aarch64: MTE compatible strncmp

Add support for MTE to strncmp. Regression tested with xcheck and benchmarked
with glibc's benchtests on the Cortex-A53, Cortex-A72, and Neoverse N1.

The existing implementation assumes that any access to the pages in which the
string resides is safe. This assumption is not true when MTE is enabled. This
patch updates the algorithm to ensure that accesses remain within the bounds
of an MTE tag (16-byte chunks) and improves overall performance.

Co-authored-by: Branislav Rankov <branislav.rankov@arm.com>
Co-authored-by: Wilco Dijkstra <wilco.dijkstra@arm.com>

1 files changed, 145 insertions, 99 deletions

diff --git a/sysdeps/aarch64/strncmp.S b/sysdeps/aarch64/strncmp.S
index c5141fab8a..ba2563490e 100644
--- a/sysdeps/aarch64/strncmp.S
+++ b/sysdeps/aarch64/strncmp.S
@@ -25,7 +25,6 @@
 
 #define REP8_01 0x0101010101010101
 #define REP8_7f 0x7f7f7f7f7f7f7f7f
-#define REP8_80 0x8080808080808080
 
 /* Parameters and result.  */
 #define src1		x0
@@ -46,15 +45,31 @@
 #define tmp3		x10
 #define zeroones	x11
 #define pos		x12
-#define limit_wd	x13
-#define mask		x14
-#define endloop		x15
+#define mask		x13
+#define endloop		x14
 #define count		mask
+#define offset		pos
+#define neg_offset	x15
 
-ENTRY_ALIGN_AND_PAD (strncmp, 6, 7)
-	DELOUSE (0)
-	DELOUSE (1)
-	DELOUSE (2)
+/* Define endian dependent shift operations.
+   On big-endian early bytes are at MSB and on little-endian LSB.
+   LS_FW means shifting towards early bytes.
+   LS_BK means shifting towards later bytes.
+   */
+#ifdef __AARCH64EB__
+#define LS_FW lsl
+#define LS_BK lsr
+#else
+#define LS_FW lsr
+#define LS_BK lsl
+#endif
+
+	.text
+	.p2align 6
+	.rep 9
+	nop	/* Pad so that the loop below fits a cache line.  */
+	.endr
+ENTRY_ALIGN (strncmp, 0)
 	cbz	limit, L(ret0)
 	eor	tmp1, src1, src2
 	mov	zeroones, #REP8_01
@@ -62,9 +77,6 @@ ENTRY_ALIGN_AND_PAD (strncmp, 6, 7)
 	and	count, src1, #7
 	b.ne	L(misaligned8)
 	cbnz	count, L(mutual_align)
-	/* Calculate the number of full and partial words -1.  */
-	sub	limit_wd, limit, #1	/* limit != 0, so no underflow.  */
-	lsr	limit_wd, limit_wd, #3	/* Convert to Dwords.  */
 
 	/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
 	   (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
@@ -74,56 +86,52 @@ L(loop_aligned):
 	ldr	data1, [src1], #8
 	ldr	data2, [src2], #8
 L(start_realigned):
-	subs	limit_wd, limit_wd, #1
+	subs	limit, limit, #8
 	sub	tmp1, data1, zeroones
 	orr	tmp2, data1, #REP8_7f
 	eor	diff, data1, data2	/* Non-zero if differences found.  */
-	csinv	endloop, diff, xzr, pl	/* Last Dword or differences.  */
+	csinv	endloop, diff, xzr, hi	/* Last Dword or differences.  */
 	bics	has_nul, tmp1, tmp2	/* Non-zero if NUL terminator.  */
 	ccmp	endloop, #0, #0, eq
 	b.eq	L(loop_aligned)
 	/* End of performance-critical section  -- one 64B cache line.  */
 
