Re: powerpc32: rearrange instructions order in ip_fast_csum()
From: christophe leroy
Date: Tue Apr 28 2015 - 15:08:00 EST
Le 25/03/2015 02:22, Scott Wood a écrit :
On Tue, Feb 03, 2015 at 12:39:27PM +0100, LEROY Christophe wrote:
Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
---
arch/powerpc/lib/checksum_32.S | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/arch/powerpc/lib/checksum_32.S b/arch/powerpc/lib/checksum_32.S
index 6d67e05..5500704 100644
--- a/arch/powerpc/lib/checksum_32.S
+++ b/arch/powerpc/lib/checksum_32.S
@@ -26,13 +26,17 @@
_GLOBAL(ip_fast_csum)
lwz r0,0(r3)
lwzu r5,4(r3)
- addic. r4,r4,-2
+ addic. r4,r4,-4
addc r0,r0,r5
mtctr r4
blelr-
-1: lwzu r4,4(r3)
- adde r0,r0,r4
+ lwzu r5,4(r3)
+ lwzu r4,4(r3)
The blelr is pointless since len is guaranteed to be >= 5 (assuming that
comment is accurate), but now it's both pointless and in the wrong place,
since you haven't yet finished the four words that you subtracted from
r4.
The blelr is just there to protect the function against negative value
of r4 hence ctr.
In any case, the returned result in that case in not correct, has we do
not touch r3.
How about keeping the blelr, without the -, moving it after the initial
words, and changing the number of inital words to 5?
We can't just do blelr, we would need to fold the result first.
But indeed, this would be useless because I quickly checked and it seems
that all functions calling ip_fast_csum()
check that the length is not lower than 5.
So I will just remove the blelr
Also maybe do all
the loads up front, since many PPC chips have a three cycle load latency
rather than two.
ok
Christophe
---
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