性能優化篇（5）：NEON優化案例——圖像顏色轉換之RGBA到RGB（aarch64版）
Author:stormQ
Sunday, 8. December 2019 1 12:47AM
實現一個基本的圖像轉換函數
如何將一個大小爲672x376的RGBA圖像轉換成相同大小的RGB圖像呢？
首先實現一個最簡單的轉換函數simple_rgba2rgb()，完整實現如下：
#define SRC_CHANNELS 4
#define DST_CHANNELS 3
void simple_rgba2rgb(const uint8_t *rgba_img, uint8_t *rgb_img,
int height, int width)
{
const int total_pixels = height * width;
for (int i = 0; i < total_pixels; i++)
{
const int src_idx = i * SRC_CHANNELS;
const int dst_idx = i * DST_CHANNELS;
*(rgb_img + dst_idx) = *(rgba_img + src_idx);
*(rgb_img + dst_idx + 1) = *(rgba_img + src_idx + 1);
*(rgb_img + dst_idx + 2) = *(rgba_img + src_idx + 2);
}
}
simple_rgba2rgb()函數將一個二維的圖像看作是一維的，意味着只需要一個循環就可以遍歷所有的像素。這種用法的好處在於，遵循空間局部性原理以高效利用cache從而改善程序性能，而不必關心二維圖像在內存中的存儲順序——是行主序還是列主序。如果用兩個循環的話會比較繁瑣，移植性也不好。另外，由於源圖像和目的圖像的大小相同但所佔的字節數不同，所以採用每次遍歷一個像素的方式，這樣便於處理。
利用 NEON 加速圖像轉換
首先，實現一個基本的利用NEON加速圖像轉換的函數rgba2rgb_with_neon()，完整實現如下：
void rgba2rgb_with_neon(const uint8_t *rgba_img, uint8_t *rgb_img,
int height, int width)
{
const int total_pixels = height * width;
const int stride_pixels = 16;
for (int i = 0; i < total_pixels; i += stride_pixels)
{
const uint8_t *src = rgba_img + i * SRC_CHANNELS;
uint8_t *dst = rgb_img + i * DST_CHANNELS;
uint8x16x4_t a = vld4q_u8(src);
uint8x16x3_t b;
b.val[0] = a.val[0];
b.val[1] = a.val[1];
b.val[2] = a.val[2];
vst3q_u8(dst, b);
}
}
上述函數沒有考慮圖像像素個數非整除時的情形，這個放到後面進行。該函數的實現用到了兩個NEON Intrinsics，分別是vld4q_u8和vst3q_u8，兩者都定義在arm_neon.h文件中。
vld4q_u8的函數原型爲uint8x16x4_t vld4q_u8 (const uint8_t * __a)，作用爲以步長爲 4 交叉地加載數據到四個連續的 128-bit 的向量寄存器。具體地：將內存地址__a、__a+4、…、__a+60處的內容分別賦值給向量寄存器Vn的lane[0]、lane[1]、…、lane[15]，將內存地址__a+1、__a+5、…、__a+61處的內容分別賦值給向量寄存器Vn+1的lane[0]、lane[1]、…、lane[15]，將內存地址__a+2、__a+6、…、__a+62處的內容分別賦值給向量寄存器Vn+2的lane[0]、lane[1]、…、lane[15]，將內存地址__a+3、__a+7、…、__a+63處的內容分別賦值給向量寄存器Vn+3的lane[0]、lane[1]、…、lane[15]。也就是說，vld4q_u8在上述函數中的作用爲：將連續 16 個像素的 R 通道： R0、R1、… 、R15 加載到向量寄存器Vn，將連續 16 個像素的 G 通道： G0、G1、… 、G15 加載到向量寄存器Vn+1，將連續 16 個像素的 B 通道： B0、B1、… 、B15 加載到向量寄存器Vn+2，將連續 16 個像素的 Alpha 通道： A0、A1、… 、A15 加載到向量寄存器Vn+3。
vst3q_u8的函數原型爲void vst3q_u8 (uint8_t * __a, uint8x16x3_t val)，作用爲以步長爲 3 交叉地存儲數據到內存中。
由於每一次迭代可以同時處理 16 個連續的像素。所以，變量stride_pixels的值爲 16。
由於aarch64指令集中沒有ARM 指令集中的VSWP指令。因此，這裏引入一個類型爲uint8x16x3_t的臨時向量b用於存儲R、G和B通道。 g++ 帶優化選項-Og編譯時，語句b.val[0] = a.val[2]; b.val[1] = a.val[1]; b.val[2] = a.val[0];對應三條MOV指令。
接着用語句vst3q_u8(dst, b);將轉換後的結果寫入內存。這樣，一次圖像轉換操作就完成了。
現在考慮圖像像素個數非整除時的情形，完整實現如下：
void rgba2rgb_with_neon(const uint8_t *rgba_img, uint8_t *rgb_img,
int height, int width)
{
const int total_pixels = height * width;
const int stride_pixels = 16;
const int left_pixels = total_pixels % stride_pixels;
const int multiply_pixels = total_pixels - left_pixels;
for (int i = 0; i < multiply_pixels; i += stride_pixels)
{
const uint8_t *src = rgba_img + i * SRC_CHANNELS;
uint8_t *dst = rgb_img + i * DST_CHANNELS;
uint8x16x4_t a = vld4q_u8(src);
uint8x16x3_t b;
b.val[0] = a.val[0];
b.val[1] = a.val[1];
b.val[2] = a.val[2];
vst3q_u8(dst, b);
}
// handling non-multiply array lengths
for (int i = multiply_pixels; i < total_pixels; i++)
