25. Integer SIMD
13장에서 VFPv2 와 floating point number를 vector operation 할 수 있음을 확인했다. integer number도 벡터연산이 가능한가? 이장에서는 그것을 확인한다.
SIMD
SIMD는 문자 그대로 Single Instruction 으로 한번에 Multible Data를 처리한다는 의미이다.
13 #14장에서
참고: NEON
Arm NEON technology is an advanced SIMD (single instruction multiple data) architecture extension for the Arm Cortex-A series and Cortex-R52 processors.
NEON technology was introduced to the Armv7-A and Armv7-R profiles. It is also now an extension to the Armv8-A and Armv8-R profiles.
NEON technology is intended to improve the multimedia user experience by accelerating audio and video encoding/decoding, user interface, 2D/3D graphics or gaming. NEON can also accelerate signal processing algorithms and functions to speed up applications such as audio and video processing, voice and facial recognition, computer vision and deep learning.
Motivating example
이러한 기능이 왜 필요한지 다음의 예제로 살펴보자. 먼저 16bit PCM audio signal 이 샘플링 되어있다고 하자. 그리고 다음의 코드처럼 그 중 두개의 signal의 평균을 구한다고 해보자.
short int channel1[num_samples]; // in our environment a 'short int' is a half-word
short int channel2[num_samples];
short int channel_out[num_samples];
for (i = 0; i < num_samples; i++)
{
channel_out[i] = (channel1[i] + channel2[i]) / 2;
}
short int 즉 2byte 사용 예제임을 주의
그리고 assembly 명령어로는 다음과 같이 번역될 수 있다.
naive_channel_mixing:
/* r0 contains the base address of channel1 */
/* r1 contains the base address of channel2 */
/* r2 contains the base address of channel_out */
/* r3 is the number of samples */
/* r4 is the number of the current sample
so it holds that 0 ← r4 < r3 */
mov r4, #0 /* r4 ← 0 */
b .Lcheck_loop /* branch to check_loop */
.Lloop:
mov r5, r4, LSL #1 /* r5 ← r4 << 1 (this is r5 ← r4 * 2) */
/* a halfword takes two bytes, so multiply
the index by two. We do this here because
ldrsh does not allow an addressing mode
like [r0, r5, LSL #1] */
ldrsh r6, [r0, r5] /* r6 ← *{signed half}(r0 + r5) */
ldrsh r7, [r1, r5] /* r7 ← *{signed half}(r1 + r5) */
add r8, r6, r7 /* r8 ← r6 + r7 */
mov r8, r8, ASR #1 /* r8 ← r8 >> 1 (this is r8 ← r8 / 2)*/
strh r8, [r2, r5] /* *{half}(r2 + r5) ← r8 */
add r4, r4, #1 /* r4 ← r4 + 1 */
.Lcheck_loop:
cmp r4, r3 /* compute r4 - r3 and update cpsr */
blt .Lloop /* if r4 < r3 jump to the
beginning of the loop */
이 코드는 우리가 익숙해서 좋지만, 다음 장 처럼 병렬연산으로 개선 해야한다.
참고,sh? signed half, 2byte를 사용한다. 21장 Subword data참고
병렬 연산 (더하기, 빼기)
통째로 더하거나 빼기 위해서는 아래와 같은 명령을 사용한다.
- Halfwords
- Signed: sadd16, ssub16
- Unsigned: uadd16, usub16
- Bytes
- Signed: sadd8, ssub8
- Unsigned: uadd8, usub8
다음의 C 코드가 있다고 하자.
// unsigned char is an unsigned byte in our environment
// a, b and c are arrays of N unsigned chars
unsigned char a[N], b[N], c[N];
int i;
for (i = 0; i < N; i++)
{
c[i] = a[i] + b[i];
}
이것을 순수하게 loop를 이용해 더하면 아래와 같다.
