Dive into the assembly

Function in c:

PHPAPI char *php_pcre_replace(char *regex,   int regex_len,
                              char *subject, int subject_len,
                              zval *replace_val, int is_callable_replace,
                              int *result_len, int limit, int *replace_count TSRMLS_DC)
{
    pc开发者_运维百科re_cache_entry    *pce;               /* Compiled regular expression */

    /* Compile regex or get it from cache. */
    if ((pce = pcre_get_compiled_regex_cache(regex, regex_len TSRMLS_CC)) == NULL) {
        return NULL;
    }

    ....
}

Its assembly:

php5ts!php_pcre_replace:
1015db70 8b442408        mov     eax,dword ptr [esp+8]
1015db74 8b4c2404        mov     ecx,dword ptr [esp+4]
1015db78 56              push    esi
1015db79 8b74242c        mov     esi,dword ptr [esp+2Ch]
1015db7d 56              push    esi
1015db7e 50              push    eax
1015db7f 51              push    ecx
1015db80 e8cbeaffff      call    php5ts!pcre_get_compiled_regex_cache (1015c650)
1015db85 83c40c          add     esp,0Ch
1015db88 85c0            test    eax,eax
1015db8a 7502            jne     php5ts!php_pcre_replace+0x1e (1015db8e)

php5ts!php_pcre_replace+0x1c:
1015db8c 5e              pop     esi
1015db8d c3              ret

The c function call pcre_get_compiled_regex_cache(regex, regex_len TSRMLS_CC) corresponds to 1015db7d~1015db80 which pushes the 3 parameters to the stack and call it.

But my doubt is,among so many registers,how does the compiler decide to use eax,ecx and esi(this is special,as it's restored before using,why?) as the intermediate to carry to the stack?

There must be some hidden indication in c that tells the compiler to do it this way,right?

No, there is no hidden indication.

This is a typical strategy for generating 80x86 instructions used by many compiler implementations, C and otherwise. For example, the 1980s Intel Fortran-77 compiler, when optimization was turned on, did the same thing.

That is uses eax and ecx preferentially is probably an artifact of avoiding use of esi and edi since those registers cannot directly be used to load byte operands.

Why not ebx and edx? Well, those are preferred by many code generators for holding intermediate pointers in evaluating complex structure evaluation, which is to say, there isn't much reason at all. The compiler just looked for two available registers to use and overwrote them to buffer the values.

Why not reuse eax like this?:

   push    esi
   mov     eax,dword ptr [esp+2Ch]
   push    eax
   mov     eax,dword ptr [esp+8]
   push    eax
   mov     eax,dword ptr [esp+4]
   push    eax

Because that causes pipeline stalls waiting for eax to complete previous memory cycles, in 80x86s since the 80586 (maybe 80486—it's too long ago to be sure off the top of my head).

The x86 architecture is a strange beast. Each register, though promoted as being "general purpose" by Intel, has its quirks (cx/ecx is tied to the loop instruction for example, and eax:edx is tied to the multiply instruction). That combined with the peculiar ways to optimize execution to avoid cache misses and pipeline stalls often leads to inscrutable generated code by a code generator which factors all that in.