{
const int src_idx = i * SRC_CHANNELS;
const int dst_idx = i * DST_CHANNELS;
*(rgb_img + dst_idx) = *(rgba_img + src_idx);
*(rgb_img + dst_idx + 1) = *(rgba_img + src_idx + 1);
*(rgb_img + dst_idx + 2) = *(rgba_img + src_idx + 2);
}
}
爲了驗證rgba2rgb_with_neon()函數的正確性，這裏引入兩個函數：init()和check()。init()函數用於初始化圖像，check()函數用於檢查轉換後的圖像是否正確。完整實現如下：
#define RED 255
#define GREEN 125
#define BLUE 80
#define ALPHA 100
#define CHECK(img, height, width) \
if (check(img, height, width)) \
{ \
std::cout << "Correct:img.\n"; \
} \
else \
{ \
std::cout << "Error:img.\n"; \
}
void init(uint8_t *img, int height, int width)
{
const int total_bytes = height * width * SRC_CHANNELS;
for (int i = 0; i < total_bytes; )
{
*(img + i++) = RED;
*(img + i++) = GREEN;
*(img + i++) = BLUE;
*(img + i++) = ALPHA;
}
}
bool check(const uint8_t *img, int height, int width)
{
const int total_bytes = height * width * DST_CHANNELS;
for (int i = 0; i < total_bytes; i += DST_CHANNELS)
{
if (*(img + i) != RED || *(img + i + 1) != GREEN ||
*(img + i + 2) != BLUE)
{
return false;
}
}
return true;
}
完整的程序爲main.cpp：
#include "arm_neon.h"
#include <iostream>
#define HEIGHT 376
#define WIDTH 672
#define SRC_CHANNELS 4
#define DST_CHANNELS 3
#define RED 255
#define GREEN 125
#define BLUE 80
#define ALPHA 100
#define CHECK(img, height, width) \
if (check(img, height, width)) \
{ \
std::cout << "Correct:img.\n"; \
} \
else \
{ \
std::cout << "Error:img.\n"; \
}
void init(uint8_t *img, int height, int width)
{
const int total_bytes = height * width * SRC_CHANNELS;
for (int i = 0; i < total_bytes; )
{
*(img + i++) = RED;
*(img + i++) = GREEN;
*(img + i++) = BLUE;
*(img + i++) = ALPHA;
}
}
bool check(const uint8_t *img, int height, int width)
{
const int total_bytes = height * width * DST_CHANNELS;
for (int i = 0; i < total_bytes; i += DST_CHANNELS)
{
if (*(img + i) != RED || *(img + i + 1) != GREEN ||
*(img + i + 2) != BLUE)
{
return false;
}
}
return true;
}
void simple_rgba2rgb(const uint8_t *rgba_img, uint8_t *rgb_img,
int height, int width)
{
const int total_pixels = height * width;
for (int i = 0; i < total_pixels; i++)
{
const int src_idx = i * SRC_CHANNELS;
const int dst_idx = i * DST_CHANNELS;
*(rgb_img + dst_idx) = *(rgba_img + src_idx);
*(rgb_img + dst_idx + 1) = *(rgba_img + src_idx + 1);
*(rgb_img + dst_idx + 2) = *(rgba_img + src_idx + 2);
}
}
void rgba2rgb_with_neon(const uint8_t *rgba_img, uint8_t *rgb_img,
int height, int width)
{
const int total_pixels = height * width;
const int stride_pixels = 16;
const int left_pixels = total_pixels % stride_pixels;
const int multiply_pixels = total_pixels - left_pixels;
for (int i = 0; i < multiply_pixels; i += stride_pixels)
{
const uint8_t *src = rgba_img + i * SRC_CHANNELS;
uint8_t *dst = rgb_img + i * DST_CHANNELS;
uint8x16x4_t a = vld4q_u8(src);
uint8x16x3_t b;
b.val[0] = a.val[0];
b.val[1] = a.val[1];
b.val[2] = a.val[2];
vst3q_u8(dst, b);
}
// handling non-multiply array lengths
for (int i = multiply_pixels; i < total_pixels; i++)
{
const int src_idx = i * SRC_CHANNELS;
const int dst_idx = i * DST_CHANNELS;
