From e2ec1eb12ef20b2fb6a496d93837009d168b11b9 Mon Sep 17 00:00:00 2001 From: "Ryan C. Gordon" Date: Tue, 15 May 2018 01:04:11 -0400 Subject: [PATCH] audio: converting int32 to/from float shouldn't use doubles. The concern is that a massive int sample, like 0x7FFFFFFF, won't fit in a float32, which doesn't have enough bits to hold a whole number this large, just to divide it to get a value between 0 and 1. Previously we would convert to double, to get more bits, do the division, and cast back to a float, but this is expensive. Casting to double is more accurate, but it's 2x to 3x slower. Shifting out the least significant byte of an int32, so it'll definitely fit in a float, and dividing by 0x7FFFFF is still accurate to about 5 decimal places, and the difference doesn't appear to be perceptable. --- src/audio/SDL_audiotypecvt.c | 42 +++++++++++------------------------- 1 file changed, 12 insertions(+), 30 deletions(-) diff --git a/src/audio/SDL_audiotypecvt.c b/src/audio/SDL_audiotypecvt.c index 2fbd916e5..fe8a89f7a 100644 --- a/src/audio/SDL_audiotypecvt.c +++ b/src/audio/SDL_audiotypecvt.c @@ -62,7 +62,7 @@ SDL_AudioFilter SDL_Convert_F32_to_S32 = NULL; #define DIVBY128 0.0078125f #define DIVBY32768 0.000030517578125f -#define DIVBY2147483648 0.00000000046566128730773926 +#define DIVBY8388607 0.00000011920930376163766f #if NEED_SCALAR_CONVERTER_FALLBACKS @@ -152,7 +152,7 @@ SDL_Convert_S32_to_F32_Scalar(SDL_AudioCVT *cvt, SDL_AudioFormat format) LOG_DEBUG_CONVERT("AUDIO_S32", "AUDIO_F32"); for (i = cvt->len_cvt / sizeof (Sint32); i; --i, ++src, ++dst) { - *dst = (float) (((double) *src) * DIVBY2147483648); + *dst = ((float) (*src>>8)) * DIVBY8388607; } if (cvt->filters[++cvt->filter_index]) { @@ -280,7 +280,7 @@ SDL_Convert_F32_to_S32_Scalar(SDL_AudioCVT *cvt, SDL_AudioFormat format) } else if (sample < -1.0f) { *dst = -2147483647; } else { - *dst = (Sint32)((double)sample * 2147483647.0); + *dst = ((Sint32)(sample * 8388607.0f)) << 8; } } @@ -509,16 +509,6 @@ SDL_Convert_U16_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) } } -#if defined(__GNUC__) && (__GNUC__ < 4) -/* these were added as of gcc-4.0: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19418 */ -static inline __m128 _mm_castsi128_ps(__m128i __A) { - return (__m128) __A; -} -static inline __m128i _mm_castps_si128(__m128 __A) { - return (__m128i) __A; -} -#endif - static void SDLCALL SDL_Convert_S32_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) { @@ -530,7 +520,7 @@ SDL_Convert_S32_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) /* Get dst aligned to 16 bytes */ for (i = cvt->len_cvt / sizeof (Sint32); i && (((size_t) dst) & 15); --i, ++src, ++dst) { - *dst = (float) (((double) *src) * DIVBY2147483648); + *dst = ((float) (*src>>8)) * DIVBY8388607; } SDL_assert(!i || ((((size_t) dst) & 15) == 0)); @@ -538,15 +528,11 @@ SDL_Convert_S32_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) { /* Aligned! Do SSE blocks as long as we have 16 bytes available. */ - const __m128d divby2147483648 = _mm_set1_pd(DIVBY2147483648); + const __m128 divby8388607 = _mm_set1_ps(DIVBY8388607); const __m128i *mmsrc = (const __m128i *) src; while (i >= 4) { /* 4 * sint32 */ - const __m128i ints = _mm_load_si128(mmsrc); - /* bitshift the whole register over, so _mm_cvtepi32_pd can read the top ints in the bottom of the vector. */ - const __m128d doubles1 = _mm_mul_pd(_mm_cvtepi32_pd(_mm_srli_si128(ints, 8)), divby2147483648); - const __m128d doubles2 = _mm_mul_pd(_mm_cvtepi32_pd(ints), divby2147483648); - /* convert to float32, bitshift/or to get these into a vector to store. */ - _mm_store_ps(dst, _mm_castsi128_ps(_mm_or_si128(_mm_slli_si128(_mm_castps_si128(_mm_cvtpd_ps(doubles1)), 8), _mm_castps_si128(_mm_cvtpd_ps(doubles2))))); + /* shift out lowest bits so int fits in a float32. Small precision loss, but much faster. */ + _mm_store_ps(dst, _mm_mul_ps(_mm_cvtepi32_ps(_mm_srli_epi32(_mm_load_si128(mmsrc), 8)), divby8388607)); i -= 4; mmsrc++; dst += 4; } src = (const Sint32 *) mmsrc; @@ -554,7 +540,7 @@ SDL_Convert_S32_to_F32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) /* Finish off any leftovers with scalar operations. */ while (i) { - *dst = (float) (((double) *src) * DIVBY2147483648); + *dst = ((float) (*src>>8)) * DIVBY8388607; i--; src++; dst++; } @@ -755,7 +741,7 @@ SDL_Convert_F32_to_S32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) /* Get dst aligned to 16 bytes */ for (i = cvt->len_cvt / sizeof (float); i && (((size_t) dst) & 15); --i, ++src, ++dst) { - *dst = (Sint32) (((double) *src) * 2147483647.0); + *dst = ((Sint32)(*src * 8388607.0f)) << 8; } SDL_assert(!i || ((((size_t) dst) & 15) == 0)); @@ -763,14 +749,10 @@ SDL_Convert_F32_to_S32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) { /* Aligned! Do SSE blocks as long as we have 16 bytes available. */ - const __m128d mulby2147483647 = _mm_set1_pd(2147483647.0); + const __m128 mulby8388607 = _mm_set1_ps(8388607.0f); __m128i *mmdst = (__m128i *) dst; while (i >= 4) { /* 4 * float32 */ - const __m128 floats = _mm_load_ps(src); - /* bitshift the whole register over, so _mm_cvtps_pd can read the top floats in the bottom of the vector. */ - const __m128d doubles1 = _mm_mul_pd(_mm_cvtps_pd(_mm_castsi128_ps(_mm_srli_si128(_mm_castps_si128(floats), 8))), mulby2147483647); - const __m128d doubles2 = _mm_mul_pd(_mm_cvtps_pd(floats), mulby2147483647); - _mm_store_si128(mmdst, _mm_or_si128(_mm_slli_si128(_mm_cvtpd_epi32(doubles1), 8), _mm_cvtpd_epi32(doubles2))); + _mm_store_si128(mmdst, _mm_slli_epi32(_mm_cvtps_epi32(_mm_mul_ps(_mm_load_ps(src), mulby8388607)), 8)); i -= 4; src += 4; mmdst++; } dst = (Sint32 *) mmdst; @@ -778,7 +760,7 @@ SDL_Convert_F32_to_S32_SSE2(SDL_AudioCVT *cvt, SDL_AudioFormat format) /* Finish off any leftovers with scalar operations. */ while (i) { - *dst = (Sint32) (((double) *src) * 2147483647.0); + *dst = ((Sint32)(*src * 8388607.0f)) << 8; i--; src++; dst++; }