[PATCH 1/3] powerpc: Optimise 64bit csum_partial
Anton Blanchard
anton at samba.org
Tue Aug 3 16:08:34 EST 2010
The main loop of csum_partial runs very slowly on recent POWER CPUs. After some
analysis on both POWER6 and POWER7 I came up with routine below. First we get
the source aligned to a double word, ignoring any odd alignment to keep things
simple. Then we do 64 bytes at a time, with an entry and exit limb of a further
64 bytes. On both POWER6 and POWER7 this should be as fast as we can go since
we are limited by the latency of the adde instructions.
To test this I forced checksumming on over loopback and ran socklib (a
simple TCP benchmark). On a POWER6 575 throughput improved by 11% with
this patch.
Signed-off-by: Anton Blanchard <anton at samba.org>
--
Index: powerpc.git/arch/powerpc/lib/checksum_64.S
===================================================================
--- powerpc.git.orig/arch/powerpc/lib/checksum_64.S 2010-08-03 13:32:45.291991557 +1000
+++ powerpc.git/arch/powerpc/lib/checksum_64.S 2010-08-03 15:16:59.600753385 +1000
@@ -65,55 +65,168 @@ _GLOBAL(csum_tcpudp_magic)
srwi r3,r3,16
blr
+#define STACKFRAMESIZE 256
+#define STK_REG(i) (112 + ((i)-14)*8)
+
/*
* Computes the checksum of a memory block at buff, length len,
* and adds in "sum" (32-bit).
*
- * This code assumes at least halfword alignment, though the length
- * can be any number of bytes. The sum is accumulated in r5.
- *
* csum_partial(r3=buff, r4=len, r5=sum)
*/
_GLOBAL(csum_partial)
- subi r3,r3,8 /* we'll offset by 8 for the loads */
- srdi. r6,r4,3 /* divide by 8 for doubleword count */
- addic r5,r5,0 /* clear carry */
- beq 3f /* if we're doing < 8 bytes */
- andi. r0,r3,2 /* aligned on a word boundary already? */
- beq+ 1f
- lhz r6,8(r3) /* do 2 bytes to get aligned */
- addi r3,r3,2
- subi r4,r4,2
- addc r5,r5,r6
- srdi. r6,r4,3 /* recompute number of doublewords */
- beq 3f /* any left? */
-1: mtctr r6
-2: ldu r6,8(r3) /* main sum loop */
- adde r5,r5,r6
- bdnz 2b
- andi. r4,r4,7 /* compute bytes left to sum after doublewords */
-3: cmpwi 0,r4,4 /* is at least a full word left? */
- blt 4f
- lwz r6,8(r3) /* sum this word */
+ addic r0,r5,0 /* clear carry */
+
+ srdi. r6,r4,3 /* less than 8 bytes? */
+ beq .Lcsum_tail_word
+
+ /*
+ * If only halfword aligned, align to a double word. Since odd
+ * aligned addresses should be rare and they would require more
+ * work to calculate the correct checksum, we ignore that case
+ * and take the potential slowdown of unaligned loads.
+ */
+ rldicl. r6,r3,64-1,64-2 /* r6 = (r3 & 0x3) >> 1 */
+ beq .Lcsum_aligned
+
+ li r7,4
+ sub r6,r7,r6
+ mtctr r6
+
+1:
+ lhz r6,0(r3) /* align to doubleword */
+ subi r4,r4,2
+ addi r3,r3,2
+ adde r0,r0,r6
+ bdnz 1b
+
+.Lcsum_aligned:
+ /*
+ * We unroll the loop such that each iteration is 64 bytes with an
+ * entry and exit limb of 64 bytes, meaning a minimum size of
+ * 128 bytes.