*(rgb_img + dst_idx) = *(rgba_img + src_idx);
*(rgb_img + dst_idx + 1) = *(rgba_img + src_idx + 1);
*(rgb_img + dst_idx + 2) = *(rgba_img + src_idx + 2);
}
}
uint8_t *rgba_img = nullptr;
uint8_t *rgb_img1 = nullptr;
uint8_t *rgb_img2 = nullptr;
int main()
{
rgba_img = new uint8_t[HEIGHT * WIDTH * SRC_CHANNELS];
rgb_img1 = new uint8_t[HEIGHT * WIDTH * DST_CHANNELS];
rgb_img2 = new uint8_t[HEIGHT * WIDTH * DST_CHANNELS];
init(rgba_img, HEIGHT, WIDTH);
simple_rgba2rgb(rgba_img, rgb_img1, HEIGHT, WIDTH);
rgba2rgb_with_neon(rgba_img, rgb_img2, HEIGHT, WIDTH);
CHECK(rgb_img1, HEIGHT, WIDTH);
CHECK(rgb_img2, HEIGHT, WIDTH);
delete [] rgb_img2;
delete [] rgb_img1;
delete [] rgba_img;
return 0;
}
編譯程序（On Jetson TX2）：
g++ -std=c++11 -g -Og -o main_Og main.cpp
| 啓動程序方式 | 第一次執行耗時(us) | 第二次執行耗時(us) | 第三次執行耗時(us) | 第四次執行耗時(us) | 第五次執行耗時(us) |
|---|---|---|---|---|---|
| ./main_Og |
從統計結果中可以看出，rgba2rgb_with_neon()函數的執行速度比simple_rgba2rgb()函數快 2 到 3 倍左右。
那麼 NEON 版的實現爲什麼能達到這樣一個加速呢？ 下面從 cache 的角度進行分析。
統計 cache 性能數據：
--------------------------------------------------------------------------------
I1 cache: 16384 B, 64 B, 4-way associative
D1 cache: 16384 B, 64 B, 4-way associative
LL cache: 262144 B, 64 B, 8-way associative
Command: ./main_Og
Data file: cachegrind.out.26634
Events recorded: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
Events shown: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
Event sort order: Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
Thresholds: 0.1 100 100 100 100 100 100 100 100
Include dirs:
User annotated: main.cpp
Auto-annotation: on
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
--------------------------------------------------------------------------------
18,330,632 1,581 1,388 2,833,111 71,198 64,190 2,014,469 42,002 41,005 PROGRAM TOTALS
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw file:function
--------------------------------------------------------------------------------
8,085,518 0 0 1,516,032 23,690 23,690 0 0 0 /home/test2/main.cpp:check(unsigned char const*, int, int)
4,548,101 3 3 758,016 15,793 15,793 758,016 11,845 11,845 /home/test2/main.cpp:simple_rgba2rgb(unsigned char const*, unsigned char*, int, int)
3,790,086 2 2 0 0 0 1,010,688 15,792 15,792 /home/test2/main.cpp:init(unsigned char*, int, int)
545,429 12 11 148,897 2,038 1,708 50,257 29 16 /build/glibc-BinVK7/glibc-2.23/elf/dl-lookup.c:_dl_lookup_symbol_x
537,013 36 36 191,329 5,065 1,024 93,645 43 24 /build/glibc-BinVK7/glibc-2.23/elf/dl-lookup.c:do_lookup_x
211,319 28 28 48,062 3,377 2,954 20,608 1,748 886 /build/glibc-BinVK7/glibc-2.23/elf/../sysdeps/aarch64/dl-machine.h:_dl_relocate_object
126,348 0 0 0 0 0 0 0 0 /home/test2/main.cpp:rgba2rgb_with_neon(unsigned char const*, unsigned char*, int, int)
108,209 8 6 37,471 330 238 14,588 25 11 /build/glibc-BinVK7/glibc-2.23/elf/dl-lookup.c:check_match
97,629 5 5 12,840 221 96 0 0 0 /build/glibc-BinVK7/glibc-2.23/string/../sysdeps/aarch64/strcmp.S:strcmp
