Optimizing RGBA8888 to RGB565 conversion with NEON

Question

I'm trying to optimize an image format conversion on iOS using the NEON vector instruction set. I assumed this would map well to that because it processes a bunch of similar data.

My attempts haven't gone that well, though, achieving only a marginal speedup vs the naive c implementation:

for(int i = 0; i < pixelCount; ++i, ++inPixel32) {
    const unsigned int r = ((*inPixel32 >> 0 ) & 0xFF);
    const unsigned int g = ((*inPixel32 >> 8 ) & 0xFF);
    const unsigned int b = ((*inPixel32 >> 16) & 0xFF);
    *outPixel16++ = ((r >> 3) << 11) | ((g >> 2) << 5) | ((b >> 3) << 0);
}

1 megapixel image array on iPad 2:

format is [min avg max n=number of timer samples] in milliseconds

C: [14.446 14.632 18.405 n=1000]ms

NEON: [11.920 12.032 15.336 n=1000]ms

My attempt at a NEON implementation is below:

    int i;
const int pixelsPerLoop = 8;
for(i = 0; i < pixelCount; i += pixelsPerLoop, inPixel32 += pixelsPerLoop, outPixel16 += pixelsPerLoop) {
    //Read all r,g,b pixels into 3 registers
    uint8x8x4_t rgba  = vld4_u8(inPixel32);
    //Right-shift r,g,b as appropriate
    uint8x8_t r = vshr_n_u8(rgba.val[0], 3);
    uint8x8_t g = vshr_n_u8(rgba.val[1], 2);
    uint8x8_t b = vshr_n_u8(rgba.val[2], 3);

    //Widen b
    uint16x8_t r5_g6_b5 = vmovl_u8(b);
    //Widen r
    uint16x8_t r16 = vmovl_u8(r);
    //Left shift into position within 16-bit int
    r16 = vshlq_n_u16(r16, 11);
    r5_g6_b5 |= r16;

    //Widen g
    uint16x8_t g16 = vmovl_u8(g);
    //Left shift into position within 16-bit int
    g16 = vshlq_n_u16(g16, 5);

    r5_g6_b5 |= g16;

    //Now write back to memory
    vst1q_u16(outPixel16, r5_g6_b5);        
}
//Do the remainder on normal flt hardware

Code was compiled via LLVM 3.0 into the following (.loc and extra labels removed):

_DNConvert_ARGB8888toRGB565:
    push    {r4, r5, r7, lr}
    mov r9, r1
    mov.w   r12, #0
    add r7, sp, #8
    cmp r2, #0
    mov.w   r1, #0
    it  ne
    movne   r1, #1
    cmp r0, #0
    mov.w   r3, #0
    it  ne
    movne   r3, #1
    cmp.w   r9, #0
    mov.w   r4, #0
    it  ne
    movne   r4, #1
    tst.w   r9, #3
    bne LBB0_8
    ands    r1, r3
    ands    r1, r4
    cmp r1, #1
    bne LBB0_8
    movs    r1, #0
    lsr.w   lr, r9, #2
    cmp.w   r1, r9, lsr #2
    bne LBB0_9
    mov r3, r2
    mov r5, r0
    b   LBB0_5
LBB0_4:
    movw    r1, #65528
    add.w   r0, lr, #7
    movt    r1, #32767
    ands    r1, r0
LBB0_5:
    mov.w   r12, #1
    cmp r1, lr
    bhs LBB0_8
    rsb r0, r1, r9, lsr #2
    mov.w   r9, #63488
    mov.w   lr, #2016
    mov.w   r12, #1
LBB0_7:
    ldr r2, [r5], #4
    subs    r0, #1
    and.w   r1, r9, r2, lsl #8
    and.w   r4, lr, r2, lsr #5
    ubfx    r2, r2, #19, #5
    orr.w   r2, r2, r4
    orr.w   r1, r1, r2
    strh    r1, [r3], #2
    bne LBB0_7
LBB0_8:
    mov r0, r12
    pop {r4, r5, r7, pc}
LBB0_9:
    sub.w   r1, lr, #1
    movs    r3, #32
    add.w   r3, r3, r1, lsl #2
    bic r3, r3, #31
    adds    r5, r0, r3
    movs    r3, #16
    add.w   r1, r3, r1, lsl #1
    bic r1, r1, #15
    adds    r3, r2, r1
    movs    r1, #0
LBB0_10:
    vld4.8  {d16, d17, d18, d19}, [r0]!
    adds    r1, #8
    cmp r1, lr
    vshr.u8 d20, d16, #3
    vshr.u8 d21, d17, #2
    vshr.u8 d16, d18, #3
    vmovl.u8    q11, d20
    vmovl.u8    q9, d21
    vmovl.u8    q8, d16
    vshl.i16    q10, q11, #11
    vshl.i16    q9, q9, #5
    vorr    q8, q8, q10
    vorr    q8, q8, q9
    vst1.16 {d16, d17}, [r2]!
Ltmp28:
    blo LBB0_10
    b   LBB0_4

Full code is available at https://github.com/darknoon/DNImageConvert I would appreciate any help, thanks!