-	/* Not reached the limit, must have found the end or a diff.  */
-	tbz	limit_wd, #63, L(not_limit)
-
-	/* Limit % 8 == 0 => all bytes significant.  */
-	ands	limit, limit, #7
-	b.eq	L(not_limit)
-
-	lsl	limit, limit, #3	/* Bits -> bytes.  */
-	mov	mask, #~0
-#ifdef __AARCH64EB__
-	lsr	mask, mask, limit
-#else
-	lsl	mask, mask, limit
-#endif
-	bic	data1, data1, mask
-	bic	data2, data2, mask
-
-	/* Make sure that the NUL byte is marked in the syndrome.  */
-	orr	has_nul, has_nul, mask
-
-L(not_limit):
+L(full_check):
+#ifndef __AARCH64EB__
 	orr	syndrome, diff, has_nul
-
-#ifndef	__AARCH64EB__
+	add	limit, limit, 8	/* Rewind limit to before last subs. */
+L(syndrome_check):
+	/* Limit was reached. Check if the NUL byte or the difference
+	   is before the limit. */
 	rev	syndrome, syndrome
 	rev	data1, data1
-	/* The MS-non-zero bit of the syndrome marks either the first bit
-	   that is different, or the top bit of the first zero byte.
-	   Shifting left now will bring the critical information into the
-	   top bits.  */
 	clz	pos, syndrome
 	rev	data2, data2
 	lsl	data1, data1, pos
+	cmp	limit, pos, lsr #3
 	lsl	data2, data2, pos
 	/* But we need to zero-extend (char is unsigned) the value and then
 	   perform a signed 32-bit subtraction.  */
 	lsr	data1, data1, #56
 	sub	result, data1, data2, lsr #56
-	RET
+	csel result, result, xzr, hi
+	ret
 #else
+	/* Not reached the limit, must have found the end or a diff.  */
+	tbz	limit, #63, L(not_limit)
+	add	tmp1, limit, 8
+	cbz	limit, L(not_limit)
+
+	lsl	limit, tmp1, #3	/* Bits -> bytes.  */
+	mov	mask, #~0
+	lsr	mask, mask, limit
+	bic	data1, data1, mask
+	bic	data2, data2, mask
+
+	/* Make sure that the NUL byte is marked in the syndrome.  */
+	orr	has_nul, has_nul, mask
+
+L(not_limit):
 	/* For big-endian we cannot use the trick with the syndrome value
 	   as carry-propagation can corrupt the upper bits if the trailing
 	   bytes in the string contain 0x01.  */
@@ -134,7 +142,7 @@ L(not_limit):
 	cmp	data1, data2
 	cset	result, ne
 	cneg	result, result, lo
-	RET
+	ret
 1:
 	/* Re-compute the NUL-byte detection, using a byte-reversed value.  */
 	rev	tmp3, data1
@@ -144,17 +152,18 @@ L(not_limit):
 	rev	has_nul, has_nul
 	orr	syndrome, diff, has_nul
 	clz	pos, syndrome
-	/* The MS-non-zero bit of the syndrome marks either the first bit
-	   that is different, or the top bit of the first zero byte.
+	/* The most-significant-non-zero bit of the syndrome marks either the
+	   first bit that is different, or the top bit of the first zero byte.
 	   Shifting left now will bring the critical information into the
 	   top bits.  */
+L(end_quick):
 	lsl	data1, data1, pos
 	lsl	data2, data2, pos
 	/* But we need to zero-extend (char is unsigned) the value and then
 	   perform a signed 32-bit subtraction.  */
 	lsr	data1, data1, #56
 	sub	result, data1, data2, lsr #56
-	RET
+	ret
 #endif
 
