audio: allow stereo Sint16 resampling fast path in SDL_AudioStream.

This currently favors libsamplerate over the fast path (quality over speed),
but I'm not sure that's the correct approach, as there may be surprising
changes in performance metrics depending on what packages are available on
a user's system. That being said, currently, the only thing with access to
SDL_AudioStream is an SDL audio device's thread, and it might be mostly idle
otherwise, so maybe this is generally good.

src/audio/SDL_audiocvt.c

@@ -858,7 +858,7 @@ SDL_BuildAudioCVT(SDL_AudioCVT * cvt,
     return (cvt->needed);
 }
 
-typedef int (*SDL_ResampleAudioStreamFunc)(SDL_AudioStream *stream, const float *inbuf, const int inbuflen, float *outbuf, const int outbuflen);
+typedef int (*SDL_ResampleAudioStreamFunc)(SDL_AudioStream *stream, const void *inbuf, const int inbuflen, void *outbuf, const int outbuflen);
 typedef void (*SDL_ResetAudioStreamResamplerFunc)(SDL_AudioStream *stream);
 typedef void (*SDL_CleanupAudioStreamResamplerFunc)(SDL_AudioStream *stream);
 
@@ -890,8 +890,10 @@ struct SDL_AudioStream
 
 #ifdef HAVE_LIBSAMPLERATE_H
 static int
-SDL_ResampleAudioStream_SRC(SDL_AudioStream *stream, const float *inbuf, const int inbuflen, float *outbuf, const int outbuflen)
+SDL_ResampleAudioStream_SRC(SDL_AudioStream *stream, const void *_inbuf, const int inbuflen, void *_outbuf, const int outbuflen)
 {
+    const float *inbuf = (const float *) _inbuf;
+    float *outbuf = (float *) _outbuf;
     const int framelen = sizeof(float) * stream->pre_resample_channels;
     SRC_STATE *state = (SRC_STATE *)stream->resampler_state;
     SRC_DATA data;
@@ -970,26 +972,48 @@ SetupLibSampleRateResampling(SDL_AudioStream *stream)
 typedef struct
 {
     SDL_bool resampler_seeded;
-    float resampler_state[8];
+    union
+    {
+        float f[8];
+        Sint16 si16[2];
+    } resampler_state;
 } SDL_AudioStreamResamplerState;
 
 static int
-SDL_ResampleAudioStream(SDL_AudioStream *stream, const float *inbuf, const int inbuflen, float *outbuf, const int outbuflen)
+SDL_ResampleAudioStream(SDL_AudioStream *stream, const void *_inbuf, const int inbuflen, void *_outbuf, const int outbuflen)
 {
+    const float *inbuf = (const float *) _inbuf;
+    float *outbuf = (float *) _outbuf;
     SDL_AudioStreamResamplerState *state = (SDL_AudioStreamResamplerState*)stream->resampler_state;
     const int chans = (int)stream->pre_resample_channels;
 
-    SDL_assert(chans <= SDL_arraysize(state->resampler_state));
+    SDL_assert(chans <= SDL_arraysize(state->resampler_state.f));
 
     if (!state->resampler_seeded) {
-        int i;
+        SDL_memcpy(state->resampler_state.f, inbuf, chans * sizeof (float));
-        for (i = 0; i < chans; i++) {
-            state->resampler_state[i] = inbuf[i];
-        }
         state->resampler_seeded = SDL_TRUE;
     }
 
-    return SDL_ResampleAudioSimple(chans, stream->rate_incr, state->resampler_state, inbuf, inbuflen, outbuf, outbuflen);
+    return SDL_ResampleAudioSimple(chans, stream->rate_incr, state->resampler_state.f, inbuf, inbuflen, outbuf, outbuflen);
+}
+
+static int
+SDL_ResampleAudioStream_si16_c2(SDL_AudioStream *stream, const void *_inbuf, const int inbuflen, void *_outbuf, const int outbuflen)
+{
+    const Sint16 *inbuf = (const Sint16 *) _inbuf;
+    Sint16 *outbuf = (Sint16 *) _outbuf;
+    SDL_AudioStreamResamplerState *state = (SDL_AudioStreamResamplerState*)stream->resampler_state;
+    const int chans = (int)stream->pre_resample_channels;
+
+    SDL_assert(chans <= SDL_arraysize(state->resampler_state.si16));
+
+    if (!state->resampler_seeded) {
+        state->resampler_state.si16[0] = inbuf[0];
+        state->resampler_state.si16[1] = inbuf[1];
+        state->resampler_seeded = SDL_TRUE;
+    }
+
+    return SDL_ResampleAudioSimple_si16_c2(stream->rate_incr, state->resampler_state.si16, inbuf, inbuflen, outbuf, outbuflen);
 }
 
