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Subject[PATCH] x86: Optimise x86 IP checksum code
Date
Performance improvements to the amd64 IP checksum code.
Summing to alternate registers almost doubles the performance
(probably 4 to 6.2 bytes/clock) on Ivy Bridge cpu.
Loop carrying the carry flag improves Haswell from 7 to 8 bytes/clock.
Older cpu will still approach 4 bytes/clock.
All achieved with a less loop unrolling - improving the performance
for small buffers that are not a multiple of the loop span

Signed-off-by: David Laight <david.laight@aculab.com>
--

I spent far too long looking at this for some code that has to calculate the
UDP checksum before sending packets through a raw IPv4 socket.

Prior to Ivy (maybe Sandy) bridge adc always took two clocks, so the adc chain
can only run at 4 bytes/clock (the same as 32bit adds to a 64 bit register).
In Ivy bridge the 'carry' flag is available a clock earlier so 8 bytes/clock
is a potential limit.

In order to 'loop carry' the carry flag some ingenuity is needed.

I did get about 12 bytes/clock using adox/adcx but that would need run-time
patching and some AMD cpu that support the instructions run them very slowly.

arch/x86/lib/csum-partial_64.c | 192 +++++++++++++++++++++--------------------
1 file changed, 98 insertions(+), 94 deletions(-)

diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
index e7925d6..7f25ab2 100644
--- a/arch/x86/lib/csum-partial_64.c
+++ b/arch/x86/lib/csum-partial_64.c
@@ -10,113 +10,118 @@
#include <linux/export.h>
#include <asm/checksum.h>

-static inline unsigned short from32to16(unsigned a)
-{
- unsigned short b = a >> 16;
- asm("addw %w2,%w0\n\t"
- "adcw $0,%w0\n"
- : "=r" (b)
- : "0" (b), "r" (a));
- return b;
-}
-
/*
* Do a 64-bit checksum on an arbitrary memory area.
* Returns a 32bit checksum.
*
* This isn't as time critical as it used to be because many NICs
* do hardware checksumming these days.
+ *
+ * All Intel cpus prior to Ivy Bridge (mayby Sandy Bridge) have a 2 clock
+ * latency on the result of adc.
+ * This limits any adc loop to 4 bytes/clock - the same as a C loop
+ * that adds 32 bit values to a 64 bit register.
+ * On Ivy bridge the adc result latency is still 2 clocks, but the carry
+ * latency is reduced to 1 clock.
+ * So summing to alternate registers increases the throughput to approaching
+ * 8 bytes/clock.
+ * Older cpu (eg core 2) have a 6+ clock delay accessing any of the flags
+ * after a partial update (eg adc after inc).
+ * The stange 'jecxz' loop avoids this.
+ * The Ivy bridge then needs the loop unrolling once more to approach
+ * 8 bytes per clock.
*
- * Things tried and found to not make it faster:
- * Manual Prefetching
- * Unrolling to an 128 bytes inner loop.
- * Using interleaving with more registers to break the carry chains.
+ * On 7th gen cpu using adox/adoc can get 12 bytes/clock (maybe 16?)
+ * provided the loop is unrolled further than the one below.
+ * But some other cpu that support the adx take 6 clocks for each.
+ *
+ * The 'sum' value on entry is added in, it can exceed 32 bits but
+ * must not get to 56 bits.
*/
-static unsigned do_csum(const unsigned char *buff, unsigned len)
+static unsigned do_csum(const unsigned char *buff, long len, u64 sum)
{
- unsigned odd, count;
- unsigned long result = 0;
+ unsigned int src_align;
+ u64 sum_0 = 0, sum_1;
+ long len_tmp;
+ bool odd = false;

- if (unlikely(len == 0))
- return result;
- odd = 1 & (unsigned long) buff;
- if (unlikely(odd)) {
- result = *buff << 8;
- len--;
- buff++;
- }
- count = len >> 1; /* nr of 16-bit words.. */
- if (count) {
- if (2 & (unsigned long) buff) {
- result += *(unsigned short *)buff;
- count--;
- len -= 2;
- buff += 2;
+ /* 64bit align the base address */
+ src_align = (unsigned long)buff & 7;
+ if (src_align) {
+ if (unlikely(src_align & 1)) {
+ sum <<= 8;
+ /* The extra flag generates better code! */
+ odd = true;
}
- count >>= 1; /* nr of 32-bit words.. */
- if (count) {
- unsigned long zero;
- unsigned count64;
- if (4 & (unsigned long) buff) {
- result += *(unsigned int *) buff;
- count--;
- len -= 4;
- buff += 4;
- }
- count >>= 1; /* nr of 64-bit words.. */
+ buff -= src_align;
+ len += src_align;
+ if (likely(src_align == 4))
+ sum_0 = *(u32 *)(buff + 4);
+ else
+ /* Mask off unwanted low bytes from full 64bit word */
+ sum_0 = *(u64 *)buff & (~0ull << (src_align * 8));
+ if (unlikely(len < 8)) {
+ /* Mask off the unwanted high bytes */
+ sum += sum_0 & ~(~0ull << (len * 8));
+ goto reduce_32;
+ }
+ len -= 8;
+ buff += 8;
+ }

