[XENVIF PATCH RESEND] Faster checksum for x64 with carry flag renaming

["Tu Dinh" <ngoc-tu.dinh@xxxxxxxxxx>]

Modern CPUs are capable of renaming CF independently of other arithmetic
flags. We can better exploit ILP by maintaining two carry chains in the
hot checksum loop. This gives a ~20% speed boost for packets larger than
64 bytes (as tested on Zen 3).

Suggested-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
---
 src/xenvif/amd64/checksum_amd64.asm | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/xenvif/amd64/checksum_amd64.asm 
b/src/xenvif/amd64/checksum_amd64.asm
index 8fbc241..37bbb7e 100644
--- a/src/xenvif/amd64/checksum_amd64.asm
+++ b/src/xenvif/amd64/checksum_amd64.asm
@@ -26,10 +26,12 @@ l64:
     adc rax, [rdx + 8]
     adc rax, [rdx + 16]
     adc rax, [rdx + 24]
-    adc rax, [rdx + 32]
-    adc rax, [rdx + 40]
-    adc rax, [rdx + 48]
-    adc rax, [rdx + 56]
+    adc rax, 0
+    mov r9, [rdx + 32]
+    add r9, [rdx + 40]
+    adc r9, [rdx + 48]
+    adc r9, [rdx + 56]
+    adc rax, r9
     adc rax, 0
 
     sub r8, 64
-- 
2.51.0.windows.2



--
Ngoc Tu Dinh | Vates XCP-ng Developer

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