src/opts/SkOpts_hsw.cpp

/*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

// It is not safe to #include any header file here unless it has been vetted for ODR safety:
// all symbols used must be file-scoped static or in an anonymous namespace.  This applies
// to _all_ header files:  C standard library, C++ standard library, Skia... everything.

#include <immintrin.h>   // ODR safe
#include <stdint.h>      // ODR safe

#if defined(__AVX2__)

namespace hsw {

    void convolve_vertically(const int16_t* filter, int filterLen,
                             uint8_t* const* srcRows, int width,
                             uint8_t* out, bool hasAlpha) {
        // It's simpler to work with the output array in terms of 4-byte pixels.
        auto dst = (int*)out;

        // Output up to eight pixels per iteration.
        for (int x = 0; x < width; x += 8) {
            // Accumulated result for 4 (non-adjacent) pairs of pixels,
            // with each channel in signed 17.14 fixed point.
            auto accum04 = _mm256_setzero_si256(),
                 accum15 = _mm256_setzero_si256(),
                 accum26 = _mm256_setzero_si256(),
                 accum37 = _mm256_setzero_si256();

            // Convolve with the filter.  (This inner loop is where we spend ~all our time.)
            // While we can, we consume 2 filter coefficients and 2 rows of 8 pixels each at a time.
            auto convolve_16_pixels = [&](__m256i interlaced_coeffs,
                                          __m256i pixels_01234567, __m256i pixels_89ABCDEF) {
                // Interlaced R0R8 G0G8 B0B8 A0A8 R1R9 G1G9... 32 8-bit values each.
                auto _08194C5D = _mm256_unpacklo_epi8(pixels_01234567, pixels_89ABCDEF),
                     _2A3B6E7F = _mm256_unpackhi_epi8(pixels_01234567, pixels_89ABCDEF);

                // Still interlaced R0R8 G0G8... as above, each channel expanded to 16-bit lanes.
                auto _084C = _mm256_unpacklo_epi8(_08194C5D, _mm256_setzero_si256()),
                     _195D = _mm256_unpackhi_epi8(_08194C5D, _mm256_setzero_si256()),
                     _2A6E = _mm256_unpacklo_epi8(_2A3B6E7F, _mm256_setzero_si256()),
                     _3B7F = _mm256_unpackhi_epi8(_2A3B6E7F, _mm256_setzero_si256());

                // accum0_R += R0*coeff0 + R8*coeff1, etc.
                accum04 = _mm256_add_epi32(accum04, _mm256_madd_epi16(_084C, interlaced_coeffs));
                accum15 = _mm256_add_epi32(accum15, _mm256_madd_epi16(_195D, interlaced_coeffs));
                accum26 = _mm256_add_epi32(accum26, _mm256_madd_epi16(_2A6E, interlaced_coeffs));
                accum37 = _mm256_add_epi32(accum37, _mm256_madd_epi16(_3B7F, interlaced_coeffs));
            };

            int i = 0;
            for (; i < filterLen/2*2; i += 2) {
                convolve_16_pixels(_mm256_set1_epi32(*(const int32_t*)(filter+i)),
                                   _mm256_loadu_si256((const __m256i*)(srcRows[i+0] + x*4)),
                                   _mm256_loadu_si256((const __m256i*)(srcRows[i+1] + x*4)));
            }
            if (i < filterLen) {
                convolve_16_pixels(_mm256_set1_epi32(*(const int16_t*)(filter+i)),
                                   _mm256_loadu_si256((const __m256i*)(srcRows[i] + x*4)),
                                   _mm256_setzero_si256());
            }

            // Trim the fractional parts off the accumulators.
            accum04 = _mm256_srai_epi32(accum04, 14);
            accum15 = _mm256_srai_epi32(accum15, 14);
            accum26 = _mm256_srai_epi32(accum26, 14);
            accum37 = _mm256_srai_epi32(accum37, 14);

            // Pack back down to 8-bit channels.
            auto pixels = _mm256_packus_epi16(_mm256_packs_epi32(accum04, accum15),
                                              _mm256_packs_epi32(accum26, accum37));

            if (hasAlpha) {
                // Clamp alpha to the max of r,g,b to make sure we stay premultiplied.
                __m256i max_rg  = _mm256_max_epu8(pixels, _mm256_srli_epi32(pixels,  8)),
                        max_rgb = _mm256_max_epu8(max_rg, _mm256_srli_epi32(pixels, 16));
                pixels = _mm256_max_epu8(pixels, _mm256_slli_epi32(max_rgb, 24));
            } else {
                // Force opaque.
                pixels = _mm256_or_si256(pixels, _mm256_set1_epi32(0xff000000));
            }

            // Normal path to store 8 pixels.
            if (x + 8 <= width) {
                _mm256_storeu_si256((__m256i*)dst, pixels);
                dst += 8;
                continue;
            }

            // Store one pixel at a time on the last iteration.
            for (int i = x; i < width; i++) {
                *dst++ = _mm_cvtsi128_si32(_mm256_castsi256_si128(pixels));
                pixels = _mm256_permutevar8x32_epi32(pixels, _mm256_setr_epi32(1,2,3,4,5,6,7,0));
            }
        }
    }

}

namespace SkOpts {
    // See SkOpts.h, writing SkConvolutionFilter1D::ConvolutionFixed as the underlying type.
    extern void (*convolve_vertically)(const int16_t* filter, int filterLen,
                                       uint8_t* const* srcRows, int width,
                                       uint8_t* out, bool hasAlpha);
    void Init_hsw() {
        convolve_vertically = hsw::convolve_vertically;
    }
}

#else  // defined(__AVX2__) is not true...

namespace SkOpts { void Init_hsw() {} }

#endif