AArch64: Optimize memcmp

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State New
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  • AArch64: Optimize memcmp
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Commit Message

H.J. Lu via Libc-alpha Nov. 17, 2021, 2:12 p.m.
Rewrite memcmp to improve performance. On small and medium inputs performance
is 10-20% better. Large inputs use a SIMD loop processing 64 bytes per iteration,
which is 30-50% faster depending on the size.

Passes regress, OK for commit?

---

Comments

H.J. Lu via Libc-alpha Dec. 1, 2021, 11:14 a.m. | #1
The 11/17/2021 14:12, Wilco Dijkstra via Libc-alpha wrote:
> Rewrite memcmp to improve performance. On small and medium inputs performance

> is 10-20% better. Large inputs use a SIMD loop processing 64 bytes per iteration,

> which is 30-50% faster depending on the size.

> 

> Passes regress, OK for commit?


thanks, this is ok for master.

Patch

diff --git a/sysdeps/aarch64/memcmp.S b/sysdeps/aarch64/memcmp.S
index c1937f6f5c103a6f74383d7d40aeca1b5ad6ff59..c7d56a8af01b4f5e7fd5cc45407d52dbdfa91e98 100644
--- a/sysdeps/aarch64/memcmp.S
+++ b/sysdeps/aarch64/memcmp.S
@@ -22,105 +22,79 @@ 
 
 /* Assumptions:
  *
- * ARMv8-a, AArch64, unaligned accesses.
+ * ARMv8-a, AArch64, Advanced SIMD, unaligned accesses.
  */
 
-/* Parameters and result.  */
-#define src1		x0
-#define src2		x1
-#define limit		x2
-#define result		w0
-
-/* Internal variables.  */
-#define data1		x3
-#define data1w		w3
-#define data1h		x4
-#define data2		x5
-#define data2w		w5
-#define data2h		x6
-#define tmp1		x7
-#define tmp2		x8
-
-ENTRY_ALIGN (memcmp, 6)
+#define src1	x0
+#define src2	x1
+#define limit	x2
+#define result	w0
+
+#define data1	x3
+#define data1w	w3
+#define data2	x4
+#define data2w	w4
+#define data3	x5
+#define data3w	w5
+#define data4	x6
+#define data4w	w6
+#define tmp	x6
+#define src1end	x7
+#define src2end	x8
+
+
+ENTRY (memcmp)
 	PTR_ARG (0)
 	PTR_ARG (1)
 	SIZE_ARG (2)
 
-	subs	limit, limit, 16
+	cmp	limit, 16
 	b.lo	L(less16)
-
-	ldp	data1, data1h, [src1], 16
-	ldp	data2, data2h, [src2], 16
+	ldp	data1, data3, [src1]
+	ldp	data2, data4, [src2]
 	ccmp	data1, data2, 0, ne
-	ccmp	data1h, data2h, 0, eq
-	b.ne	L(return64)
+	ccmp	data3, data4, 0, eq
+	b.ne	L(return2)
 
-	subs	limit, limit, 16
+	add	src1end, src1, limit
+	add	src2end, src2, limit
+	cmp	limit, 32
 	b.ls	L(last_bytes)
-	cmp	limit, 112
-	b.lo	L(loop16)
-
-	and	tmp1, src1, 15
-	add	limit, limit, tmp1
-	sub	src1, src1, tmp1
-	sub	src2, src2, tmp1
-	subs	limit, limit, 48
+	cmp	limit, 160
+	b.hs	L(loop_align)
+	sub	limit, limit, 32
 
-	/* Compare 128 up bytes using aligned access. */
 	.p2align 4
-L(loop64):
-	ldp	data1, data1h, [src1]
-	ldp	data2, data2h, [src2]
-	cmp	data1, data2
-	ccmp	data1h, data2h, 0, eq
-	b.ne	L(return64)
-
-	ldp	data1, data1h, [src1, 16]
-	ldp	data2, data2h, [src2, 16]
-	cmp	data1, data2
-	ccmp	data1h, data2h, 0, eq
-	b.ne	L(return64)
-
-	ldp	data1, data1h, [src1, 32]
-	ldp	data2, data2h, [src2, 32]
-	cmp	data1, data2
-	ccmp	data1h, data2h, 0, eq
-	b.ne	L(return64)
-
-	ldp	data1, data1h, [src1, 48]
-	ldp	data2, data2h, [src2, 48]
+L(loop32):
+	ldp	data1, data3, [src1, 16]
+	ldp	data2, data4, [src2, 16]
 	cmp	data1, data2
-	ccmp	data1h, data2h, 0, eq
-	b.ne	L(return64)
+	ccmp	data3, data4, 0, eq
+	b.ne	L(return2)
+	cmp	limit, 16
+	b.ls	L(last_bytes)
 
-	subs	limit, limit, 64
-	add	src1, src1, 64
-	add	src2, src2, 64
-	b.pl	L(loop64)
-	adds	limit, limit, 48
-	b.lo	L(last_bytes)
-
-L(loop16):
-	ldp	data1, data1h, [src1], 16
-	ldp	data2, data2h, [src2], 16
+	ldp	data1, data3, [src1, 32]
+	ldp	data2, data4, [src2, 32]
 	cmp	data1, data2
-	ccmp	data1h, data2h, 0, eq
-	b.ne	L(return64)
+	ccmp	data3, data4, 0, eq
+	b.ne	L(return2)
+	add	src1, src1, 32
+	add	src2, src2, 32
+L(last64):
+	subs	limit, limit, 32
+	b.hi	L(loop32)
 
