Re: Confused at usage of stack for MIPS arch

[Andrew Haley <aph at redhat dot com>]

PRC writes:
 > 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?

They're not saved, they're moved to local stack slots.

 > By the way, why sp+4, sp+8 and sp+12 are left unused?

Alognment.

Have a good look at 

http://www.sco.com/developers/devspecs/mipsabi.pdf

Pages 3-11 onwards.  If there's anything you still don't understand,
let us know.

Andrew.

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