Optimize color xforms with 2.2 gammas for SSE2

src/opts/SkColorXform_opts.h

Index: src/opts/SkColorXform_opts.h
diff --git a/src/opts/SkColorXform_opts.h b/src/opts/SkColorXform_opts.h
new file mode 100644
index 0000000000000000000000000000000000000000..b1acfb3b6163ad75f134c88322994a85cb9d8ebf
--- /dev/null
+++ b/src/opts/SkColorXform_opts.h
@@ -0,0 +1,181 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkColorXform_opts_DEFINED
+#define SkColorXform_opts_DEFINED
+
+#include "SkColorPriv.h"
+
+namespace SK_OPTS_NS {
+
+static uint8_t clamp_float_to_byte(float v) {
+    if (v >= 254.5f) {
+        return 255;
+    } else if (v < 0.5f) {
+        return 0;
+    } else {
+        return (uint8_t) (v + 0.5f);
+    }
+}
+
+static void color_xform_2Dot2_RGBA_to_8888_portable(uint32_t* dst, const uint32_t* src, int len,
+                                                    const float matrix[16]) {
+    while (len-- > 0) {
+        float srcFloats[3];
+        srcFloats[0] = (float) ((*src >>  0) & 0xFF);
+        srcFloats[1] = (float) ((*src >>  8) & 0xFF);
+        srcFloats[2] = (float) ((*src >> 16) & 0xFF);
+
+        // Convert to linear.
+        // TODO (msarett):
+        // We should use X^2.2 here instead of X^2.  What is the impact on correctness?
+        // We should be able to get closer to 2.2 at a small performance cost.
+        srcFloats[0] = srcFloats[0] * srcFloats[0];
+        srcFloats[1] = srcFloats[1] * srcFloats[1];
+        srcFloats[2] = srcFloats[2] * srcFloats[2];
+
+        // Convert to dst gamut.
+        float dstFloats[3];
+        // TODO (msarett): matrix[12], matrix[13], and matrix[14] are almost always zero.
+        // Should we have another optimized path that avoids the extra addition when they
+        // are zero?
+        dstFloats[0] = srcFloats[0] * matrix[0] + srcFloats[1] * matrix[4] +
+                       srcFloats[2] * matrix[8] + matrix[12];
+        dstFloats[1] = srcFloats[0] * matrix[1] + srcFloats[1] * matrix[5] +
+                       srcFloats[2] * matrix[9] + matrix[13];
+        dstFloats[2] = srcFloats[0] * matrix[2] + srcFloats[1] * matrix[6] +
+                       srcFloats[2] * matrix[10] + matrix[14];
+
+        // Convert to dst gamma.
+        // TODO (msarett):
+        // We should use X^(1/2.2) here instead of X^(1/2).  What is the impact on correctness?
+        // We should be able to get closer to (1/2.2) at a small performance cost.
+        dstFloats[0] = sqrtf(dstFloats[0]);
+        dstFloats[1] = sqrtf(dstFloats[1]);
+        dstFloats[2] = sqrtf(dstFloats[2]);
+
+        *dst = SkPackARGB32NoCheck(((*src >> 24) & 0xFF),
+                                   clamp_float_to_byte(dstFloats[0]),
+                                   clamp_float_to_byte(dstFloats[1]),
+                                   clamp_float_to_byte(dstFloats[2]));
+
+        dst++;
+        src++;
+    }
+}
+
+#if SK_CPU_SSE_LEVEL >= SK_CPU_SSE_LEVEL_SSE2
+
+static void color_xform_2Dot2_RGBA_to_8888(uint32_t* dst, const uint32_t* src, int len,
+                                           const float matrix[16]) {
+    // Load transformation matrix.
+    __m128 rXgXbX = _mm_loadu_ps(&matrix[0]);
+    __m128 rYgYbY = _mm_loadu_ps(&matrix[4]);
+    __m128 rZgZbZ = _mm_loadu_ps(&matrix[8]);
+    __m128 rQgQbQ = _mm_loadu_ps(&matrix[12]);
+
+    while (len >= 4) {
+        // Load 4 pixels and convert them to floats.
+        __m128i rgba = _mm_loadu_si128((const __m128i*) src);
+        __m128i byteMask = _mm_set1_epi32(0xFF);
+        __m128 reds   = _mm_cvtepi32_ps(_mm_and_si128(               rgba,      byteMask));
+        __m128 greens = _mm_cvtepi32_ps(_mm_and_si128(_mm_srli_epi32(rgba,  8), byteMask));
+        __m128 blues  = _mm_cvtepi32_ps(_mm_and_si128(_mm_srli_epi32(rgba, 16), byteMask));
+
+        // Convert to linear.
+        // FIXME (msarett):
+        // Should we be more accurate?
+        reds   = _mm_mul_ps(reds, reds);
+        greens = _mm_mul_ps(greens, greens);
+        blues  = _mm_mul_ps(blues, blues);
+
+        // Apply the transformation matrix to dst gamut.
+        // FIXME (msarett):
+        // rQ, gQ, and bQ are almost always zero.  Can we save a couple instructions?
+
+        // Splat rX, rY, rZ, and rQ each across a register.
+        __m128 rX = _mm_shuffle_ps(rXgXbX, rXgXbX, 0x00);
+        __m128 rY = _mm_shuffle_ps(rYgYbY, rYgYbY, 0x00);
+        __m128 rZ = _mm_shuffle_ps(rZgZbZ, rZgZbZ, 0x00);
+        __m128 rQ = _mm_shuffle_ps(rQgQbQ, rQgQbQ, 0x00);
+
+        // dstReds = rX * reds + rY * greens + rZ * blues + rQ
+        __m128 dstReds =                     _mm_mul_ps(reds,   rX);
+               dstReds = _mm_add_ps(dstReds, _mm_mul_ps(greens, rY));
+               dstReds = _mm_add_ps(dstReds, _mm_mul_ps(blues,  rZ));
+               dstReds = _mm_add_ps(dstReds,                    rQ);
+
+        // Splat gX, gY, gZ, and gQ each across a register.
+        __m128 gX = _mm_shuffle_ps(rXgXbX, rXgXbX, 0x55);
+        __m128 gY = _mm_shuffle_ps(rYgYbY, rYgYbY, 0x55);
+        __m128 gZ = _mm_shuffle_ps(rZgZbZ, rZgZbZ, 0x55);
+        __m128 gQ = _mm_shuffle_ps(rQgQbQ, rQgQbQ, 0x55);
+
+        // dstGreens = gX * reds + gY * greens + gZ * blues + gQ
+        __m128 dstGreens =                       _mm_mul_ps(reds,   gX);
+               dstGreens = _mm_add_ps(dstGreens, _mm_mul_ps(greens, gY));
+               dstGreens = _mm_add_ps(dstGreens, _mm_mul_ps(blues,  gZ));
+               dstGreens = _mm_add_ps(dstGreens,                    gQ);
+
+        // Splat bX, bY, bZ, and bQ each across a register.
+        __m128 bX = _mm_shuffle_ps(rXgXbX, rXgXbX, 0xAA);
+        __m128 bY = _mm_shuffle_ps(rYgYbY, rYgYbY, 0xAA);
+        __m128 bZ = _mm_shuffle_ps(rZgZbZ, rZgZbZ, 0xAA);
+        __m128 bQ = _mm_shuffle_ps(rQgQbQ, rQgQbQ, 0xAA);
+
+        // dstBlues = bX * reds + bY * greens + bZ * blues + bQ
+        __m128 dstBlues =                      _mm_mul_ps(reds,   bX);
+               dstBlues = _mm_add_ps(dstBlues, _mm_mul_ps(greens, bY));
+               dstBlues = _mm_add_ps(dstBlues, _mm_mul_ps(blues,  bZ));
+               dstBlues = _mm_add_ps(dstBlues,                    bQ);
+
+        // Convert to dst gamma.
+        // Note that the reciprocal of the reciprocal sqrt, is just a fast sqrt.
+        // FIXME (msarett):
+        // Should we be more accurate?
+        dstReds   = _mm_rcp_ps(_mm_rsqrt_ps(dstReds));
+        dstGreens = _mm_rcp_ps(_mm_rsqrt_ps(dstGreens));
+        dstBlues  = _mm_rcp_ps(_mm_rsqrt_ps(dstBlues));
+
+        // Clamp floats to 0-255 range.
+        dstReds   = _mm_min_ps(_mm_max_ps(dstReds,   _mm_setzero_ps()), _mm_set1_ps(255.0f));

