我們從匯編來看，各種變量在內存中的存放位置

1、使用交叉編譯器將C語言轉換成匯編

寫一個C語言程序：

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)
使用交叉編譯器將其編譯成.o文件 : arm-linux-androideabi-gcc-4.9 -c main.c
使用objdump工具將.o反匯編:arm-linux-androideabi-objdump -d main.o

hehezhou@buildsrv-165:~/workspace/test/test1$ arm-linux-androideabi-objdump -D main.omain.o: file format elf32-littlearmDisassembly of section .text:00000000 <main>:0: e52db004 push {fp} ; (str fp, [sp, #-4]!)4: e28db000 add fp, sp, #08: e24dd00c sub sp, sp, #12c: e59f3054 ldr r3, [pc, #84] ; 68 <main+0x68>10: e08f3003 add r3, pc, r314: e59f2050 ldr r2, [pc, #80] ; 6c <main+0x6c>18: e08f2002 add r2, pc, r21c: e5922000 ldr r2, [r2]20: e50b2008 str r2, [fp, #-8]24: e59f2044 ldr r2, [pc, #68] ; 70 <main+0x70>28: e08f2002 add r2, pc, r22c: 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, #058: e1a00003 mov r0, r35c: e24bd000 sub sp, fp, #060: e49db004 pop {fp} ; (ldr fp, [sp], #4)64: e12fff1e bx lr68: 00000050 andeq r0, r0, r0, asr r06c: 0000004c andeq r0, r0, ip, asr #3270: 00000040 andeq r0, r0, r0, asr #32...Disassembly of section .data:00000000 <y>:0: 0000000a andeq r0, r0, sl00000004 <w>:4: 00000014 andeq r0, r0, r4, lsl r0Disassembly of section .bss:00000000 <x>:0: 00000000 andeq r0, r0, r0Disassembly of section .comment:00000000 <.comment>:0: 43434700 movtmi r4, #14080 ; 0x37004: 4728203a ; <UNDEFINED> instruction: 0x4728203a8: 2029554e eorcs r5, r9, lr, asr #10c: 2e392e34 mrccs 14, 1, r2, cr9, cr4, {1}10: 30322078 eorscc r2, r2, r8, ror r014: 31303531 teqcc r0, r1, lsr r518: 28203332 stmdacs r0!, {r1, r4, r5, r8, r9, ip, sp}1c: 72657270 rsbvc r7, r5, #112, 420: 61656c65 cmnvs r5, r5, ror #2424: 00296573 eoreq r6, r9, r3, ror r5Disassembly of section .ARM.attributes:00000000 <.ARM.attributes>:0: 00002a41 andeq r2, r0, r1, asr #204: 61656100 cmnvs r5, r0, lsl #28: 01006962 tsteq r0, r2, ror #18c: 00000020 andeq r0, r0, r0, lsr #3210: 45543505 ldrbmi r3, [r4, #-1285] ; 0xfffffafb14: 08040600 stmdaeq r4, {r9, sl}18: 12010901 andne r0, r1, #16384 ; 0x40001c: 15011404 strne r1, [r1, #-1028] ; 0xfffffbfc20: 18031701 stmdane r3, {r0, r8, r9, sl, ip}24: 1a011901 bne 46430 <w+0x4642c>28: Address 0x0000000000000028 is out of bounds.

(arm64)
使用交叉編譯器將其編譯成.o文件 : aarch64-linux-android-gcc-4.9 -c main.c
使用objdump工具將.o反匯編:aarch64-linux-android-objdump -D main.o

hehezhou@buildsrv-165:~/workspace/test/test1$ aarch64-linux-android-objdump -D main.omain.o: file format elf64-littleaarch64Disassembly of section .text:0000000000000000 <main>:0: d10043ff sub sp, sp, #0x104: 90000000 adrp x0, 0 <main>8: 91000000 add x0, x0, #0x0c: b9400000 ldr w0, [x0]10: b9000fe0 str w0, [sp,#12]14: 90000000 adrp x0, 0 <main>18: 91000000 add x0, x0, #0x01c: 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 // #048: 910043ff add sp, sp, #0x104c: d65f03c0 retDisassembly of section .data:0000000000000000 <y>:0: 0000000a .word 0x0000000a0000000000000004 <w>:4: 00000014 .word 0x00000014Disassembly of section .bss:0000000000000000 <x>:0: 00000000 .word 0x00000000Disassembly of section .comment:0000000000000000 <.comment>:0: 43434700 .inst 0x43434700 ; undefined4: 4728203a .inst 0x4728203a ; undefined8: 2029554e .inst 0x2029554e ; undefinedc: 2e392e34 uqsub v20.8b, v17.8b, v25.8b10: 30322078 adr x24, 6441d <w+0x64419>14: 31303531 adds w17, w9, #0xc0d18: 28203332 stnp w18, w12, [x25,#-256]1c: 72657270 .inst 0x72657270 ; undefined20: 61656c65 .inst 0x61656c65 ; undefined24: 00296573 .inst 0x00296573 ; NYI hehezhou@buildsrv-165:~/workspace/test/test1$

2 以aarch64的匯編分析

（1）、可以看到帶有初始值的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 0x0000000a0000000000000004 <w>:4: 00000014 .word 0x00000014Disassembly of section .bss:0000000000000000 <x>:0: 00000000 .word 0x00000000

（2）、我們再來看匯編代碼

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, #0x104: 90000000 adrp x0, 0 <main>8: 91000000 add x0, x0, #0x0c: b9400000 ldr w0, [x0]10: b9000fe0 str w0, [sp,#12]14: 90000000 adrp x0, 0 <main>18: 91000000 add x0, x0, #0x01c: 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 // #048: 910043ff add sp, sp, #0x104c: d65f03c0 ret

總結

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