I'm a little confused at how stack is used for mips arch by gcc.
Here is the C source code:
char *
strcpy(char *dest, char *src)
{
char *tmp = dest;
while( (*dest++ = *src++) != '\0' ) ;
return tmp;
}
And the assembly code:
-------------------------------------------------------
1.o: file format elf32-littlemips
Disassembly of section .text:
00000000 <strcpy>:
char *
strcpy(char *dest, char *src)
{
0: 27bdfff0 addiu sp,sp,-16
4: afbe0008 sw s8,8(sp)
8: 03a0f021 move s8,sp
c: afc40010 sw a0,16(s8)
10: afc50014 sw a1,20(s8)
char *tmp = dest;
14: 8fc20010 lw v0,16(s8)
18: afc20000 sw v0,0(s8)
while( (*dest++ = *src++) != '\0' ) ;
1c: 8fc30010 lw v1,16(s8)
20: 27c40014 addiu a0,s8,20
24: 8c820000 lw v0,0(a0)
28: 90450000 lbu a1,0(v0)
2c: 24420001 addiu v0,v0,1
30: ac820000 sw v0,0(a0)
34: 00601021 move v0,v1
38: a0450000 sb a1,0(v0)
3c: 24630001 addiu v1,v1,1
40: afc30010 sw v1,16(s8)
44: 00051600 sll v0,a1,0x18
48: 00021603 sra v0,v0,0x18
4c: 10400003 beqz v0,5c <strcpy+0x5c>
50: 00000000 nop
54: 08000007 j 1c <strcpy+0x1c>
58: 00000000 nop
return tmp;
5c: 8fc20000 lw v0,0(s8)
} 60: 03c0e821 move sp,s8
64: 8fbe0008 lw s8,8(sp)
68: 27bd0010 addiu sp,sp,16
6c: 03e00008 jr ra
70: 00000000 nop
-------------------------------------------------------
The usage of stack inside the function should look like:
sp' +-----+
| tmp | sp+0
|_____|
| | sp+4
| |
|_____|
| | sp+8
| |
|_____|
| | sp+12
| |
|_____|
sp | | sp+16
| a0 |
|_____|
| | sp+20
| a1 |
|_____|
As a gerneral rule, the units above the stack top should be protected
during the call of the function. But why the stack top is used to
save the a0 and a1 registers here? By the way, why sp+4, sp+8 and sp+12
are left unused?
My gcc is
-------------------
Reading specs from /usr/local/lib/gcc/mips-elf/3.4.0-macraigor1/specs
Configured with: /rel/share/gnu/src/gcc-3.4.0-macraigor1/configure --host=i686-pc-cygwin --target=mips-elf --disable-nls --with-stabs --disable-install-libiberty --disable-install-libbfd --disable-dependency-tracking --enable-64-bit-bfd --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
Thread model: single
gcc version 3.4.0-macraigor1
which is available on http://www.ocdemon.com/gnutools-mips-elf-2.05.exe
