Hi Luke,
On 12/05/16 16:34, Luke Starrett wrote:
Hi Robin,
I pulled this in to a userspace test app expecting that the __uint128_t
type might cause GCC to emit 'ldp'. Seems like that was that your
intent based on your commit note. Instead I see two 64b loads (ldr Xn),
and a single 32b load (ldr Wn) for the trailing 4B. This was with
Linaro GCC 4.9-2015.06.
GCC 5 happily emits ldp there, but indeed I couldn't figure out how to
convince GCC 4 to do so. From a quick ferret around in the GCC Git, it
looks like the relevant optimisations may have only gone in post-4.9.
Otherwise, the C cycle count looks good enough compared to the asm version.
Yeah, compiling as standalone functions with GCC 5 I get 19 instructions
vs. 17 for the asm, but the loop logic gets optimised out completely
when ihl is a compile-time constant (e.g. inet_gro_receive())
Cheers,
Robin.
Thanks,
Luke
On 5/12/2016 10:26 AM, Robin Murphy wrote:
AArch64 is capable of 128-bit memory accesses without alignment
restrictions, which makes it both possible and highly practical to slurp
up a typical 20-byte IP header in just 2 loads. Implement our own
version of ip_fast_checksum() to take advantage of that, resulting in
considerably fewer instructions and memory accesses than the generic
version. We can also get more optimal code generation for csum_fold() by
defining it a slightly different way round from the generic version, so
throw that into the mix too.
Suggested-by: Luke Starrett <luke.starr...@broadcom.com>
Signed-off-by: Robin Murphy <robin.mur...@arm.com>
---
Having spent an evening poring over disassembler output, it turns out
that there's really no need to drop into asm for this, hurrah! I've
taken it for a spin on both big and little-endian, with no ill effects.
arch/arm64/include/asm/checksum.h | 49
+++++++++++++++++++++++++++++++++++++++
1 file changed, 49 insertions(+)
create mode 100644 arch/arm64/include/asm/checksum.h
diff --git a/arch/arm64/include/asm/checksum.h
b/arch/arm64/include/asm/checksum.h
new file mode 100644
index 000000000000..7e975ffce837
--- /dev/null
+++ b/arch/arm64/include/asm/checksum.h
@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2016 ARM Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#ifndef __ASM_CHECKSUM_H
+#define __ASM_CHECKSUM_H
+
+static inline __sum16 csum_fold(__wsum csum)
+{
+ u32 sum = (__force u32)csum;
+ sum += (sum >> 16) | (sum << 16);
+ return ~(__force __sum16)(sum >> 16);
+}
+#define csum_fold csum_fold
+
+static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
+{
+ __uint128_t tmp;
+ u64 sum;
+
+ tmp = *(const __uint128_t *)iph;
+ iph += 16;
+ ihl -= 4;
+ tmp += ((tmp >> 64) | (tmp << 64));
+ sum = tmp >> 64;
+ do {
+ sum += *(const u32 *)iph;
+ iph += 4;
+ } while (--ihl);
+
+ sum += ((sum >> 32) | (sum << 32));
+ return csum_fold(sum >> 32);
+}
+#define ip_fast_csum ip_fast_csum
+
+#include <asm-generic/checksum.h>
+
+#endif /* __ASM_CHECKSUM_H */