 L(mutual_align):
@@ -169,22 +178,12 @@ L(mutual_align):
 	neg	tmp3, count, lsl #3	/* 64 - bits(bytes beyond align). */
 	ldr	data2, [src2], #8
 	mov	tmp2, #~0
-	sub	limit_wd, limit, #1	/* limit != 0, so no underflow.  */
-#ifdef __AARCH64EB__
-	/* Big-endian.  Early bytes are at MSB.  */
-	lsl	tmp2, tmp2, tmp3	/* Shift (count & 63).  */
-#else
-	/* Little-endian.  Early bytes are at LSB.  */
-	lsr	tmp2, tmp2, tmp3	/* Shift (count & 63).  */
-#endif
-	and	tmp3, limit_wd, #7
-	lsr	limit_wd, limit_wd, #3
-	/* Adjust the limit. Only low 3 bits used, so overflow irrelevant.  */
-	add	limit, limit, count
-	add	tmp3, tmp3, count
+	LS_FW	tmp2, tmp2, tmp3	/* Shift (count & 63).  */
+	/* Adjust the limit and ensure it doesn't overflow.  */
+	adds	limit, limit, count
+	csinv	limit, limit, xzr, lo
 	orr	data1, data1, tmp2
 	orr	data2, data2, tmp2
-	add	limit_wd, limit_wd, tmp3, lsr #3
 	b	L(start_realigned)
 
 	.p2align 6
@@ -203,18 +202,15 @@ L(byte_loop):
 	b.eq	L(byte_loop)
 L(done):
 	sub	result, data1, data2
-	RET
-
+	ret
 	/* Align the SRC1 to a dword by doing a bytewise compare and then do
 	   the dword loop.  */
 L(try_misaligned_words):
-	lsr	limit_wd, limit, #3
-	cbz	count, L(do_misaligned)
+	cbz	count, L(src1_aligned)
 
 	neg	count, count
 	and	count, count, #7
 	sub	limit, limit, count
-	lsr	limit_wd, limit, #3
 
 L(page_end_loop):
 	ldrb	data1w, [src1], #1
@@ -225,48 +221,98 @@ L(page_end_loop):
 	subs	count, count, #1
 	b.hi	L(page_end_loop)
 
-L(do_misaligned):
-	/* Prepare ourselves for the next page crossing.  Unlike the aligned
-	   loop, we fetch 1 less dword because we risk crossing bounds on
-	   SRC2.  */
-	mov	count, #8
-	subs	limit_wd, limit_wd, #1
-	b.lo	L(done_loop)
+	/* The following diagram explains the comparison of misaligned strings.
+	   The bytes are shown in natural order. For little-endian, it is
+	   reversed in the registers. The "x" bytes are before the string.
+	   The "|" separates data that is loaded at one time.
+	   src1     | a a a a a a a a | b b b c c c c c | . . .
+	   src2     | x x x x x a a a   a a a a a b b b | c c c c c . . .
+	   After shifting in each step, the data looks like this:
+	                STEP_A              STEP_B              STEP_C
+	   data1    a a a a a a a a     b b b c c c c c     b b b c c c c c
+	   data2    a a a a a a a a     b b b 0 0 0 0 0     0 0 0 c c c c c
+	   The bytes with "0" are eliminated from the syndrome via mask.
+	   Align SRC2 down to 16 bytes. This way we can read 16 bytes at a
+	   time from SRC2. The comparison happens in 3 steps. After each step
+	   the loop can exit, or read from SRC1 or SRC2. */
+L(src1_aligned):
+	/* Calculate offset from 8 byte alignment to string start in bits. No
+	   need to mask offset since shifts are ignoring upper bits. */
+	lsl	offset, src2, #3
+	bic	src2, src2, #0xf
+	mov	mask, -1
+	neg	neg_offset, offset
+	ldr	data1, [src1], #8
+	ldp	tmp1, tmp2, [src2], #16
+	LS_BK	mask, mask, neg_offset
+	and	neg_offset, neg_offset, #63	/* Need actual value for cmp later. */
+	/* Skip the first compare if data in tmp1 is irrelevant. */
+	tbnz	offset, 6, L(misaligned_mid_loop)
+
 L(loop_misaligned):
-	and	tmp2, src2, #0xff8
-	eor	tmp2, tmp2, #0xff8
-	cbz	tmp2, L(page_end_loop)
+	/* STEP_A: Compare full 8 bytes when there is enough data from SRC2.*/
+	LS_FW	data2, tmp1, offset
+	LS_BK	tmp1, tmp2, neg_offset
+	subs	limit, limit, #8
+	orr	data2, data2, tmp1	/* 8 bytes from SRC2 combined from two regs.*/
+	sub	has_nul, data1, zeroones
+	eor	diff, data1, data2	/* Non-zero if differences found.  */
+	orr	tmp3, data1, #REP8_7f
+	csinv	endloop, diff, xzr, hi	/* If limit, set to all ones. */
+	bic	has_nul, has_nul, tmp3	/* Non-zero if NUL byte found in SRC1. */
+	orr	tmp3, endloop, has_nul
+	cbnz	tmp3, L(full_check)
 