naive_byte_array_addition:
/* r0 contains the base address of a */
/* r1 contains the base address of b */
/* r2 contains the base address of c */
/* r3 is N */
/* r4 is the number of the current item
so it holds that 0 ← r4 < r3 */
mov r4, #0 /* r4 ← 0 */
b .Lcheck_loop0 /* branch to check_loop0 */
.Lloop0:
ldrb r5, [r0, r4] /* r5 ← *{unsigned byte}(r0 + r4) */
ldrb r6, [r1, r4] /* r6 ← *{unsigned byte}(r1 + r4) */
add r7, r5, r6 /* r7 ← r5 + r6 */
strb r7, [r2, r4] /* *{unsigned byte}(r2 + r4) ← r7 */
add r4, r4, #1 /* r4 ← r4 + 1 */
^^^^
.Lcheck_loop0:
cmp r4, r3 /* perform r4 - r3 and update cpsr */
blt .Lloop0 /* if cpsr means that r4 < r3 jump to loop0 */
sadd8 을 이용하기
simd_byte_array_addition_0:
/* r0 contains the base address of a */
/* r1 contains the base address of b */
/* r2 contains the base address of c */
/* r3 is N */
/* r4 is the number of the current item
so it holds that 0 ← r4 < r3 */
mov r4, #0 /* r4 ← 0 */
b .Lcheck_loop1 /* branch to check_loop1 */
.Lloop1:
ldr r5, [r0, r4] /* r5 ← *(r0 + r4) */
ldr r6, [r1, r4] /* r6 ← *(r1 + r4) */
sadd8 r7, r5, r6 /* r7[7:0] ← r5[7:0] + r6[7:0] */
^^^^^^^^^^^^^^^^ /* r7[15:8] ← r5[15:8] + r6[15:8] */
/* r7[23:16] ← r5[23:16] + r6[23:16] */
/* r7[31:24] ← r5[31:24] + r6[31:24] */
/* rA[x:y] means bits x to y of the register rA */
str r7, [r2, r4] /* *(r2 + r4) ← r7 */
add r4, r4, #4 /* r4 ← r4 + 4 */
^^^^
.Lcheck_loop1:
cmp r4, r3 /* perform r4 - r3 and update cpsr */
blt .Lloop1 /* if cpsr means that r4 < r3 jump to loop1 */
sadd8 을 이용하면 4byte씩 한번에 더한다.
하지만 이 예제는 N이 4의 배수일때만 사용된다. N이 4의 배수가 아닌경우는 다음과 같이한다.
simd_byte_array_addition_2:
/* r0 contains the base address of a */
/* r1 contains the base address of b */
/* r2 contains the base address of c */
/* r3 is N */
/* r4 is the number of the current item
so it holds that 0 ← r4 < r3 */
mov r4, #0 /* r4 ← 0 */
sub r8, r3, #3 /* r8 ← r3 - 3
^^^^^^^^^^^^^^ this is r8 ← N - 3 */
b .Lcheck_loop2 /* branch to check_loop2 */
.Lloop2:
ldr r5, [r0, r4] /* r5 ← *(r0 + r4) */
ldr r6, [r1, r4] /* r6 ← *(r1 + r4) */
sadd8 r7, r5, r6 /* r7[7:0] ← r5[7:0] + r6[7:0] */
/* r7[15:8] ← r5[15:8] + r6[15:8] */
/* r7[23:16] ← r5[23:16] + r6[23:16] */
/* r7[31:24] ← r5[31:24] + r6[31:24] */
str r7, [r2, r4] /* *(r2 + r4) ← r7 */
add r4, r4, #4 /* r4 ← r4 + 4 */
.Lcheck_loop2:
cmp r4, r8 /* perform r4 - r8 and update cpsr */
blt .Lloop2 /* if cpsr means that r4 < r8 jump to loop2 */
/* i.e. if r4 < N - 3 jump to loop2 */
N - 3으로 비교한뒤 나머지 값들은 다음 코드에서 byte단위로 더해서 마무리한다.
/* epilog loop */
b .Lcheck_loop3 /* branch to check_loop3 */
.Lloop3:
ldrb r5, [r0, r4] /* r5 ← *{unsigned byte}(r0 + r4) */
ldrb r6, [r1, r4] /* r6 ← *{unsigned byte}(r1 + r4) */
add r7, r5, r6 /* r7 ← r5 + r6 */
strb r7, [r2, r4] /* *{unsigned byte}(r2 + r4) ← r7 */
add r4, r4, #1 /* r4 ← r4 + 1 */
.Lcheck_loop3:
cmp r4, r3 /* perform r4 - r3 and update cpsr */
blt .Lloop3 /* if cpsr means that r4 < r3 jump to loop 3 */
Halving instructions
half word 에 대해서도 다음의 연산이 존재한다.
