Checksum code for i386 in the kernel.

From: Perianayagam Somasundaram (somu_at_CS.Arizona.EDU)
Date: 10/25/05 

Date: Tue, 25 Oct 2005 13:27:44 -0700

Hi,

I was just looking at the csum_partial in
arch/i386/lib/checksum.S.

For sake of simplicity, I am going to paste code where I have
some have some questions.

139 csum_partial:
    140 pushl %esi
    141 pushl %ebx
    142 movl 20(%esp),%eax # Function arg: unsigned int sum
    143 movl 16(%esp),%ecx # Function arg: int len
    144 movl 12(%esp),%esi # Function arg: const unsigned char *buf
    145
    146 testl $3, %esi
    147 jnz 25f
    148 10:
    149 movl %ecx, %edx
    150 movl %ecx, %ebx
    151 andl $0x7c, %ebx
    152 shrl $7, %ecx
    153 addl %ebx,%esi
    154 shrl $2, %ebx
    155 negl %ebx
    156 lea 45f(%ebx,%ebx,2), %ebx
    157 testl %esi, %esi
    158 jmp *%ebx
    ...
    ...
    ...
    194 addl -128(%esi), %eax
    195 adcl -124(%esi), %eax
    ...
    198 adcl -112(%esi), %eax
    ...
    223 adcl -12(%esi), %eax
    224 adcl -8(%esi), %eax
    225 adcl -4(%esi), %eax
    226 45:
    227 lea 128(%esi), %esi
    228 adcl $0, %eax
    229 dec %ecx
    230 jge 40b

So I do understand the loop unrolling part for performance based
optimizations. My questions is in pretext to calculation of the
address using ebx register in line <156>. The routine cannot make
any assumptions about the len being passed. But it uses it in the
calculation of ebx, which then is translated into a offset. Now
this offset is added to the address of "45f" and stored into ebx
and then the indirect jmp.

I also have noticed that the offset varies by an amount of 3
bytes. Coincidentally, that also happens to be the length of
"adcl x(%esi), %eax". Is there an implicit assumption of the
instruction length in bytes of "adcl x(%esi), %eax"? That is what
I infer from all the groundwork I have done.

On the other hand, given my inference is wrong, then this code
can throw a panic if the instruction length of "adcl x(%esi),
%eax" changes ( which it may not ).

cheers
Somu
*************************************************************************
"I've never understood how God could expect his creatures
 to pick the one true religion by faith - it strikes me as a
 sloppy way to run a universe."
Robert A. Heinlein
STRANGER IN A STRANGE LAND
*************************************************************************