78,960 1 1 63,168 15,793 15,793 47,376 11,845 11,845 /usr/lib/gcc/aarch64-linux-gnu/5/include/arm_neon.h:rgba2rgb_with_neon(unsigned char const*, unsigned char*, int, int)
56,685 3 3 16,081 2,184 1,017 3,209 2 0 /build/glibc-BinVK7/glibc-2.23/elf/do-rel.h:_dl_relocate_object
44,112 8 8 13,412 1,022 986 12 1 0 /build/glibc-BinVK7/glibc-2.23/elf/dl-addr.c:_dl_addr
--------------------------------------------------------------------------------
-- User-annotated source: main.cpp
--------------------------------------------------------------------------------
No information has been collected for main.cpp
--------------------------------------------------------------------------------
-- Auto-annotated source: /usr/lib/gcc/aarch64-linux-gnu/5/include/arm_neon.h
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
-- line 16574 ----------------------------------------
. . . . . . . . . return ret;
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . __extension__ static __inline uint8x16x4_t __attribute__ ((__always_inline__))
. . . . . . . . . vld4q_u8 (const uint8_t * __a)
. . . . . . . . . {
. . . . . . . . . uint8x16x4_t ret;
. . . . . . . . . __builtin_aarch64_simd_xi __o;
15,792 0 0 63,168 15,793 15,793 0 0 0 __o = __builtin_aarch64_ld4v16qi ((const __builtin_aarch64_simd_qi *) __a);
. . . . . . . . . ret.val[0] = (uint8x16_t) __builtin_aarch64_get_qregxiv16qi (__o, 0);
. . . . . . . . . ret.val[1] = (uint8x16_t) __builtin_aarch64_get_qregxiv16qi (__o, 1);
. . . . . . . . . ret.val[2] = (uint8x16_t) __builtin_aarch64_get_qregxiv16qi (__o, 2);
. . . . . . . . . ret.val[3] = (uint8x16_t) __builtin_aarch64_get_qregxiv16qi (__o, 3);
. . . . . . . . . return ret;
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . __extension__ static __inline uint16x8x4_t __attribute__ ((__always_inline__))
-- line 16590 ----------------------------------------
-- line 23620 ----------------------------------------
. . . . . . . . . __o = __builtin_aarch64_set_qregciv2di (__o, (int64x2_t) val.val[2], 2);
. . . . . . . . . __builtin_aarch64_st3v2di ((__builtin_aarch64_simd_di *) __a, __o);
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . __extension__ static __inline void __attribute__ ((__always_inline__))
. . . . . . . . . vst3q_u8 (uint8_t * __a, uint8x16x3_t val)
. . . . . . . . . {
. . . . . . . . . __builtin_aarch64_simd_ci __o;
15,792 0 0 0 0 0 0 0 0 __o = __builtin_aarch64_set_qregciv16qi (__o, (int8x16_t) val.val[0], 0);
15,792 1 1 0 0 0 0 0 0 __o = __builtin_aarch64_set_qregciv16qi (__o, (int8x16_t) val.val[1], 1);
15,792 0 0 0 0 0 0 0 0 __o = __builtin_aarch64_set_qregciv16qi (__o, (int8x16_t) val.val[2], 2);
15,792 0 0 0 0 0 47,376 11,845 11,845 __builtin_aarch64_st3v16qi ((__builtin_aarch64_simd_qi *) __a, __o);
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . __extension__ static __inline void __attribute__ ((__always_inline__))
. . . . . . . . . vst3q_u16 (uint16_t * __a, uint16x8x3_t val)
. . . . . . . . . {
. . . . . . . . . __builtin_aarch64_simd_ci __o;
. . . . . . . . . __o = __builtin_aarch64_set_qregciv8hi (__o, (int16x8_t) val.val[0], 0);