- Halfwords
- Signed: shadd16, shsub16
- Unsigned: uhadd16, uhsub16
- Bytes
- Signed: shadd8, shsub8
- Unsigned: uhadd8, uhsub8
위의 motivation example은 다음과 같이 번역될 수 있다. 이 방식은 loop종료 이후 추가 연산이 필요없다.
better_channel_mixing:
/* r0 contains the base address of channel1 */
/* r1 contains the base address of channel2 */
/* r2 contains the base address of channel_out */
/* r3 is the number of samples */
/* r4 is the number of the current sample
so it holds that 0 ← r4 < r3 */
mov r4, #0 /* r4 ← 0 */
b .Lcheck_loop /* branch to check_loop */
.Lloop:
ldr r6, [r0, r4] /* r6 ← *(r0 + r4) */
ldr r7, [r1, r4] /* r7 ← *(r1 + r4) */
shadd16 r8, r6, r7 /* r8[15:0] ← (r6[15:0] + r7[15:0]) >> 1*/
/* r8[31:16] ← (r6[31:16] + r7[31:16]) >> 1*/
str r8, [r2, r4] /* *(r2 + r4) ← r8 */
add r4, r4, #2 /* r4 ← r4 + 2 */
.Lcheck_loop:
cmp r4, r3 /* compute r4 - r3 and update cpsr */
blt .Lloop /* if r4 < r3 jump to the
beginning of the loop */
Saturating arithmetic
만약 2byte에서 더한값이 overflow된다면? 그것을 확인하는 동작이 필요하다. 아래와 같은 함수를 이용할 수 있다.
.data
max16bit: .word 32767
.text
clipped_add16bit:
/* first operand is in r0 */
/* second operand is in r1 */
/* result is left in r0 */
push {r4, lr} /* keep registers */
ldr r4, addr_of_max16bit /* r4 ← &max16bit */
ldr r4, [r4] /* r4 ← *r4 */
/* now r4 == 32767 (i.e. 2^15 - 1) */
add r0, r0, r1 /* r0 ← r0 + r1 */
cmp r0, r4 /* perform r0 - r4 and update cpsr */
movgt r0, r4 /* if r0 > r4 then r0 ← r4 */
bgt end /* if r0 > r4 then branch to end */
mvn r4, r4 /* r4 ← ~r4
now r4 == -32768 (i.e. -2^15) */
cmp r0, r4 /* perform r0 - r4 and update cpsr */
movlt r0, r4 /* if r0 < r4 then r0 ← r4 */
end:
pop {r4, lr} /* restore registers */
bx lr /* return */
addr_of_max16bit: .word max16bit
기존의 명령어로 처리하기 위해서는 상당히 많은 라인이 필요하지만 다행이도 Saturated arithmetics 명령어가 이를 해결해준다.
- Halfwords
- Signed: qadd16, qsub16
- Unsigned: uqadd16, uqsub16
- Bytes
- Signed: qadd8, qsub8
- Unsigned: uqadd8, uqsub8
more_realistic_channel_mixing:
/* r0 contains the base address of channel1 */
/* r1 contains the base address of channel2 */
/* r2 contains the base address of channel_out */
/* r3 is the number of samples */
/* r4 is the number of the current sample
so it holds that 0 ← r4 < r3 */
mov r4, #0 /* r4 ← 0 */
b .Lcheck_loop /* branch to check_loop */
.Lloop:
ldr r6, [r0, r4] /* r6 ← *(r0 + r4) */
ldr r7, [r1, r4] /* r7 ← *(r1 + r4) */
qadd16 r8, r6, r7 /* r8[15:0] ← saturated_sum_16(r6[15:0], r7[15:0]) */
/* r8[31:16] ← saturated_sum_16(r6[31:16], r7[31:16]) */
str r8, [r2, r4] /* *(r2 + r4) ← r8 */
add r4, r4, #2 /* r4 ← r4 + 2 */
.Lcheck_loop:
cmp r4, r3 /* compute r4 - r3 and update cpsr */
blt .Lloop /* if r4 < r3 jump to the
beginning of the loop */