BGRA to RGBA

BGRA2RGBA

// Really awesome code taken from: http://apangborn.com/2011/05/pixel-processing-using-arm-assembly/
inline static void neon_rgba_to_bgra(unsigned char *src, unsigned char *dst, int numPixels)
{
#ifdef __ARM_NEON__
    int simd_pixels = numPixels & ~7; // round down to nearest 8
    int simd_iterations = simd_pixels >> 3;
    int col;
    if(simd_iterations) { // make sure at least 1 iteration
        __asm__ __volatile__ ("1: \n\t"
                              // structured load of 8 pixels into d0-d3 (64-bit) NEON registers
                              "vld4.8 {d0, d1, d2, d3}, [%[source]]! \n\t" // the "!" increments the pointer by number of bytes read
                              "vswp d0, d2 \n\t" // swap registers d0 and d2 (swaps red and blue, 8 pixels at a time)
                              "vst4.8 {d0, d1, d2, d3}, [%[dest]]! \n\t" // structured store the 8 pixels back, the "!" increments the pointer by number of bytes written
                              "subs %[iterations],%[iterations],#1 \n\t"
                              "bne 1b" // jump to label "1", "b" suffix means the jump is back/behind the current statement
                              : [source]"+r"(src), [dest] "+r"(dst), [iterations]"+r"(simd_iterations) // output parameters, we list read-write, "+", value as outputs. Read-write so that the auto-increment actually affects the 'src' and 'dst'
                              :  // no input parameters, they're all read-write so we put them in the output parameters
                              : "memory", "d0", "d1", "d2", "d3" // clobbered registers
                              );
    }

    // swap the leftover pixels
    // assembly with "load word, rotate right 1 byte, reverse word, store word" might be faster - exercise for the ambitious reader
    unsigned char r;
    for (col = simd_pixels; col < numPixels; col++, src += 4, dst += 4)     {
        r = src[0];
        dst[0] = src[2];
        dst[1] = src[1];
        dst[2] = r;
        dst[3] = src[3];
    }
#else
    for (NSUInteger i = 0; i < numPixels; i += 4) {
        char temp = src[i];
        dst[i] = src[i + 2];
        dts[i + 2] = temp;
    }
#endif
}