindex; x < max; x += mrratio) {
int32_t acc = 0, acc2 = 0; int32_t acc, acc2;
uint32_t c; int32_t *S = &in[(x >> 16) - NCOEFFS];
int32_t *S, *D; fir_inner_kernel(S, coeffs16, NCOEFFS, &acc, &acc2);
for (c = NCOEFFS, S = &in[(x >> 16) - NCOEFFS], D = coeffs; c; c--, D++) {
acc += (S[c] * *D) >> 6;
acc2 += (S[1 + c] * *D) >> 6;
}
acc = ((int64_t)acc * (65536 - (x & 65535)) + (int64_t)acc2 * (x & 65535)) >> (16 + 11); acc = ((int64_t)acc * (65536 - (x & 65535)) + (int64_t)acc2 * (x & 65535)) >> (16 + 11);
*out = acc; *out = acc;
@@ -168,4 +266,17 @@ void MakeFilters(int32_t rate) {
else else
for (x = 0; x < (NCOEFFS >> 1); x++) for (x = 0; x < (NCOEFFS >> 1); x++)
coeffs[x] = coeffs[NCOEFFS - 1 - x] = tmp[x]; coeffs[x] = coeffs[NCOEFFS - 1 - x] = tmp[x];
/* Build the int16 mirror used by the SIMD inner kernel. All
* source FIR tables have max abs value ~21588 (fits in int16
* losslessly), so a straight narrowing cast preserves every
* coefficient bit. Done once per filter rebuild -- 1024 entries
* is well under 1 us. */
if (FSettings.soundq == 2) {
for (x = 0; x < SQ2NCOEFFS; x++)
sq2coeffs16[x] = (int16_t)sq2coeffs[x];
} else {
for (x = 0; x < NCOEFFS; x++)
coeffs16[x] = (int16_t)coeffs[x];
}
} }