- /* main loop using 64byte blocks */
- zero = 0;
- count64 = count >> 3;
- while (count64) {
- asm("addq 0*8(%[src]),%[res]\n\t"
- "adcq 1*8(%[src]),%[res]\n\t"
- "adcq 2*8(%[src]),%[res]\n\t"
- "adcq 3*8(%[src]),%[res]\n\t"
- "adcq 4*8(%[src]),%[res]\n\t"
- "adcq 5*8(%[src]),%[res]\n\t"
- "adcq 6*8(%[src]),%[res]\n\t"
- "adcq 7*8(%[src]),%[res]\n\t"
- "adcq %[zero],%[res]"
- : [res] "=r" (result)
- : [src] "r" (buff), [zero] "r" (zero),
- "[res]" (result));
- buff += 64;
- count64--;
- }
+ /* Read first 8 bytes to 16 byte align the loop below */
+ sum_1 = len & 8 ? *(u64 *)buff : 0;

- /* last up to 7 8byte blocks */
- count %= 8;
- while (count) {
- asm("addq %1,%0\n\t"
- "adcq %2,%0\n"
- : "=r" (result)
- : "m" (*(unsigned long *)buff),
- "r" (zero), "0" (result));
- --count;
- buff += 8;
- }
- result = add32_with_carry(result>>32,
- result&0xffffffff);
+ /* The main loop uses negative offsets from the end of the buffer */
+ buff += len;

- if (len & 4) {
- result += *(unsigned int *) buff;
- buff += 4;
- }
- }
- if (len & 2) {
- result += *(unsigned short *) buff;
- buff += 2;
- }
+ /* Add in trailing bytes to 64bit align the length */
+ if (len & 7) {
+ unsigned int tail_len = len & 7;
+ buff -= tail_len;
+ if (likely(tail_len == 4))
+ sum += *(u32 *)buff;
+ else
+ /* Mask off the unwanted high bytes */
+ sum += *(u64 *)buff & ~(~0ull << (tail_len * 8));
}
- if (len & 1)
- result += *buff;
- result = add32_with_carry(result>>32, result & 0xffffffff);
- if (unlikely(odd)) {
- result = from32to16(result);
- result = ((result >> 8) & 0xff) | ((result & 0xff) << 8);
- }
- return result;
+
+ /* Align and negate len so that we need to sum [buff[len]..buf[0]) */
+ len = -(len & ~15);
+
+ /*
+ * Align the byte count to a multiple of 16 then
+ * add 64 bit words to alternating registers.
+ * Finally reduce to 64 bits.
+ */
+ asm( " bt $4, %[len]\n"
+ " jnc 10f\n"
+ " add (%[buff], %[len]), %[sum_0]\n"
+ " adc 8(%[buff], %[len]), %[sum_1]\n"
+ " lea 16(%[len]), %[len]\n"
+ "10: jecxz 20f\n"
+ " adc (%[buff], %[len]), %[sum_0]\n"
+ " adc 8(%[buff], %[len]), %[sum_1]\n"
+ " lea 32(%[len]), %[len_tmp]\n"
+ " adc 16(%[buff], %[len]), %[sum_0]\n"
+ " adc 24(%[buff], %[len]), %[sum_1]\n"
+ " mov %[len_tmp], %[len]\n"
+ " jmp 10b\n"
+ "20: adc %[sum_0], %[sum]\n"
+ " adc %[sum_1], %[sum]\n"
+ " adc $0, %[sum]\n"
+ : [sum] "+&r" (sum), [sum_0] "+&r" (sum_0), [sum_1] "+&r" (sum_1),
+ [len] "+&c" (len), [len_tmp] "=&r" (len_tmp)
+ : [buff] "r" (buff)
+ : "memory" );
+
+reduce_32:
+ sum = add32_with_carry(sum>>32, sum & 0xffffffff);
+
+ if (unlikely(odd))
+ return __builtin_bswap32(sum);
+
+ return sum;
}

/*
@@ -133,8 +138,7 @@ static unsigned do_csum(const unsigned char *buff, unsigned len)
*/
__wsum csum_partial(const void *buff, int len, __wsum sum)
{
- return (__force __wsum)add32_with_carry(do_csum(buff, len),
- (__force u32)sum);
+ return do_csum(buff, len, (__force u32)sum);
}
EXPORT_SYMBOL(csum_partial);

--
1.8.1.2
-
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