From 9ea11a4235b3e3521cc8bf914a27c2d0dc062db9 Mon Sep 17 00:00:00 2001 From: mtklein Date: Thu, 11 Feb 2016 05:56:08 -0800 Subject: [PATCH] SkHalfToFloat_01 / SkFloatToHalf_01 These are basically inlined, 4-at-a-time versions of our existing functions, but cut down to avoid any work that's only necessary outside [0,1]. Both f16 and f32 denorms should work fine modulo the usual ARMv7 NEON denorm==zero caveat. In exchange for a little speed, f32->f16 does not round properly. Instead it truncates, so it's never off by more than 1 bit. Support for finite values >1 or <0 is straightforward to add back. >1 might already work as-is. Getting close to _u16 performance: micros bench 261.13 xferu64_bw_1_opaque_u16 1833.51 xferu64_bw_1_alpha_u16 2762.32 ? xferu64_aa_1_opaque_u16 3334.29 xferu64_aa_1_alpha_u16 249.78 xferu64_bw_1_opaque_f16 3383.18 xferu64_bw_1_alpha_f16 4214.72 xferu64_aa_1_opaque_f16 4701.19 xferu64_aa_1_alpha_f16 BUG=skia: GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1685133005 Review URL: https://codereview.chromium.org/1685133005 --- src/core/SkHalf.h | 63 ++++++++++++++++++++++++++++++++++++++ src/core/SkXfermodeU64.cpp | 25 ++------------- tests/Float16Test.cpp | 35 +++++++++++++++++++++ 3 files changed, 100 insertions(+), 23 deletions(-) diff --git a/src/core/SkHalf.h b/src/core/SkHalf.h index 7e41c6ff0c..baf86fda3b 100644 --- a/src/core/SkHalf.h +++ b/src/core/SkHalf.h @@ -8,6 +8,7 @@ #ifndef SkHalf_DEFINED #define SkHalf_DEFINED +#include "SkNx.h" #include "SkTypes.h" // 16-bit floating point value @@ -23,4 +24,66 @@ typedef uint16_t SkHalf; float SkHalfToFloat(SkHalf h); SkHalf SkFloatToHalf(float f); +// Convert between half and single precision floating point, but pull any dirty +// trick we can to make it faster as long as it's correct enough for values in [0,1]. +static inline Sk4f SkHalfToFloat_01(uint64_t); +static inline uint64_t SkFloatToHalf_01(const Sk4f&); + +// ~~~~~~~~~~~ impl ~~~~~~~~~~~~~~ // + +// Like the serial versions in SkHalf.cpp, these are based on +// https://fgiesen.wordpress.com/2012/03/28/half-to-float-done-quic/ + +// TODO: NEON versions +static inline Sk4f SkHalfToFloat_01(uint64_t hs) { +#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2 + // Load our 16-bit floats into the bottom 16 bits of each 32-bit lane, with zeroes on top. + __m128i h = _mm_unpacklo_epi16(_mm_loadl_epi64((const __m128i*)&hs), _mm_setzero_si128()); + + // Fork into two paths, depending on whether the 16-bit float is denormalized. + __m128 is_denorm = _mm_castsi128_ps(_mm_cmplt_epi32(h, _mm_set1_epi32(0x0400))); + + // TODO: figure out, explain + const __m128 half = _mm_set1_ps(0.5f); + __m128 denorm = _mm_sub_ps(_mm_or_ps(_mm_castsi128_ps(h), half), half); + + // If we're normalized, just shift ourselves so the exponent/mantissa dividing line + // is correct, then re-bias the exponent from 15 to 127. + __m128 norm = _mm_castsi128_ps(_mm_add_epi32(_mm_slli_epi32(h, 13), + _mm_set1_epi32((127-15) << 23))); + + return _mm_or_ps(_mm_and_ps (is_denorm, denorm), + _mm_andnot_ps(is_denorm, norm)); +#else + float fs[4]; + for (int i = 0; i < 4; i++) { + fs[i] = SkHalfToFloat(hs >> (i*16)); + } + return Sk4f::Load(fs); +#endif +} + +static inline uint64_t SkFloatToHalf_01(const Sk4f& fs) { +#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2 + // Scale our floats down by a tiny power of 2 to pull up our mantissa bits, + // then shift back down to 16-bit float layout. This doesn't round, so can be 1 bit small. + // TODO: understand better. Why this scale factor? + const __m128 scale = _mm_castsi128_ps(_mm_set1_epi32(15 << 23)); + __m128i h = _mm_srli_epi32(_mm_castps_si128(_mm_mul_ps(fs.fVec, scale)), 13); + + uint64_t r; + _mm_storel_epi64((__m128i*)&r, _mm_packs_epi32(h,h)); + return r; +#else + SkHalf hs[4]; + for (int i = 0; i < 4; i++) { + hs[i] = SkFloatToHalf(fs[i]); + } + return (uint64_t)hs[3] << 48 + | (uint64_t)hs[2] << 32 + | (uint64_t)hs[1] << 16 + | (uint64_t)hs[0] << 0; +#endif +} + #endif diff --git a/src/core/SkXfermodeU64.cpp b/src/core/SkXfermodeU64.cpp index 6435e6a2e2..6de5f1b09f 100644 --- a/src/core/SkXfermodeU64.cpp +++ b/src/core/SkXfermodeU64.cpp @@ -46,36 +46,15 @@ static uint64_t store_to_u16(const Sk4f& x4) { return value; } -static Sk4f load_from_f16(uint64_t value) { - const uint16_t* u16 = reinterpret_cast(&value); - float f4[4]; - for (int i = 0; i < 4; ++i) { - f4[i] = SkHalfToFloat(u16[i]); - } - return Sk4f::Load(f4); -} - -static uint64_t store_to_f16(const Sk4f& x4) { - uint64_t value; - uint16_t* u16 = reinterpret_cast(&value); - - float f4[4]; - x4.store(f4); - for (int i = 0; i < 4; ++i) { - u16[i] = SkFloatToHalf(f4[i]); - } - return value; -} - // Returns dst in its "natural" bias (either unit-float or 16bit int) // template Sk4f load_from_dst(uint64_t dst) { - return (D == kU16_Dst) ? load_from_u16(dst) : load_from_f16(dst); + return (D == kU16_Dst) ? load_from_u16(dst) : SkHalfToFloat_01(dst); } // Assumes x4 is already in the "natural" bias (either unit-float or 16bit int) template uint64_t store_to_dst(const Sk4f& x4) { - return (D == kU16_Dst) ? store_to_u16(x4) : store_to_f16(x4); + return (D == kU16_Dst) ? store_to_u16(x4) : SkFloatToHalf_01(x4); } /////////////////////////////////////////////////////////////////////////////////////////////////// diff --git a/tests/Float16Test.cpp b/tests/Float16Test.cpp index f437268c79..f96d904567 100644 --- a/tests/Float16Test.cpp +++ b/tests/Float16Test.cpp @@ -10,6 +10,7 @@ #include "SkHalf.h" #include "SkOpts.h" #include "SkPixmap.h" +#include "SkRandom.h" static bool eq_within_half_float(float a, float b) { const float kTolerance = 1.0f / (1 << (8 + 10)); @@ -64,3 +65,37 @@ DEF_TEST(float_to_half, reporter) { SkOpts::half_to_float(fscratch, hs, 7); REPORTER_ASSERT(reporter, 0 == memcmp(fscratch, fs, sizeof(fs))); } + +DEF_TEST(HalfToFloat_01, r) { + for (uint16_t h = 0; h < 0x8000; h++) { + float f = SkHalfToFloat(h); + if (f >= 0 && f <= 1) { + REPORTER_ASSERT(r, SkHalfToFloat_01(h)[0] == f); + REPORTER_ASSERT(r, SkFloatToHalf_01(SkHalfToFloat_01(h)) == h); + } + } +} + +DEF_TEST(FloatToHalf_01, r) { +#if 0 + for (uint32_t bits = 0; bits < 0x80000000; bits++) { +#else + SkRandom rand; + for (int i = 0; i < 1000000; i++) { + uint32_t bits = rand.nextU(); +#endif + float f; + memcpy(&f, &bits, 4); + if (f >= 0 && f <= 1) { + uint16_t h1 = (uint16_t)SkFloatToHalf_01(Sk4f(f,0,0,0)), + h2 = SkFloatToHalf(f); + bool ok = (h1 == h2 || h1 == h2-1); + REPORTER_ASSERT(r, ok); + if (!ok) { + SkDebugf("%08x (%d) -> %04x (%d), want %04x (%d)\n", + bits, bits>>23, h1, h1>>10, h2, h2>>10); + break; + } + } + } +}