[mtklein, 2016/06/08 18:49:57]

Aww man, it looks so much prettier if you write

    max(0, min(x, 255)).

The x is between 0 and 255 :P

[msarett, 2016/06/08 19:11:13]

Agreed!

+        dstGreens = _mm_min_ps(_mm_max_ps(dstGreens, _mm_setzero_ps()), _mm_set1_ps(255.0f));
+        dstBlues  = _mm_min_ps(_mm_max_ps(dstBlues,  _mm_setzero_ps()), _mm_set1_ps(255.0f));
+
+        // Convert to bytes and store to memory.
+        rgba = _mm_and_si128(_mm_set1_epi32(0xFF000000), rgba);
+#ifdef SK_PMCOLOR_IS_RGBA
+        rgba = _mm_or_si128(rgba,                _mm_cvtps_epi32(dstReds)       );
+        rgba = _mm_or_si128(rgba, _mm_slli_epi32(_mm_cvtps_epi32(dstGreens),  8));
+        rgba = _mm_or_si128(rgba, _mm_slli_epi32(_mm_cvtps_epi32(dstBlues),  16));
+#else
+        rgba = _mm_or_si128(rgba,                _mm_cvtps_epi32(dstBlues)      );
+        rgba = _mm_or_si128(rgba, _mm_slli_epi32(_mm_cvtps_epi32(dstGreens),  8));
+        rgba = _mm_or_si128(rgba, _mm_slli_epi32(_mm_cvtps_epi32(dstReds),   16));
+#endif
+        _mm_storeu_si128((__m128i*) dst, rgba);
+
+        dst += 4;
+        src += 4;
+        len -= 4;
+    }
+
+    color_xform_2Dot2_RGBA_to_8888_portable(dst, src, len, matrix);
+}
+
+#else
+
+static void color_xform_2Dot2_RGBA_to_8888(uint32_t* dst, const uint32_t* src, int len,
+                                           const float matrix[16]) {
+    color_xform_2Dot2_RGBA_to_8888_portable(dst, src, len, matrix);
+}
+
+#endif
+
+}
+
+#endif // SkColorXform_opts_DEFINED