C语言结构体传参分析

从来没有直接使用结构体传参，想了解一下编译器是如何处理的，写了个简单的C文件编译后分析一下。

test.c文件：

#include <stdio.h>struct ttt{    unsigned long w;    unsigned long h;};unsigned long test_ttt(struct ttt test){    return test.w + test.h;}int main(void){    struct ttt test;    test.w = 10;    test.h = 20;    test_ttt(test);    return 0;}

使用下面的命令生成test.S汇编文件

arm-none-linux-gnueabi-gcc -g3 test.c

 arm-none-linux-gnueabi-objdump -Slz a.out > test.S

a.out:     file format elf32-littlearmDisassembly of section .init:0000830c <_init>:_init():    830c:e92d4008 push{r3, lr}    8310:eb00001e bl8390 <call_gmon_start>    8314:e8bd4008 pop{r3, lr}    8318:e12fff1e bxlrDisassembly of section .plt:0000831c <.plt>:    831c:e52de004 push{lr}; (str lr, [sp, #-4]!)    8320:e59fe004 ldrlr, [pc, #4]; 832c <_init+0x20>    8324:e08fe00e addlr, pc, lr    8328:e5bef008 ldrpc, [lr, #8]!    832c:0000837c .word0x0000837c    8330:e28fc600 addip, pc, #0, 12    8334:e28cca08 addip, ip, #8, 20; 0x8000    8338:e5bcf37c ldrpc, [ip, #892]!; 0x37c    833c:e28fc600 addip, pc, #0, 12    8340:e28cca08 addip, ip, #8, 20; 0x8000    8344:e5bcf374 ldrpc, [ip, #884]!; 0x374    8348:e28fc600 addip, pc, #0, 12    834c:e28cca08 addip, ip, #8, 20; 0x8000    8350:e5bcf36c ldrpc, [ip, #876]!; 0x36cDisassembly of section .text:00008354 <_start>:_start():    8354:e3a0b000 movfp, #0    8358:e3a0e000 movlr, #0    835c:e49d1004 pop{r1}; (ldr r1, [sp], #4)    8360:e1a0200d movr2, sp    8364:e52d2004 push{r2}; (str r2, [sp, #-4]!)    8368:e52d0004 push{r0}; (str r0, [sp, #-4]!)    836c:e59fc010 ldrip, [pc, #16]; 8384 <_start+0x30>    8370:e52dc004 push{ip}; (str ip, [sp, #-4]!)    8374:e59f000c ldrr0, [pc, #12]; 8388 <_start+0x34>    8378:e59f300c ldrr3, [pc, #12]; 838c <_start+0x38>    837c:ebffffee bl833c <_init+0x30>    8380:ebffffea bl8330 <_init+0x24>    8384:00008554 .word0x00008554    8388:0000843c .word0x0000843c    838c:00008478 .word0x0000847800008390 <call_gmon_start>:call_gmon_start():    8390:e92d4008 push{r3, lr}    8394:e59f3018 ldrr3, [pc, #24]; 83b4 <call_gmon_start+0x24>    8398:e59f0018 ldrr0, [pc, #24]; 83b8 <call_gmon_start+0x28>    839c:e08f3003 addr3, pc, r3    83a0:e7932000 ldrr2, [r3, r0]    83a4:e3520000 cmpr2, #0    83a8:1bffffe6 blne8348 <_init+0x3c>    83ac:e8bd4008 pop{r3, lr}    83b0:e12fff1e bxlr    83b4:00008304 .word0x00008304    83b8:00000018 .word0x00000018000083bc <__do_global_dtors_aux>:__do_global_dtors_aux():    83bc:e59f3010 ldrr3, [pc, #16]; 83d4 <__do_global_dtors_aux+0x18>    83c0:e5d32000 ldrbr2, [r3]    83c4:e3520000 cmpr2, #0    83c8:03a02001 moveqr2, #1    83cc:05c32000 strbeqr2, [r3]    83d0:e12fff1e bxlr    83d4:000106cc .word0x000106cc000083d8 <frame_dummy>:frame_dummy():    83d8:e59f0024 ldrr0, [pc, #36]; 8404 <frame_dummy+0x2c>    83dc:e92d4008 push{r3, lr}    83e0:e5903000 ldrr3, [r0]    83e4:e3530000 cmpr3, #0    83e8:0a000003 beq83fc <frame_dummy+0x24>    83ec:e59f3014 ldrr3, [pc, #20]; 8408 <frame_dummy+0x30>    83f0:e3530000 cmpr3, #0    83f4:11a0e00f movnelr, pc    83f8:112fff13 bxner3    83fc:e8bd4008 pop{r3, lr}    8400:e12fff1e bxlr    8404:000105b4 .word0x000105b4    8408:00000000 .word0x000000000000840c <test_ttt>:test_ttt():/home/user/test.c:9unsigned long w;unsigned long h;};unsigned long test_ttt(struct ttt test){    840c:e52db004 push{fp}; (str fp, [sp, #-4]!)    