Revert of SkHalfToFloat_01 / SkFloatToHalf_01 (patchset #11 id:200001 of https://codereview.chromium.org/1685133005/ )

Reason for revert:
Gotta fix Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD

Original issue's description:
> 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
>
> Committed: https://skia.googlesource.com/skia/+/9ea11a4235b3e3521cc8bf914a27c2d0dc062db9

TBR=jvanverth@google.com,reed@google.com,mtklein@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1693443003

3 files changed, 23 insertions, 100 deletions

diff --git a/src/core/SkHalf.h b/src/core/SkHalf.h
index baf86fda3b..7e41c6ff0c 100644
--- a/src/core/SkHalf.h
+++ b/src/core/SkHalf.h
@@ -8,7 +8,6 @@
 #ifndef SkHalf_DEFINED
 #define SkHalf_DEFINED
 
-#include "SkNx.h"
 #include "SkTypes.h"
 
 // 16-bit floating point value
@@ -24,66 +23,4 @@ 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 6de5f1b09f..6435e6a2e2 100644
--- a/src/core/SkXfermodeU64.cpp
+++ b/src/core/SkXfermodeU64.cpp
@@ -46,15 +46,36 @@ 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<const uint16_t*>(&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<uint16_t*>(&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 <DstType D> Sk4f load_from_dst(uint64_t dst) {
-    return (D == kU16_Dst) ? load_from_u16(dst) : SkHalfToFloat_01(dst);
+    return (D == kU16_Dst) ? load_from_u16(dst) : load_from_f16(dst);
 }
 
 // Assumes x4 is already in the "natural" bias (either unit-float or 16bit int)
 template <DstType D> uint64_t store_to_dst(const Sk4f& x4) {
-    return (D == kU16_Dst) ? store_to_u16(x4) : SkFloatToHalf_01(x4);
+    return (D == kU16_Dst) ? store_to_u16(x4) : store_to_f16(x4);
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
diff --git a/tests/Float16Test.cpp b/tests/Float16Test.cpp
index f96d904567..f437268c79 100644
--- a/tests/Float16Test.cpp
+++ b/tests/Float16Test.cpp
@@ -10,7 +10,6 @@
 #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));
@@ -65,37 +64,3 @@ 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;
-            }
-        }
-    }
-}