. . . . . . . . . __o = __builtin_aarch64_set_qregciv8hi (__o, (int16x8_t) val.val[1], 1);
-- line 23639 ----------------------------------------
--------------------------------------------------------------------------------
-- Auto-annotated source: /home/test2/main.cpp
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
-- line 22 ----------------------------------------
. . . . . . . . . } \
. . . . . . . . . else \
. . . . . . . . . { \
. . . . . . . . . std::cout << "Error:img.\n"; \
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . void init(uint8_t *img, int height, int width)
. . . . . . . . . {
2 0 0 0 0 0 0 0 0 const int total_bytes = height * width * SRC_CHANNELS;
758,020 2 2 0 0 0 0 0 0 for (int i = 0; i < total_bytes; )
. . . . . . . . . {
758,016 0 0 0 0 0 252,672 15,792 15,792 *(img + i++) = RED;
758,016 0 0 0 0 0 252,672 0 0 *(img + i++) = GREEN;
758,016 0 0 0 0 0 252,672 0 0 *(img + i++) = BLUE;
758,016 0 0 0 0 0 252,672 0 0 *(img + i++) = ALPHA;
. . . . . . . . . }
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . bool check(const uint8_t *img, int height, int width)
. . . . . . . . . {
4 0 0 0 0 0 0 0 0 const int total_bytes = height * width * DST_CHANNELS;
2,021,382 0 0 0 0 0 0 0 0 for (int i = 0; i < total_bytes; i += DST_CHANNELS)
. . . . . . . . . {
5,053,440 0 0 1,010,688 15,794 15,794 0 0 0 if (*(img + i) != RED || *(img + i + 1) != GREEN ||
1,010,688 0 0 505,344 7,896 7,896 0 0 0 *(img + i + 2) != BLUE)
. . . . . . . . . {
. . . . . . . . . return false;
. . . . . . . . . }
. . . . . . . . . }
4 0 0 0 0 0 0 0 0 return true;
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . void simple_rgba2rgb(const uint8_t *rgba_img, uint8_t *rgb_img,
. . . . . . . . . int height, int width)
. . . . . . . . . {
1 1 1 0 0 0 0 0 0 const int total_pixels = height * width;
1,010,692 1 1 0 0 0 0 0 0 for (int i = 0; i < total_pixels; i++)
. . . . . . . . . {
252,672 1 1 0 0 0 0 0 0 const int src_idx = i * SRC_CHANNELS;
252,672 0 0 0 0 0 0 0 0 const int dst_idx = i * DST_CHANNELS;
1,010,688 0 0 252,672 15,793 15,793 252,672 3,949 3,949 *(rgb_img + dst_idx) = *(rgba_img + src_idx);
1,010,688 0 0 252,672 0 0 252,672 3,948 3,948 *(rgb_img + dst_idx + 1) = *(rgba_img + src_idx + 1);
1,010,688 0 0 252,672 0 0 252,672 3,948 3,948 *(rgb_img + dst_idx + 2) = *(rgba_img + src_idx + 2);
. . . . . . . . . }
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . void rgba2rgb_with_neon(const uint8_t *rgba_img, uint8_t *rgb_img,
. . . . . . . . . int height, int width)
. . . . . . . . . {
1 0 0 0 0 0 0 0 0 const int total_pixels = height * width;
. . . . . . . . . const int stride_pixels = 16;
4 0 0 0 0 0 0 0 0 const int left_pixels = total_pixels % stride_pixels;
1 0 0 0 0 0 0 0 0 const int multiply_pixels = total_pixels - left_pixels;
. . . . . . . . .
63,171 0 0 0 0 0 0 0 0 for (int i = 0; i < multiply_pixels; i += stride_pixels)
. . . . . . . . . {
31,584 0 0 0 0 0 0 0 0 const uint8_t *src = rgba_img + i * SRC_CHANNELS;
31,584 0 0 0 0 0 0 0 0 uint8_t *dst = rgb_img + i * DST_CHANNELS;
. . . . . . . . .
