[PATCH] aarch64: Improve strcmp unaligned performance

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

Replace the simple byte-wise compare in the misaligned case with a
dword compare with page boundary checks in place.  For simplicity I've
chosen a 4K page boundary so that we don't have to query the actual
page size on the system.

This results in up to 3x improvement in performance in the unaligned
case on falkor and about 2.5x improvement on mustang as measured using
bench-strcmp.

	* sysdeps/aarch64/strcmp.S (misaligned8): Compare dword at a
	time whenever possible.
---
 sysdeps/aarch64/strcmp.S | 31 +++++++++++++++++++++++++++++--
 1 file changed, 29 insertions(+), 2 deletions(-)

diff --git a/sysdeps/aarch64/strcmp.S b/sysdeps/aarch64/strcmp.S
index e99d662..c260e1d 100644
--- a/sysdeps/aarch64/strcmp.S
+++ b/sysdeps/aarch64/strcmp.S
@@ -72,6 +72,7 @@ L(start_realigned):
 	cbz	syndrome, L(loop_aligned)
 	/* End of performance-critical section  -- one 64B cache line.  */
 
+L(end):
 #ifndef	__AARCH64EB__
 	rev	syndrome, syndrome
 	rev	data1, data1
@@ -145,12 +146,38 @@ L(mutual_align):
 	b	L(start_realigned)
 
 L(misaligned8):
-	/* We can do better than this.  */
+	/* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
+	   checking to make sure that we don't access beyond page boundary in
+	   SRC2.  */
+	tst	src1, #7
+	b.eq	L(loop_misaligned)
+L(do_misaligned):
 	ldrb	data1w, [src1], #1
 	ldrb	data2w, [src2], #1
 	cmp	data1w, #1
 	ccmp	data1w, data2w, #0, cs	/* NZCV = 0b0000.  */
-	b.eq	L(misaligned8)
+	b.ne	L(done)
+	tst	src1, #7
+	b.ne	L(misaligned8)
+
+L(loop_misaligned):
+	/* Test if we are within the last dword of the end of a 4K page.  If
+	   yes then jump back to the misaligned loop to copy a byte at a time.  */
+	and	tmp1, src2, #0xff8
+	eor	tmp1, tmp1, #0xff8
+	cbz	tmp1, L(do_misaligned)
+	ldr	data1, [src1], #8
+	ldr	data2, [src2], #8
+
+	sub	tmp1, data1, zeroones
+	orr	tmp2, data1, #REP8_7f
+	eor	diff, data1, data2	/* Non-zero if differences found.  */
+	bic	has_nul, tmp1, tmp2	/* Non-zero if NUL terminator.  */
+	orr	syndrome, diff, has_nul
+	cbz	syndrome, L(loop_misaligned)
+	b	L(end)
+
+L(done):
 	sub	result, data1, data2
 	RET
 END(strcmp)
-- 
2.7.5