Optimizing RGBA8888 to RGB565 conversion with NEON

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®;
//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

[To see links please register here]

I would appreciate any help, thanks!

You might want to use vld4q_u8() instead of vld4_u8() and adjust the rest of your code accordingly. It's hard to tell where the problem might be, but the assembler doesn't look too bad otherwise.

(I'm not familiar with NEON, nor deeply with the memory system of the Ipad2, but this is what we used to do with 88110 pixel-ops, which were an early precursor to today's SIMD extensions)

How big is the memory latency?

Could you hide it by unrolling the inner loop and running the NEON instructions on the "previous" values while the ARM pulls the "next" values from memory? A brief scan of the NEON manual implies you can run ARM and NEON instructions in parallel.

I don't think converting vld4_u8 to vld4q_u8 would lead to any bettering of the performance.

The code seems simple enough. I am not good at ASM and so it would take some time to look into it deeply.

The neon seems simple enough. But I am not quiet sure about **r5_g6_b5 |= g16** being used instead of **vorrq_u16**

Please have a look at the optimization level too. As far as what I heard neon code optimization level goes to a maximum of 1. So the performance may differ when default optimization is being taken into account for both the reference code and neon code, as the level of optimization of reference by DEFAULT may be different.

I doesnt find any area in neon that can better the current code.

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

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.