-	subs	limit, limit, 16
-	b.hi	L(loop16)
 	/* Compare last 1-16 bytes using unaligned access.  */
 L(last_bytes):
-	add	src1, src1, limit
-	add	src2, src2, limit
-	ldp	data1, data1h, [src1]
-	ldp	data2, data2h, [src2]
+	ldp	data1, data3, [src1end, -16]
+	ldp	data2, data4, [src2end, -16]
+L(return2):
+	cmp	data1, data2
+	csel	data1, data1, data3, ne
+	csel	data2, data2, data4, ne
 
 	/* Compare data bytes and set return value to 0, -1 or 1.  */
-L(return64):
-	cmp	data1, data2
-	csel	data1, data1, data1h, ne
-	csel	data2, data2, data2h, ne
 L(return):
 #ifndef __AARCH64EB__
 	rev	data1, data1
@@ -133,45 +107,98 @@  L(return):
 
 	.p2align 4
 L(less16):
-	adds	limit, limit, 8
-	b.lo	L(less8)		//lo:<
+	add	src1end, src1, limit
+	add	src2end, src2, limit
+	tbz	limit, 3, L(less8)
 	ldr	data1, [src1]
 	ldr	data2, [src2]
-	/* equal 8 optimized */
-	ccmp	data1, data2, 0, ne
-	b.ne	L(return)
-
-	ldr	data1, [src1, limit]
-	ldr	data2, [src2, limit]
-	b	L(return)
+	ldr	data3, [src1end, -8]
+	ldr	data4, [src2end, -8]
+	b	L(return2)
 
 	.p2align 4
 L(less8):
-	adds	limit, limit, 4
-	b.lo	L(less4)
+	tbz	limit, 2, L(less4)
 	ldr	data1w, [src1]
 	ldr	data2w, [src2]
-	ccmp	data1w, data2w, 0, ne
-	b.ne	L(return)
-	ldr	data1w,	[src1, limit]
-	ldr	data2w,	[src2, limit]
-	b	L(return)
+	ldr	data3w, [src1end, -4]
+	ldr	data4w, [src2end, -4]
+	b	L(return2)
 
-	.p2align 4
 L(less4):
-	adds	limit, limit, 4
-	b.eq	L(ret_0)
-
-L(byte_loop):
-	ldrb	data1w, [src1], 1
-	ldrb	data2w, [src2], 1
-	subs	limit, limit, 1
-	ccmp	data1w, data2w, 0, ne	/* NZCV = 0b0000.  */
-	b.eq	L(byte_loop)
+	tbz	limit, 1, L(less2)
+	ldrh	data1w, [src1]
+	ldrh	data2w, [src2]
+	cmp	data1w, data2w
+	b.ne	L(return)
+L(less2):
+	mov	result, 0
+	tbz	limit, 0, L(return_zero)
+	ldrb	data1w, [src1end, -1]
+	ldrb	data2w, [src2end, -1]
 	sub	result, data1w, data2w
+L(return_zero):
 	ret
-L(ret_0):
-	mov	result, 0
+
+L(loop_align):
+	ldp	data1, data3, [src1, 16]
+	ldp	data2, data4, [src2, 16]
+	cmp	data1, data2
+	ccmp	data3, data4, 0, eq
+	b.ne	L(return2)
+
+	/* Align src2 and adjust src1, src2 and limit.  */
+	and	tmp, src2, 15
+	sub	tmp, tmp, 16
+	sub	src2, src2, tmp
+	add	limit, limit, tmp
+	sub	src1, src1, tmp
+	sub	limit, limit, 64 + 16
+
+	.p2align 4
+L(loop64):
+	ldr	q0, [src1, 16]
+	ldr	q1, [src2, 16]
+	subs	limit, limit, 64
+	ldr	q2, [src1, 32]
+	ldr	q3, [src2, 32]
+	eor	v0.16b, v0.16b, v1.16b
+	eor	v1.16b, v2.16b, v3.16b
+	ldr	q2, [src1, 48]
+	ldr	q3, [src2, 48]
+	umaxp	v0.16b, v0.16b, v1.16b
+	ldr	q4, [src1, 64]!
+	ldr	q5, [src2, 64]!
+	eor	v1.16b, v2.16b, v3.16b
+	eor	v2.16b, v4.16b, v5.16b
+	umaxp	v1.16b, v1.16b, v2.16b
+	umaxp	v0.16b, v0.16b, v1.16b
+	umaxp	v0.16b, v0.16b, v0.16b
+	fmov	tmp, d0
+	ccmp	tmp, 0, 0, hi
+	b.eq	L(loop64)
+
+	/* If equal, process last 1-64 bytes using scalar loop.  */
+	add	limit, limit, 64 + 16
+	cbz	tmp, L(last64)
+
+	/* Determine the 8-byte aligned offset of the first difference.  */
+#ifdef __AARCH64EB__
+	rev16	tmp, tmp
+#endif
+	rev	tmp, tmp
+	clz	tmp, tmp
+	bic	tmp, tmp, 7
+	sub	tmp, tmp, 48
+	ldr	data1, [src1, tmp]
+	ldr	data2, [src2, tmp]
+#ifndef __AARCH64EB__
+	rev	data1, data1
+	rev	data2, data2
+#endif
+	mov	result, 1
+	cmp	data1, data2
+	cneg	result, result, lo
 	ret
 
 END (memcmp)