 	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.  */
-	bics	has_nul, tmp1, tmp2	/* Non-zero if NUL terminator.  */
-	ccmp	diff, #0, #0, eq
-	b.ne	L(not_limit)
-	subs	limit_wd, limit_wd, #1
-	b.pl	L(loop_misaligned)
+L(misaligned_mid_loop):
+	/* STEP_B: Compare first part of data1 to second part of tmp2. */
+	LS_FW	data2, tmp2, offset
+#ifdef __AARCH64EB__
+	/* For big-endian we do a byte reverse to avoid carry-propagation
+	problem described above. This way we can reuse the has_nul in the
+	next step and also use syndrome value trick at the end. */
+	rev	tmp3, data1
+	#define data1_fixed tmp3
+#else
+	#define data1_fixed data1
+#endif
+	sub	has_nul, data1_fixed, zeroones
+	orr	tmp3, data1_fixed, #REP8_7f
+	eor	diff, data2, data1	/* Non-zero if differences found.  */
+	bic	has_nul, has_nul, tmp3	/* Non-zero if NUL terminator.  */
+#ifdef __AARCH64EB__
+	rev	has_nul, has_nul
+#endif
+	cmp	limit, neg_offset, lsr #3
+	orr	syndrome, diff, has_nul
+	bic	syndrome, syndrome, mask	/* Ignore later bytes. */
+	csinv	tmp3, syndrome, xzr, hi	/* If limit, set to all ones. */
+	cbnz	tmp3, L(syndrome_check)
 
-L(done_loop):
-	/* We found a difference or a NULL before the limit was reached.  */
-	and	limit, limit, #7
-	cbz	limit, L(not_limit)
-	/* Read the last word.  */
-	sub	src1, src1, 8
-	sub	src2, src2, 8
-	ldr	data1, [src1, limit]
-	ldr	data2, [src2, limit]
-	sub	tmp1, data1, zeroones
-	orr	tmp2, data1, #REP8_7f
-	eor	diff, data1, data2	/* Non-zero if differences found.  */
-	bics	has_nul, tmp1, tmp2	/* Non-zero if NUL terminator.  */
-	ccmp	diff, #0, #0, eq
-	b.ne	L(not_limit)
+	/* STEP_C: Compare second part of data1 to first part of tmp1. */
+	ldp	tmp1, tmp2, [src2], #16
+	cmp	limit, #8
+	LS_BK	data2, tmp1, neg_offset
+	eor	diff, data2, data1	/* Non-zero if differences found.  */
+	orr	syndrome, diff, has_nul
+	and	syndrome, syndrome, mask	/* Ignore earlier bytes. */
+	csinv	tmp3, syndrome, xzr, hi	/* If limit, set to all ones. */
+	cbnz	tmp3, L(syndrome_check)
+
+	ldr	data1, [src1], #8
+	sub	limit, limit, #8
+	b	L(loop_misaligned)
+
+#ifdef	__AARCH64EB__
+L(syndrome_check):
+	clz	pos, syndrome
+	cmp	pos, limit, lsl #3
+	b.lo	L(end_quick)
+#endif
 
 L(ret0):
 	mov	result, #0
-	RET
+	ret
 
 END (strncmp)
 libc_hidden_builtin_def (strncmp)