+ */
+ srdi. r6,r4,7
+ beq .Lcsum_tail_doublewords /* len < 128 */
+
+ srdi r6,r4,6
+ subi r6,r6,1
+ mtctr r6
+
+ stdu r1,-STACKFRAMESIZE(r1)
+ std r14,STK_REG(r14)(r1)
+ std r15,STK_REG(r15)(r1)
+ std r16,STK_REG(r16)(r1)
+
+ ld r6,0(r3)
+ ld r9,8(r3)
+
+ ld r10,16(r3)
+ ld r11,24(r3)
+
+ /*
+ * On POWER6 and POWER7 back to back addes take 2 cycles because of
+ * the XER dependency. This means the fastest this loop can go is
+ * 16 cycles per iteration. The scheduling of the loop below has
+ * been shown to hit this on both POWER6 and POWER7.
+ */
+ .align 5
+2:
+ adde r0,r0,r6
+ ld r12,32(r3)
+ ld r14,40(r3)
+
+ adde r0,r0,r9
+ ld r15,48(r3)
+ ld r16,56(r3)
+ addi r3,r3,64
+
+ adde r0,r0,r10
+
+ adde r0,r0,r11
+
+ adde r0,r0,r12
+
+ adde r0,r0,r14
+
+ adde r0,r0,r15
+ ld r6,0(r3)
+ ld r9,8(r3)
+
+ adde r0,r0,r16
+ ld r10,16(r3)
+ ld r11,24(r3)
+ bdnz 2b
+
+
+ adde r0,r0,r6
+ ld r12,32(r3)
+ ld r14,40(r3)
+
+ adde r0,r0,r9
+ ld r15,48(r3)
+ ld r16,56(r3)
+ addi r3,r3,64
+
+ adde r0,r0,r10
+ adde r0,r0,r11
+ adde r0,r0,r12
+ adde r0,r0,r14
+ adde r0,r0,r15
+ adde r0,r0,r16
+
+ ld r14,STK_REG(r14)(r1)
+ ld r15,STK_REG(r15)(r1)
+ ld r16,STK_REG(r16)(r1)
+ addi r1,r1,STACKFRAMESIZE
+
+ andi. r4,r4,63
+
+.Lcsum_tail_doublewords: /* Up to 127 bytes to go */
+ srdi. r6,r4,3
+ beq .Lcsum_tail_word
+
+ mtctr r6
+3:
+ ld r6,0(r3)
+ addi r3,r3,8
+ adde r0,r0,r6
+ bdnz 3b
+
+ andi. r4,r4,7
+
+.Lcsum_tail_word: /* Up to 7 bytes to go */
+ srdi. r6,r4,2
+ beq .Lcsum_tail_halfword
+
+ lwz r6,0(r3)
addi r3,r3,4
+ adde r0,r0,r6
subi r4,r4,4
- adde r5,r5,r6
-4: cmpwi 0,r4,2 /* is at least a halfword left? */
- blt+ 5f
- lhz r6,8(r3) /* sum this halfword */
- addi r3,r3,2
- subi r4,r4,2
- adde r5,r5,r6
-5: cmpwi 0,r4,1 /* is at least a byte left? */
- bne+ 6f
- lbz r6,8(r3) /* sum this byte */
- slwi r6,r6,8 /* this byte is assumed to be the upper byte of a halfword */
- adde r5,r5,r6
-6: addze r5,r5 /* add in final carry */
- rldicl r4,r5,32,0 /* fold two 32-bit halves together */
- add r3,r4,r5
- srdi r3,r3,32
- blr
+
+.Lcsum_tail_halfword: /* Up to 3 bytes to go */
+ srdi. r6,r4,1
+ beq .Lcsum_tail_byte
+
+ lhz r6,0(r3)
+ addi r3,r3,2
+ adde r0,r0,r6
+ subi r4,r4,2
+
+.Lcsum_tail_byte: /* Up to 1 byte to go */
+ andi. r6,r4,1
+ beq .Lcsum_finish
+
+ lbz r6,0(r3)
+ sldi r9,r6,8 /* Pad the byte out to 16 bits */
+ adde r0,r0,r9
+
+.Lcsum_finish:
+ addze r0,r0 /* add in final carry */
+ rldicl r4,r0,32,0 /* fold two 32 bit halves together */
+ add r3,r4,r0
+ srdi r3,r3,32
+ blr
/*
* Computes the checksum of a memory block at src, length len,
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