. . . . . . . . . uint8x16x4_t a = vld4q_u8(src);
. . . . . . . . . uint8x16x3_t b;
. . . . . . . . . b.val[0] = a.val[0];
. . . . . . . . . b.val[1] = a.val[1];
. . . . . . . . . b.val[2] = a.val[2];
. . . . . . . . . vst3q_u8(dst, b);
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . // handling non-multiply array lengths
3 0 0 0 0 0 0 0 0 for (int i = multiply_pixels; i < total_pixels; i++)
. . . . . . . . . {
. . . . . . . . . const int src_idx = i * SRC_CHANNELS;
. . . . . . . . . const int dst_idx = i * DST_CHANNELS;
. . . . . . . . . *(rgb_img + dst_idx) = *(rgba_img + src_idx);
. . . . . . . . . *(rgb_img + dst_idx + 1) = *(rgba_img + src_idx + 1);
. . . . . . . . . *(rgb_img + dst_idx + 2) = *(rgba_img + src_idx + 2);
. . . . . . . . . }
. . . . . . . . . }
. . . . . . . . .
. . . . . . . . . uint8_t *rgba_img = nullptr;
. . . . . . . . . uint8_t *rgb_img1 = nullptr;
. . . . . . . . . uint8_t *rgb_img2 = nullptr;
. . . . . . . . .
. . . . . . . . . int main()
3 1 1 0 0 0 4 0 0 {
6 1 1 0 0 0 1 1 0 rgba_img = new uint8_t[HEIGHT * WIDTH * SRC_CHANNELS];
5 0 0 0 0 0 1 1 1 rgb_img1 = new uint8_t[HEIGHT * WIDTH * DST_CHANNELS];
3 0 0 0 0 0 1 0 0 rgb_img2 = new uint8_t[HEIGHT * WIDTH * DST_CHANNELS];
. . . . . . . . .
4 0 0 1 0 0 0 0 0 init(rgba_img, HEIGHT, WIDTH);
. . . . . . . . .
5 1 1 2 2 2 0 0 0 simple_rgba2rgb(rgba_img, rgb_img1, HEIGHT, WIDTH);
5 0 0 2 2 2 0 0 0 rgba2rgb_with_neon(rgba_img, rgb_img2, HEIGHT, WIDTH);
. . . . . . . . .
12 1 1 1 1 1 0 0 0 CHECK(rgb_img1, HEIGHT, WIDTH);
14 1 1 1 1 1 0 0 0 CHECK(rgb_img2, HEIGHT, WIDTH);
. . . . . . . . .
5 0 0 1 1 1 0 0 0 delete [] rgb_img2;
5 1 1 1 0 0 0 0 0 delete [] rgb_img1;
5 0 0 1 0 0 0 0 0 delete [] rgba_img;
. . . . . . . . . return 0;
22 3 2 9 0 0 5 0 0 }
-- line 123 ----------------------------------------
--------------------------------------------------------------------------------
The following files chosen for auto-annotation could not be found:
--------------------------------------------------------------------------------
/build/glibc-BinVK7/glibc-2.23/elf/do-rel.h
/build/glibc-BinVK7/glibc-2.23/elf/dl-lookup.c
/build/glibc-BinVK7/glibc-2.23/string/../sysdeps/aarch64/strcmp.S
/build/glibc-BinVK7/glibc-2.23/elf/dl-addr.c
/build/glibc-BinVK7/glibc-2.23/elf/../sysdeps/aarch64/dl-machine.h
--------------------------------------------------------------------------------
Ir I1mr ILmr Dr D1mr DLmr Dw D1mw DLmw
--------------------------------------------------------------------------------
91 1 1 82 78 86 90 94 96 percentage of events annotated
分析統計結果：
| 函數名稱 | 內存讀操作數量(Dr列) | 一級數據緩存讀不命中次數(D1mr列) | 最後一級數據緩存讀不命中次數(DLmr列) | 內存寫操作數量(Dw列) | 一級數據緩存寫不命中次數(D1mw列) | 最後一級數據緩存寫不命中次數(DLmw列) |
|---|---|---|---|---|---|---|
| simple_rgba2rgb | 758,016 | 15,793 | 15,793 | 758,016 | 11,845 | 11,845 |
| rgba2rgb_with_neon | 63,168 | 15,793 | 15,793 | 47,376 | 11,845 | 11,845 |
可以看出，rgba2rgb_with_neon函數的內存讀操作數量和內存寫操作數量分別爲simple_rgba2rgb函數的 0.0833 倍、0.0625 倍，其他的都是相同的。因此，從 cache 的角度來看，NEON 大大減少了內存讀寫操作數量，從而改善程序性能。
最後，能否降低 cache 不命中率從而進一步改善程序性能呢？…
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