大对象的传递和返回

    如果一个对象很小，寄存器可以放得下，一般是通过寄存器返回的，这个寄存器一般是EAX,但是如果对象足够大，以至于寄存器放不下，那么编译器是如何处理这个对象的传递和放回的呢？

    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