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GCC编译器的优化: fprefetch-loop-arrays

以MIPS架构为基础进行分析
预取指令:pref

指令预取,是指提前将所需要的数据取出来,在使用时可用

for loop

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volatile unsigned char aa[4096];

int func()
{
int i;

__asm__ __volatile__("ssnop\n\t");
__asm__ __volatile__("ssnop\n\t");

for(i = 0; i < 4096; i++) {
aa[i] = i;
}

__asm__ __volatile__("ssnop\n\t");
__asm__ __volatile__("ssnop\n\t");
return 0;
}

编译:

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>mips-linux-gnu-gcc -c for_loop.c -o for_loop.o
>

反汇编:

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mips-linux-gnu-objdump -d for_loop.o > for_loop.s
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00000000 <func>:
0: 27bdffe8 addiu sp,sp,-24
4: afbe0014 sw s8,20(sp)
8: 03a0f025 move s8,sp
c: 00000040 ssnop
10: 00000040 ssnop
14: afc00008 sw zero,8(s8)
18: 1000000b b 48 <func+0x48>
1c: 00000000 nop
20: 8fc20008 lw v0,8(s8)
24: 304300ff andi v1,v0,0xff
28: 3c020000 lui v0,0x0
2c: 24440000 addiu a0,v0,0
30: 8fc20008 lw v0,8(s8)
34: 00821021 addu v0,a0,v0
38: a0430000 sb v1,0(v0)
3c: 8fc20008 lw v0,8(s8)
40: 24420001 addiu v0,v0,1
44: afc20008 sw v0,8(s8)
48: 8fc20008 lw v0,8(s8)
4c: 28421000 slti v0,v0,4096
50: 1440fff3 bnez v0,20 <func+0x20>
54: 00000000 nop
58: 00000040 ssnop
5c: 00000040 ssnop
60: 00001025 move v0,zero
64: 03c0e825 move sp,s8
68: 8fbe0014 lw s8,20(sp)
6c: 27bd0018 addiu sp,sp,24
70: 03e00008 jr ra
74: 00000000 nop
...

优化

for循环的优化,对大数组的赋值

-fprefetch-loop-arrays 生成数组预读取指令,对于使用巨大数组的程序可以加快代码执行速度,适合数据库相关的大型软件等

gcc默认没有开预取指令的优化,需要通过-O3

编译:

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>mips-linux-gnu-gcc -c for_loop.c -o for_loop.o -O3 -fprefetch-loop-arrays
>

反汇编:

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00000000 <func>:
0: 27bdff00 addiu sp,sp,-256
4: afbe00fc sw s8,252(sp)
...
24: afb000dc sw s0,220(sp)
28: 00000040 ssnop
2c: 00000040 ssnop
30: 3c020000 lui v0,0x0
34: 00002025 move a0,zero
38: 00001825 move v1,zero
3c: 24460000 addiu a2,v0,0
40: 306200ff andi v0,v1,0xff
44: afa20008 sw v0,8(sp)
...
a0: afbe0054 sw s8,84(sp)
a4: 24960019 addiu s6,a0,25
a8: 00d7f021 addu s8,a2,s7
ac: 93b70010 lbu s7,16(sp)
b0: 24820004 addiu v0,a0,4
...
424: a0b40000 sb s4,0(a1)
428: 8fa500a8 lw a1,168(sp)
42c: 93b40010 lbu s4,16(sp)
430: a0b40000 sb s4,0(a1)
434: 8fa500ac lw a1,172(sp)
...
474: a12a0000 sb t2,0(t1)
478: a0e80000 sb t0,0(a3)
47c: a0450000 sb a1,0(v0)
480: 24020fe0 li v0,4064
484: 1462feee bne v1,v0,40 <func+0x40>
488: 24051000 li a1,4096
48c: 304400ff andi a0,v0,0xff
490: 00c21821 addu v1,a2,v0
494: 24420001 addiu v0,v0,1
498: 1445fffc bne v0,a1,48c <func+0x48c>
49c: a0640000 sb a0,0(v1)
4a0: 00000040 ssnop
4a4: 00000040 ssnop
4a8: 00001025 move v0,zero
4ac: 8fbe00fc lw s8,252(sp)
4b0: 8fb700f8 lw s7,248(sp)
...
4cc: 8fb000dc lw s0,220(sp)
4d0: 03e00008 jr ra
4d4: 27bd0100 addiu sp,sp,256
...

for语句汇编

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void func()
{
int i, a;

__asm__ __volatile__("ssnop\n\t");
for (i = 0; i < 88; i++)
a = i;
__asm__ __volatile__("ssnop\n\t");

}
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00000000 <func>:
0: 27bdffe8 addiu sp,sp,-24
4: afbe0014 sw s8,20(sp)
8: 03a0f025 move s8,sp
c: 00000040 ssnop
10: afc00008 sw zero,8(s8) //1. i = 0;
14: 10000006 b 30 <func+0x30> //2. 跳转到0x30,判断i,如果小于88进行(4)循环
18: 00000000 nop
1c: 8fc20008 lw v0,8(s8)
20: afc2000c sw v0,12(s8)
24: 8fc20008 lw v0,8(s8)
28: 24420001 addiu v0,v0,1 //3. i++
2c: afc20008 sw v0,8(s8)
30: 8fc20008 lw v0,8(s8)
34: 28420058 slti v0,v0,88
38: 1440fff8 bnez v0,1c <func+0x1c> //4. 如果i小于88,跳转到0x1c进行循环
3c: 00000000 nop
40: 00000040 ssnop
44: 00000000 nop
48: 03c0e825 move sp,s8
4c: 8fbe0014 lw s8,20(sp)
50: 27bd0018 addiu sp,sp,24
54: 03e00008 jr ra
58: 00000000 nop
5c: 00000000 nop
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