我有一个大数组（大约 1 MB）
要么这是一个错字，要么你的目标严重老化，要么这个压缩操作在你的应用程序的关键路径中被重复调用。
任何关于如何使其更高效或更快（希望保持可读性）的代码片段或建议都会非常有帮助。
通常，您会通过经验性地测量性能和检查生成的代码来找到最佳信息。使用分析器来确定正在执行的代码、缓存未命中和管道停顿的位置——这些可以帮助您调整算法。
例如，您选择了 4 个元素的步幅。这仅仅是因为您将四个输入元素映射到一个字节吗？您可以使用本机 SIMD 指令/内在函数一次对更多元素进行操作吗？
另外，你是如何为你的目标编译的，你的编译器对你的代码的优化能力如何？
让我们问clang一下它是否在尝试优化您的代码时发现了任何问题：
$ clang -fvectorize -O3 -Rpass-missed=licm -c tryme.c
tryme.c:11:28: remark: failed to move load with loop-invariant address because the loop may invalidate its value [-Rpass-missed=licm]
temp[small_loop] = *in; // Load into local variable
^
tryme.c:21:25: remark: failed to move load with loop-invariant address because the loop may invalidate its value [-Rpass-missed=licm]
*out = (uint8_t)((temp[0] & 0x03) << 6) |
^
tryme.c:22:25: remark: failed to move load with loop-invariant address because the loop may invalidate its value [-Rpass-missed=licm]
((temp[1] & 0x03) << 4) |
^
tryme.c:23:25: remark: failed to move load with loop-invariant address because the loop may invalidate its value [-Rpass-missed=licm]
((temp[2] & 0x03) << 2) |
^
tryme.c:24:25: remark: failed to move load with loop-invariant address because the loop may invalidate its value [-Rpass-missed=licm]
((temp[3] & 0x03));
^
我不确定，但也许别名分析是让它认为它无法移动这个负载的原因。试试看__restrict__有没有效果。
$ clang -fvectorize -O3 -Rpass-analysis=loop-vectorize -c tryme.c
tryme.c:13:13: remark: loop not vectorized: loop contains a switch statement [-Rpass-analysis=loop-vectorize]
if (temp[small_loop] == 3) // 3's are discarded
除非你改变你的算法，否则我想不出任何明显的事情可以解决这个问题。如果在不删除 s 的情况下压缩比令人满意3，您也许可以消除它。
那么生成的代码是什么样的呢？看看下面。你怎么能更好地手写呢？如果您自己可以更好地编写它，要么这样做，要么将其反馈到您的算法中以帮助指导编译器。
编译后的代码是否利用了目标的指令集和寄存器？
最重要的是——尝试执行它，看看你在哪里花费了最多的周期。因分支错误预测、未对齐负载而导致的停顿？也许你可以对这些做点什么。使用您对输入数据频率的了解为编译器提供有关编码器分支的提示。
$ objdump -d --source tryme.o
...
0000000000000000 <compressByPacking>:
#include <stdint.h>
void compressByPacking (uint8_t* out, uint8_t* in, uint32_t length)
{
for (int loop = 0; loop < length/4; loop ++, in += 4, out++)
0: c1 ea 02 shr $0x2,%edx
3: 0f 84 86 00 00 00 je 8f <compressByPacking+0x8f>
9: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
{
uint8_t temp[4];
for (int small_loop = 0; small_loop < 4; small_loop++)
{
temp[small_loop] = *in; // Load into local variable
10: 8a 06 mov (%rsi),%al
if (temp[small_loop] == 3) // 3's are discarded
12: 3c 04 cmp $0x4,%al
14: 74 3a je 50 <compressByPacking+0x50>
16: 3c 03 cmp $0x3,%al
18: 41 88 c0 mov %al,%r8b
1b: 75 03 jne 20 <compressByPacking+0x20>
1d: 45 31 c0 xor %r8d,%r8d
20: 3c 04 cmp $0x4,%al
22: 74 33 je 57 <compressByPacking+0x57>
24: 3c 03 cmp $0x3,%al
26: 88 c1 mov %al,%cl
28: 75 02 jne 2c <compressByPacking+0x2c>
2a: 31 c9 xor %ecx,%ecx
2c: 3c 04 cmp $0x4,%al
2e: 74 2d je 5d <compressByPacking+0x5d>
30: 3c 03 cmp $0x3,%al
32: 41 88 c1 mov %al,%r9b
35: 75 03 jne 3a <compressByPacking+0x3a>
37: 45 31 c9 xor %r9d,%r9d
3a: 3c 04 cmp $0x4,%al
3c: 74 26 je 64 <compressByPacking+0x64>
3e: 3c 03 cmp $0x3,%al
40: 75 24 jne 66 <compressByPacking+0x66>
42: 31 c0 xor %eax,%eax
44: eb 20 jmp 66 <compressByPacking+0x66>
46: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
4d: 00 00 00
50: 41 b0 03 mov $0x3,%r8b
53: 3c 04 cmp $0x4,%al
55: 75 cd jne 24 <compressByPacking+0x24>
57: b1 03 mov $0x3,%cl
59: 3c 04 cmp $0x4,%al
5b: 75 d3 jne 30 <compressByPacking+0x30>
5d: 41 b1 03 mov $0x3,%r9b
60: 3c 04 cmp $0x4,%al
62: 75 da jne 3e <compressByPacking+0x3e>
64: b0 03 mov $0x3,%al
temp[small_loop] = 3;
} // end small loop
// Pack the bits into write pointer
*out = (uint8_t)((temp[0] & 0x03) << 6) |
66: 41 c0 e0 06 shl $0x6,%r8b
((temp[1] & 0x03) << 4) |
6a: c0 e1 04 shl $0x4,%cl
6d: 80 e1 30 and $0x30,%cl
temp[small_loop] = 3;
} // end small loop
// Pack the bits into write pointer
*out = (uint8_t)((temp[0] & 0x03) << 6) |
70: 44 08 c1 or %r8b,%cl
((temp[1] & 0x03) << 4) |
((temp[2] & 0x03) << 2) |
73: 41 c0 e1 02 shl $0x2,%r9b
77: 41 80 e1 0c and $0xc,%r9b
((temp[3] & 0x03));
7b: 24 03 and $0x3,%al
} // end small loop
// Pack the bits into write pointer
*out = (uint8_t)((temp[0] & 0x03) << 6) |
((temp[1] & 0x03) << 4) |
7d: 44 08 c8 or %r9b,%al
((temp[2] & 0x03) << 2) |
80: 08 c8 or %cl,%al
temp[small_loop] = 3;
} // end small loop
// Pack the bits into write pointer
*out = (uint8_t)((temp[0] & 0x03) << 6) |
82: 88 07 mov %al,(%rdi)
#include <stdint.h>
void compressByPacking (uint8_t* out, uint8_t* in, uint32_t length)
{
for (int loop = 0; loop < length/4; loop ++, in += 4, out++)
84: 48 83 c6 04 add $0x4,%rsi
88: 48 ff c7 inc %rdi
8b: ff ca dec %edx
8d: 75 81 jne 10 <compressByPacking+0x10>
((temp[1] & 0x03) << 4) |
((temp[2] & 0x03) << 2) |
((temp[3] & 0x03));
} // end loop
}
8f: c3 retq