code:

#include "stdafx.h"

struct big {
char buf[100];
int i;
// long d;
} B, B2;

big bigfun(big b) {
b.i = 100;
return b;
}

int main(int argc, char* argv[])
{
B2 = bigfun(B);
return 0;
}

用vc++6.0生成的汇编代码为：
TITLE E:\code\th_in_c++\passtruc\passtruc.cpp
.386P
include listing.inc
if @Version gt 510
.model FLAT
else
_TEXT SEGMENT PARA USE32 PUBLIC 'CODE'
_TEXT ENDS
_DATA SEGMENT DWORD USE32 PUBLIC 'DATA'
_DATA ENDS
CONST SEGMENT DWORD USE32 PUBLIC 'CONST'
CONST ENDS
_BSS SEGMENT PARA USE32 PUBLIC 'BSS'
_BSS ENDS
$$SYMBOLS SEGMENT BYTE USE32 'DEBSYM'
$$SYMBOLS ENDS
$$TYPES SEGMENT BYTE USE32 'DEBTYP'
$$TYPES ENDS
_TLS SEGMENT DWORD USE32 PUBLIC 'TLS'
_TLS ENDS
; COMDAT ?bigfun@@YA?AUbig@@U1@@Z
_TEXT SEGMENT PARA USE32 PUBLIC 'CODE'
_TEXT ENDS
; COMDAT _main
_TEXT SEGMENT PARA USE32 PUBLIC 'CODE'
_TEXT ENDS
FLAT GROUP _DATA, CONST, _BSS
ASSUME CS: FLAT, DS: FLAT, SS: FLAT
endif
PUBLIC ?B@@3Ubig@@A ; B
PUBLIC ?B2@@3Ubig@@A ; B2
_BSS SEGMENT
?B@@3Ubig@@A DB 068H DUP (?) ; B
?B2@@3Ubig@@A DB 068H DUP (?) ; B2
_BSS ENDS
PUBLIC ?bigfun@@YA?AUbig@@U1@@Z ; bigfun
; COMDAT ?bigfun@@YA?AUbig@@U1@@Z
_TEXT SEGMENT
_b$ = 12
$T241 = 8
?bigfun@@YA?AUbig@@U1@@Z PROC NEAR ; bigfun, COMDAT
; File E:\code\th_in_c++\passtruc\passtruc.cpp
; Line 12
push ebp
mov ebp, esp
sub esp, 64 ; 00000040H
push ebx
push esi
push edi
lea edi, DWORD PTR [ebp-64]
mov ecx, 16 ; 00000010H
mov eax, -858993460 ; ccccccccH
rep stosd
; Line 13
mov DWORD PTR _b$[ebp+100], 100 ; 00000064H
; Line 14
mov ecx, 26 ; 0000001aH
lea esi, DWORD PTR _b$[ebp]
mov edi, DWORD PTR $T241[ebp]
rep movsd ；复制返回值
mov eax, DWORD PTR $T241[ebp] ；EAX中存放的是返回值地址
; Line 15
pop edi
pop esi
pop ebx
mov esp, ebp
pop ebp
ret 0
?bigfun@@YA?AUbig@@U1@@Z ENDP ; bigfun
_TEXT ENDS
PUBLIC _main
EXTRN __chkesp:NEAR
; COMDAT _main
_TEXT SEGMENT
$T244 = -104
$T245 = -208
_main PROC NEAR ; COMDAT
; Line 18
push ebp
mov ebp, esp
sub esp, 272 ; 00000110H
push ebx
push esi
push edi
lea edi, DWORD PTR [ebp-272]
mov ecx, 68 ; 00000044H
mov eax, -858993460 ; ccccccccH
rep stosd
; Line 19
sub esp, 104 ; 00000068H
mov ecx, 26 ; 0000001aH
mov esi, OFFSET FLAT:?B@@3Ubig@@A ; B
mov edi, esp
rep movsd ；将参数压栈
lea eax, DWORD PTR $T245[ebp]
push eax ；保存断点
call ?bigfun@@YA?AUbig@@U1@@Z ; bigfun
add esp, 108 ; 0000006cH
mov esi, eax
mov ecx, 26 ; 0000001aH
lea edi, DWORD PTR $T244[ebp]
rep movsd
mov ecx, 26 ; 0000001aH
lea esi, DWORD PTR $T244[ebp]
mov edi, OFFSET FLAT:?B2@@3Ubig@@A ; B2
rep movsd                                             ;将结果赋给B2
; Line 20
xor eax, eax
; Line 21
pop edi
pop esi
pop ebx
add esp, 272 ; 00000110H
cmp ebp, esp
call __chkesp
mov esp, ebp
pop ebp
ret 0
_main ENDP
_TEXT ENDS
END