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SubjectRe: TCP Segmentation Offloading (TSO)
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Hello, 

I updated the csum_partial() for x86.
csum_partial() for standard x86 can also handle odd buffer better.

> > > Now that is grossly inefficient ;-) since you can save one instruction by
> > > moving roll after adcl (hand edited partial patch hunk, won't apply):

I applied it.
But it will be trivial for its performance on most packets.

> > I've been doing some PPro assembly lately, and I'm reminded that
> > sometimes inserting instructions can reduce the timing by up to 8 cycles
> > or so.
>
> The one instruction that you can still be moved around easily is the
> pointer increment. But I would never try to improve code paths that I
> consider non critical.

I wish recent x86 processor can reorder instructions in it.


--- linux/arch/i386/lib/checksum.S.BUG Sun Sep 1 17:00:59 2030
+++ linux/arch/i386/lib/checksum.S Fri Sep 6 16:19:27 2030
@@ -55,8 +55,21 @@ csum_partial:
movl 20(%esp),%eax # Function arg: unsigned int sum
movl 16(%esp),%ecx # Function arg: int len
movl 12(%esp),%esi # Function arg: unsigned char *buff
- testl $2, %esi # Check alignment.
+ testl $3, %esi # Check alignment.
jz 2f # Jump if alignment is ok.
+ testl $1, %esi # Check alignment.
+ jz 10f # Jump if alignment is boundary of 2bytes.
+
+ # buf is odd
+ dec %ecx
+ jl 8f
+ movzbl (%esi), %ebx
+ adcl %ebx, %eax
+ roll $8, %eax
+ inc %esi
+ testl $2, %esi
+ jz 2f
+10:
subl $2, %ecx # Alignment uses up two bytes.
jae 1f # Jump if we had at least two bytes.
addl $2, %ecx # ecx was < 2. Deal with it.
@@ -111,6 +124,10 @@ csum_partial:
6: addl %ecx,%eax
adcl $0, %eax
7:
+ testl $1, 12(%esp)
+ jz 8f
+ roll $8, %eax
+8:
popl %ebx
popl %esi
ret
@@ -126,8 +143,8 @@ csum_partial:
movl 16(%esp),%ecx # Function arg: int len
movl 12(%esp),%esi # Function arg: const unsigned char *buf

- testl $2, %esi
- jnz 30f
+ testl $3, %esi
+ jnz 25f
10:
movl %ecx, %edx
movl %ecx, %ebx
@@ -145,6 +162,19 @@ csum_partial:
lea 2(%esi), %esi
adcl $0, %eax
jmp 10b
+25:
+ testl $1, %esi
+ jz 30f
+ # buf is odd
+ dec %ecx
+ jl 90f
+ movzbl (%esi), %ebx
+ addl %ebx, %eax
+ adcl $0, %eax
+ roll $8, %eax
+ inc %esi
+ testl $2, %esi
+ jz 10b

30: subl $2, %ecx
ja 20b
@@ -211,6 +241,10 @@ csum_partial:
addl %ebx,%eax
adcl $0,%eax
80:
+ testl $1, 12(%esp)
+ jz 90f
+ roll $8, %eax
+90:
popl %ebx
popl %esi
ret
-
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