Re: [PATCH] lib/x86: Optimise csum_partial of buffers that are not multiples of 8 bytes.

Add in the trailing bytes first so that there is no need to worry
about the sum exceeding 64 bits.

Signed-off-by: David Laight <redacted>
---

This ought to be faster - because of all the removed 'adc $0'.
Guessing how fast x86 code will run is hard!
There are other ways of handing buffers that are shorter than 8 bytes,
but I'd rather hope they don't happen in any hot paths.

Note - I've not even compile tested it.
(But have tested an equivalent change before.)

 arch/x86/lib/csum-partial_64.c | 55 ++++++++++++----------------------
 1 file changed, 19 insertions(+), 36 deletions(-)

diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index abf819dd8525..fbcc073fc2b5 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c

@@ -37,6 +37,24 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
        u64 temp64 = (__force u64)sum;
        unsigned result;

+       if (len & 7) {
+               if (unlikely(len < 8)) {
+                       /* Avoid falling off the start of the buffer */
+                       if (len & 4) {
+                               temp64 += *(u32 *)buff;
+                               buff += 4;
+                       }
+                       if (len & 2) {
+                               temp64 += *(u16 *)buff;
+                               buff += 2;
+                       }
+                       if (len & 1)
+                               temp64 += *(u8 *)buff;
+                       goto reduce_to32;
+               }
+               temp64 += *(u64 *)(buff + len - 8) << (8 - (len & 7)) * 8;

+       }
+
        while (unlikely(len >= 64)) {
                asm("addq 0*8(%[src]),%[res]\n\t"
                    "adcq 1*8(%[src]),%[res]\n\t"

@@ -82,43 +100,8 @@ __wsum csum_partial(const void *buff, int len, __wsum sum)
                        : "memory");
                buff += 8;
        }
-       if (len & 7) {
-#ifdef CONFIG_DCACHE_WORD_ACCESS
-               unsigned int shift = (8 - (len & 7)) * 8;
-               unsigned long trail;
-
-               trail = (load_unaligned_zeropad(buff) << shift) >> shift;

-               asm("addq %[trail],%[res]\n\t"
-                   "adcq $0,%[res]"
-                       : [res] "+r" (temp64)
-                       : [trail] "r" (trail));
-#else
-               if (len & 4) {
-                       asm("addq %[val],%[res]\n\t"
-                           "adcq $0,%[res]"
-                               : [res] "+r" (temp64)
-                               : [val] "r" ((u64)*(u32 *)buff)
-                               : "memory");
-                       buff += 4;
-               }
-               if (len & 2) {
-                       asm("addq %[val],%[res]\n\t"
-                           "adcq $0,%[res]"
-                               : [res] "+r" (temp64)
-                               : [val] "r" ((u64)*(u16 *)buff)
-                               : "memory");
-                       buff += 2;
-               }
-               if (len & 1) {
-                       asm("addq %[val],%[res]\n\t"
-                           "adcq $0,%[res]"
-                               : [res] "+r" (temp64)
-                               : [val] "r" ((u64)*(u8 *)buff)
-                               : "memory");
-               }
-#endif
-       }
+reduce_to32:
        result = add32_with_carry(temp64 >> 32, temp64 & 0xffffffff);
        return (__force __wsum)result;
 }
--

2.17.1

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)