我们从汇编来看，各种变量在内存中的存放位置
1、使用交叉编译器将C语言转换成汇编
写一个C语言程序：
static int x;
static int y = 10;
int z;
int w = 20;
int main()
{
int s;
s=x;
s=y;
s=z;
s=w;
return 0;
}
(arm32)
使用交叉编译器将其编译成.o文件 : arm-linux-androideabi-gcc-4.9 -c main.c
使用objdump工具将.o反汇编:arm-linux-androideabi-objdump -d main.o
hehezhou@buildsrv-165:~/workspace/test/test1$ arm-linux-androideabi-objdump -D main.o
main.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <main>:
0: e52db004 push {fp} ; (str fp, [sp, #-4]!)
4: e28db000 add fp, sp, #0
8: e24dd00c sub sp, sp, #12
c: e59f3054 ldr r3, [pc, #84] ; 68 <main+0x68>
10: e08f3003 add r3, pc, r3
14: e59f2050 ldr r2, [pc, #80] ; 6c <main+0x6c>
18: e08f2002 add r2, pc, r2
1c: e5922000 ldr r2, [r2]
20: e50b2008 str r2, [fp, #-8]
24: e59f2044 ldr r2, [pc, #68] ; 70 <main+0x70>
28: e08f2002 add r2, pc, r2
2c: e5922000 ldr r2, [r2]
30: e50b2008 str r2, [fp, #-8]
34: e59f2038 ldr r2, [pc, #56] ; 74 <main+0x74>
38: e7932002 ldr r2, [r3, r2]
3c: e5922000 ldr r2, [r2]
40: e50b2008 str r2, [fp, #-8]
44: e59f202c ldr r2, [pc, #44] ; 78 <main+0x78>
48: e7933002 ldr r3, [r3, r2]
4c: e5933000 ldr r3, [r3]
50: e50b3008 str r3, [fp, #-8]
54: e3a03000 mov r3, #0
58: e1a00003 mov r0, r3
5c: e24bd000 sub sp, fp, #0
60: e49db004 pop {fp} ; (ldr fp, [sp], #4)
64: e12fff1e bx lr
68: 00000050 andeq r0, r0, r0, asr r0
6c: 0000004c andeq r0, r0, ip, asr #32
70: 00000040 andeq r0, r0, r0, asr #32
...
Disassembly of section .data:
00000000 <y>:
0: 0000000a andeq r0, r0, sl
00000004 <w>:
4: 00000014 andeq r0, r0, r4, lsl r0
Disassembly of section .bss:
00000000 <x>:
0: 00000000 andeq r0, r0, r0
Disassembly of section .comment:
00000000 <.comment>:
0: 43434700 movtmi r4, #14080 ; 0x3700
4: 4728203a ; <UNDEFINED> instruction: 0x4728203a
8: 2029554e eorcs r5, r9, lr, asr #10
c: 2e392e34 mrccs 14, 1, r2, cr9, cr4, {1}
10: 30322078 eorscc r2, r2, r8, ror r0
14: 31303531 teqcc r0, r1, lsr r5
18: 28203332 stmdacs r0!, {r1, r4, r5, r8, r9, ip, sp}
1c: 72657270 rsbvc r7, r5, #112, 4
20: 61656c65 cmnvs r5, r5, ror #24
24: 00296573 eoreq r6, r9, r3, ror r5
Disassembly of section .ARM.attributes:
00000000 <.ARM.attributes>:
0: 00002a41 andeq r2, r0, r1, asr #20
4: 61656100 cmnvs r5, r0, lsl #2
8: 01006962 tsteq r0, r2, ror #18
c: 00000020 andeq r0, r0, r0, lsr #32
10: 45543505 ldrbmi r3, [r4, #-1285] ; 0xfffffafb
14: 08040600 stmdaeq r4, {r9, sl}
18: 12010901 andne r0, r1, #16384 ; 0x4000
1c: 15011404 strne r1, [r1, #-1028] ; 0xfffffbfc
20: 18031701 stmdane r3, {r0, r8, r9, sl, ip}
24: 1a011901 bne 46430 <w+0x4642c>
28: Address 0x0000000000000028 is out of bounds.