8410:e28db000 addfp, sp, #0    8414:e24dd00c subsp, sp, #12    8418:e24b300c subr3, fp, #12    841c:e8830003 stmr3, {r0, r1}/home/user/test.c:10return test.w + test.h;    8420:e51b200c ldrr2, [fp, #-12]    8424:e51b3008 ldrr3, [fp, #-8]    8428:e0823003 addr3, r2, r3/home/user/test.c:11}    842c:e1a00003 movr0, r3    8430:e28bd000 addsp, fp, #0    8434:e49db004 pop{fp}; (ldr fp, [sp], #4)    8438:e12fff1e bxlr0000843c <main>:main():/home/user/test.c:13int main(void){    843c:e92d4800 push{fp, lr}    8440:e28db004 addfp, sp, #4    8444:e24dd008 subsp, sp, #8/home/user/test.c:16struct ttt test;test.w = 10;    8448:e3a0300a movr3, #10    844c:e50b300c strr3, [fp, #-12]/home/user/test.c:17test.h = 20;    8450:e3a03014 movr3, #20    8454:e50b3008 strr3, [fp, #-8]/home/user/test.c:19test_ttt(test);    8458:e24b300c subr3, fp, #12    845c:e8930003 ldmr3, {r0, r1}    8460:ebffffe9 bl840c <test_ttt>/home/user/test.c:21return 0;    8464:e3a03000 movr3, #0/home/user/test.c:22}    8468:e1a00003 movr0, r3    846c:e24bd004 subsp, fp, #4    8470:e8bd4800 pop{fp, lr}    8474:e12fff1e bxlr00008478 <__libc_csu_init>:__libc_csu_init():    8478:e92d47f0 push{r4, r5, r6, r7, r8, r9, sl, lr}    847c:e1a0a000 movsl, r0    8480:e1a08001 movr8, r1    8484:e1a07002 movr7, r2    8488:e59f40bc ldrr4, [pc, #188]; 854c <__libc_csu_init+0xd4>    848c:ebffff9e bl830c <_init>    8490:e59f10b8 ldrr1, [pc, #184]; 8550 <__libc_csu_init+0xd8>    8494:e08f4004 addr4, pc, r4    8498:e08f0001 addr0, pc, r1    849c:e0649000 rsbr9, r4, r0    84a0:e1b09149 asrsr9, r9, #2    84a4:0a000026 beq8544 <__libc_csu_init+0xcc>    84a8:e3a06001 movr6, #1    84ac:e4943004 ldrr3, [r4], #4    84b0:e1a0000a movr0, sl    84b4:e1a01008 movr1, r8    84b8:e1a02007 movr2, r7    84bc:e2495001 subr5, r9, #1    84c0:e1a0e00f movlr, pc    84c4:e12fff13 bxr3    84c8:e1560009 cmpr6, r9    84cc:e0055006 andr5, r5, r6    84d0:0a00001b beq8544 <__libc_csu_init+0xcc>    84d4:e3550000 cmpr5, #0    84d8:0a000008 beq8500 <__libc_csu_init+0x88>    84dc:e494c004 ldrip, [r4], #4    84e0:e1a0000a movr0, sl    84e4:e1a01008 movr1, r8    84e8:e1a02007 movr2, r7    84ec:e3a06002 movr6, #2    84f0:e1a0e00f movlr, pc    84f4:e12fff1c bxip    84f8:e1560009 cmpr6, r9    84fc:0a000010 beq8544 <__libc_csu_init+0xcc>    8500:e1a05004 movr5, r4    8504:e4953004 ldrr3, [r5], #4    8508:e1a0000a movr0, sl    850c:e1a01008 movr1, r8    8510:e1a02007 movr2, r7    8514:e1a0e00f movlr, pc    8518:e12fff13 bxr3    851c:e2866002 addr6, r6, #2    8520:e594c004 ldrip, [r4, #4]    8524:e1a0000a movr0, sl    8528:e1a01008 movr1, r8    852c:e1a02007 movr2, r7    8530:e1a0e00f movlr, pc    8534:e12fff1c bxip    8538:e1560009 cmpr6, r9    853c:e2854004 addr4, r5, #4    8540:1affffee bne8500 <__libc_csu_init+0x88>    8544:e8bd47f0 pop{r4, r5, r6, r7, r8, r9, sl, lr}    8548:e12fff1e bxlr    854c:00008110 .word0x00008110    8550:00008110 .word0x0000811000008554 <__libc_csu_fini>:__libc_csu_fini():    8554:e12fff1e bxlrDisassembly of section .fini:00008558 <_fini>:_fini():    8558:e92d4008 push{r3, lr}    855c:e8bd4008 pop{r3, lr}    8560:e12fff1e bxlr

结论： 结构体传参是通过栈实现的，而结构体指针传参是使用寄存器。我们知道，寄存器的读写速度远远高于SDRAM读写速度，所以，尽可能使用结构体指针传参代替结构体传参。