Answer 1

Here you are, hand-optimized NEON implementation ready for XCode :

/* IT DOESN'T WORK!!! USE THE NEXT VERSION BELOW.
 * BGRA2RGB565.s
 *
 * Created by Jake "Alquimista" Lee on 11. 11. 1..
 * Copyright 2011 Jake Lee. All rights reserved.
 */


    .align 2
    .globl _bgra2rgb565_neon
    .private_extern _bgra2rgb565_neon

// unsigned int * bgra2rgb565_neon(unsigned int * pDst, unsigned int * pSrc, unsigned int count);


//ARM
pDst        .req    r0
pSrc        .req    r1
count       .req    r2

//NEON
blu         .req    d16
grn         .req    d17
red         .req    d18
alp         .req    d19
rg          .req    red
gb          .req    blu

_bgra2rgb565_neon:
    pld     [pSrc]
    tst     count, #0x7
    movne   r0, #0
    bxne    lr

loop:
    pld     [pSrc, #32]
    vld4.8  {blu, grn, red, alp}, [pSrc]!
    subs    count, count, #8
    vshr.u8 red, red, #3
    vext.8  rg, grn, red, #5
    vshr.u8 grn, grn, #2
    vext.8  gb, blu, grn, #3
    vst2.8  {gb, rg}, [pDst]!
    bgt     loop

    bx      lr

This version will be many times faster than what you suggested :

• increased cache hit rate via PLD

• conversion to "long" not necessary

• fewer instructions within the loop

There is still some room for optimizations though, you could modify the loop so that it converts 16 pixels per iteration instead of 8. Then you can schedule the instructions to avoid the two stalls completely (which is simply not possible in this 8/iteration version above) and benefit from NEON's dual-issue capability in addition.

I didn't do this because it would make the code hard to understand.

It's important to know what VEXT is supposed to do.

Now it's up to you. :)

I verified this code to be properly compiled under Xcode. Although I'm pretty sure it works correctly as well, I cannot guarantee this since I don't have the test environment. In case of malfunctioning, please let me know. I'll correct it accordingly then.

cya

==============================================================================

Well, here is the improved version.

Due to the nature of the VSRI instruction not allowing two operands other than the target, it was not possible to create a more robust one regarding the register assignment.

Please check the image format of your source image. (exact byte order of the elements)

If it's not B, G, R, A, which is the default and native one on iOS, your application will suffer heavily from internal conversions by iOS.

If it's absolutely not possible to change this for whatever the reason, let me know. I'll write a new version matching it.

PS : I forgot to remove the underscore at the start of the function prototype. Now it's gone.

/*
 * BGRA2RGB565.s
 *
 * Created by Jake "Alquimista" Lee on 11. 11. 1..
 * Copyright 2011 Jake Lee. All rights reserved.
 *
 * Version 1.1
 * - bug fix
 *
 * Version 1.0
 * - initial release
 */


    .align 2
    .globl _bgra2rgb565_neon
    .private_extern _bgra2rgb565_neon

// unsigned int * bgra2rgb565_neon(unsigned int * pDst, unsigned int * pSrc, unsigned int count);


//ARM
pDst        .req    r0
pSrc        .req    r1
count       .req    r2

//NEON
blu         .req    d16
grn         .req    d17
red         .req    d18
alp         .req    d19

gb          .req    grn
rg          .req    red

_bgra2rgb565_neon:
    pld     [pSrc]
    tst     count, #0x7
    movne   r0, #0
    bxne    lr

.loop:
    pld     [pSrc, #32]
    vld4.8  {blu, grn, red, alp}, [pSrc]!
    subs    count, count, #8

    vsri.8  red, grn, #5
    vshl.u8 gb, grn, #3
    vsri.8  gb, blu, #3

    vst2.8  {gb, rg}, [pDst]!
    bgt     .loop

    bx      lr

Answer 2

If you are on iOS or OS X, then you may be delighted to discover vImageConvert_RGBA8888toRGB565() and friends, in Accelerate.framework. This function rounds the 8-bit values to nearest 565 value.

For even better dithering, the quality of which is nearly indistinguishable from 8-bit color, try vImageConvert_AnyToAny():

vImage_CGImageFormat RGBA8888Format = 
{
    .bitsPerComponent = 8,
    .bitsPerPixel = 32,
    .bitmapInfo = kCGBitmapByteOrderDefault | kCGImageAlphaNoneSkipLast,
    .colorSpace = NULL,  // sRGB or substitute your own in
};

vImage_CGImageFormat RGB565Format = 
{
    .bitsPerComponent = 5,
    .bitsPerPixel = 16,
    .bitmapInfo = kCGBitmapByteOrder16Little | kCGImageAlphaNone,
    .colorSpace = RGBA8888Format.colorSpace,  
};


err = vImageConverterRef converter = vImageConverter_CreateWithCGImageFormat(
         &RGBA8888Format, &RGB565Format, NULL, kvImageNoFlags, &err );

err = vImageConvert_AnyToAny( converter, &src, &dest, NULL, kvImageNoFlags );

Either of these approaches will be vectorized and multithreaded for best performance.