 static void
@@ -1016,6 +1040,9 @@ SDL_NewAudioStream(const SDL_AudioFormat src_format,
     const int packetlen = 4096;  /* !!! FIXME: good enough for now. */
     Uint8 pre_resample_channels;
     SDL_AudioStream *retval;
+#ifndef HAVE_LIBSAMPLERATE_H
+    const SDL_bool SRC_available = SDL_FALSE;
+#endif
 
     retval = (SDL_AudioStream *) SDL_calloc(1, sizeof (SDL_AudioStream));
     if (!retval) {
@@ -1043,11 +1070,22 @@ SDL_NewAudioStream(const SDL_AudioFormat src_format,
     /* Not resampling? It's an easy conversion (and maybe not even that!). */
     if (src_rate == dst_rate) {
         retval->cvt_before_resampling.needed = SDL_FALSE;
-        retval->cvt_before_resampling.len_mult = 1;
         if (SDL_BuildAudioCVT(&retval->cvt_after_resampling, src_format, src_channels, dst_rate, dst_format, dst_channels, dst_rate) < 0) {
             SDL_FreeAudioStream(retval);
             return NULL;  /* SDL_BuildAudioCVT should have called SDL_SetError. */
         }
+    /* fast path special case for stereo Sint16 data that just needs resampling. */
+    } else if ((!SRC_available) && (src_channels == 2) && (dst_channels == 2) && (src_format == AUDIO_S16SYS) && (dst_format == AUDIO_S16SYS)) {
+        SDL_assert(src_rate != dst_rate);
+        retval->resampler_state = SDL_calloc(1, sizeof(SDL_AudioStreamResamplerState));
+        if (!retval->resampler_state) {
+            SDL_FreeAudioStream(retval);
+            SDL_OutOfMemory();
+            return NULL;
+        }
+        retval->resampler_func = SDL_ResampleAudioStream_si16_c2;
+        retval->reset_resampler_func = SDL_ResetAudioStreamResampler;
+        retval->cleanup_resampler_func = SDL_CleanupAudioStreamResampler;
     } else {
         /* Don't resample at first. Just get us to Float32 format. */
         /* !!! FIXME: convert to int32 on devices without hardware float. */
@@ -1136,16 +1174,16 @@ SDL_AudioStreamPut(SDL_AudioStream *stream, const void *buf, const Uint32 _bufle
 
     if (stream->dst_rate != stream->src_rate) {
         const int workbuflen = buflen * ((int) SDL_ceil(stream->rate_incr));
-        float *workbuf = (float *) EnsureBufferSize(&stream->resample_buffer, &stream->resample_buffer_len, workbuflen);
+        void *workbuf = EnsureBufferSize(&stream->resample_buffer, &stream->resample_buffer_len, workbuflen);
         if (workbuf == NULL) {
             return -1;  /* probably out of memory. */
         }
-        buflen = stream->resampler_func(stream, (float *) buf, buflen, workbuf, workbuflen);
+        buflen = stream->resampler_func(stream, buf, buflen, workbuf, workbuflen);
         buf = workbuf;
     }
 
     if (stream->cvt_after_resampling.needed) {
-        const int workbuflen = buflen * stream->cvt_before_resampling.len_mult;  /* will be "* 1" if not needed */
+        const int workbuflen = buflen * stream->cvt_after_resampling.len_mult;  /* will be "* 1" if not needed */
         Uint8 *workbuf;
 
         if (buf == stream->resample_buffer) {