(arm64)
使用交叉编译器将其编译成.o文件 : aarch64-linux-android-gcc-4.9 -c main.c
使用objdump工具将.o反汇编:aarch64-linux-android-objdump -D main.o
hehezhou@buildsrv-165:~/workspace/test/test1$ aarch64-linux-android-objdump -D main.o
main.o: file format elf64-littleaarch64
Disassembly of section .text:
0000000000000000 <main>:
0: d10043ff sub sp, sp, #0x10
4: 90000000 adrp x0, 0 <main>
8: 91000000 add x0, x0, #0x0
c: b9400000 ldr w0, [x0]
10: b9000fe0 str w0, [sp,#12]
14: 90000000 adrp x0, 0 <main>
18: 91000000 add x0, x0, #0x0
1c: b9400000 ldr w0, [x0]
20: b9000fe0 str w0, [sp,#12]
24: 90000000 adrp x0, 4 <main+0x4>
28: f9400000 ldr x0, [x0]
2c: b9400000 ldr w0, [x0]
30: b9000fe0 str w0, [sp,#12]
34: 90000000 adrp x0, 4 <main+0x4>
38: f9400000 ldr x0, [x0]
3c: b9400000 ldr w0, [x0]
40: b9000fe0 str w0, [sp,#12]
44: 52800000 mov w0, #0x0 // #0
48: 910043ff add sp, sp, #0x10
4c: d65f03c0 ret
Disassembly of section .data:
0000000000000000 <y>:
0: 0000000a .word 0x0000000a
0000000000000004 <w>:
4: 00000014 .word 0x00000014
Disassembly of section .bss:
0000000000000000 <x>:
0: 00000000 .word 0x00000000
Disassembly of section .comment:
0000000000000000 <.comment>:
0: 43434700 .inst 0x43434700 ; undefined
4: 4728203a .inst 0x4728203a ; undefined
8: 2029554e .inst 0x2029554e ; undefined
c: 2e392e34 uqsub v20.8b, v17.8b, v25.8b
10: 30322078 adr x24, 6441d <w+0x64419>
14: 31303531 adds w17, w9, #0xc0d
18: 28203332 stnp w18, w12, [x25,#-256]
1c: 72657270 .inst 0x72657270 ; undefined
20: 61656c65 .inst 0x61656c65 ; undefined
24: 00296573 .inst 0x00296573 ; NYI
hehezhou@buildsrv-165:~/workspace/test/test1$
2 以aarch64的汇编分析
（1）、可以看到带有初始值的static int y = 10 和int w = 20 都存放在data段
未初始值的static int x在bss段
static int x;
static int y = 10;
int z;
int w = 20;
Disassembly of section .data:
0000000000000000 <y>:
0: 0000000a .word 0x0000000a
0000000000000004 <w>:
4: 00000014 .word 0x00000014
Disassembly of section .bss:
0000000000000000 <x>:
0: 00000000 .word 0x00000000
（2）、我们再来看汇编代码
static int x;
static int y = 10;
int z;
int w = 20;
int main()
{
int s;
s=x;
s=y;
s=z;
s=w;
return 0;
}
0000000000000000 <main>:
0: d10043ff sub sp, sp, #0x10
4: 90000000 adrp x0, 0 <main>
8: 91000000 add x0, x0, #0x0
c: b9400000 ldr w0, [x0]
10: b9000fe0 str w0, [sp,#12]
14: 90000000 adrp x0, 0 <main>
18: 91000000 add x0, x0, #0x0
1c: b9400000 ldr w0, [x0]
20: b9000fe0 str w0, [sp,#12]
24: 90000000 adrp x0, 4 <main+0x4>
28: f9400000 ldr x0, [x0]
2c: b9400000 ldr w0, [x0]
30: b9000fe0 str w0, [sp,#12]
34: 90000000 adrp x0, 4 <main+0x4>
38: f9400000 ldr x0, [x0]
3c: b9400000 ldr w0, [x0]
40: b9000fe0 str w0, [sp,#12]
44: 52800000 mov w0, #0x0 // #0
48: 910043ff add sp, sp, #0x10
4c: d65f03c0 ret
