/* Assumptions:
*
- * ARMv8-a, AArch64, unaligned accesses, min page size 4k.
+ * ARMv8-a, AArch64, Advanced SIMD.
+ * MTE compatible.
*/
#ifndef STRLEN
# define STRLEN __strlen
#endif
-/* To test the page crossing code path more thoroughly, compile with
- -DTEST_PAGE_CROSS - this will force all calls through the slower
- entry path. This option is not intended for production use. */
-
-/* Arguments and results. */
#define srcin x0
-#define len x0
+#define result x0
-/* Locals and temporaries. */
#define src x1
-#define data1 x2
-#define data2 x3
-#define has_nul1 x4
-#define has_nul2 x5
-#define tmp1 x4
-#define tmp2 x5
-#define tmp3 x6
-#define tmp4 x7
-#define zeroones x8
-
- /* NUL detection works on the principle that (X - 1) & (~X) & 0x80
- (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
- can be done in parallel across the entire word. A faster check
- (X - 1) & 0x80 is zero for non-NUL ASCII characters, but gives
- false hits for characters 129..255. */
-
-#define REP8_01 0x0101010101010101
-#define REP8_7f 0x7f7f7f7f7f7f7f7f
-#define REP8_80 0x8080808080808080
-
-#ifdef TEST_PAGE_CROSS
-# define MIN_PAGE_SIZE 16
-#else
-# define MIN_PAGE_SIZE 4096
-#endif
-
- /* Since strings are short on average, we check the first 16 bytes
- of the string for a NUL character. In order to do an unaligned ldp
- safely we have to do a page cross check first. If there is a NUL
- byte we calculate the length from the 2 8-byte words using
- conditional select to reduce branch mispredictions (it is unlikely
- strlen will be repeatedly called on strings with the same length).
-
- If the string is longer than 16 bytes, we align src so don't need
- further page cross checks, and process 32 bytes per iteration
- using the fast NUL check. If we encounter non-ASCII characters,
- fallback to a second loop using the full NUL check.
-
- If the page cross check fails, we read 16 bytes from an aligned
- address, remove any characters before the string, and continue
- in the main loop using aligned loads. Since strings crossing a
- page in the first 16 bytes are rare (probability of
- 16/MIN_PAGE_SIZE ~= 0.4%), this case does not need to be optimized.
-
- AArch64 systems have a minimum page size of 4k. We don't bother
- checking for larger page sizes - the cost of setting up the correct
- page size is just not worth the extra gain from a small reduction in
- the cases taking the slow path. Note that we only care about
- whether the first fetch, which may be misaligned, crosses a page
- boundary. */
-
-ENTRY_ALIGN (STRLEN, 6)
+#define synd x2
+#define tmp x3
+#define wtmp w3
+#define shift x4
+
+#define data q0
+#define vdata v0
+#define vhas_nul v1
+#define vrepmask v2
+#define vend v3
+#define dend d3
+
+/* Core algorithm:
+
+ For each 16-byte chunk we calculate a 64-bit syndrome value with four bits
+ per byte. For even bytes, bits 0-3 are set if the relevant byte matched the
+ requested character or the byte is NUL. Bits 4-7 must be zero. Bits 4-7 are
+ set likewise for odd bytes so that adjacent bytes can be merged. Since the
+ bits in the syndrome reflect the order in which things occur in the original
+ string, counting trailing zeros identifies exactly which byte matched. */
+
+ENTRY (STRLEN)
DELOUSE (0)
DELOUSE (1)
- and tmp1, srcin, MIN_PAGE_SIZE - 1
- mov zeroones, REP8_01
- cmp tmp1, MIN_PAGE_SIZE - 16
- b.gt L(page_cross)
- ldp data1, data2, [srcin]
-#ifdef __AARCH64EB__
- /* For big-endian, carry propagation (if the final byte in the
- string is 0x01) means we cannot use has_nul1/2 directly.
- Since we expect strings to be small and early-exit,
- byte-swap the data now so has_null1/2 will be correct. */
- rev data1, data1
- rev data2, data2
-#endif
- sub tmp1, data1, zeroones
- orr tmp2, data1, REP8_7f
- sub tmp3, data2, zeroones
- orr tmp4, data2, REP8_7f
- bics has_nul1, tmp1, tmp2
- bic has_nul2, tmp3, tmp4
- ccmp has_nul2, 0, 0, eq
- beq L(main_loop_entry)
-
- /* Enter with C = has_nul1 == 0. */
- csel has_nul1, has_nul1, has_nul2, cc
- mov len, 8
- rev has_nul1, has_nul1
- clz tmp1, has_nul1
- csel len, xzr, len, cc
- add len, len, tmp1, lsr 3
+ bic src, srcin, 15
+ mov wtmp, 0xf00f
+ ld1 {vdata.16b}, [src]
+ dup vrepmask.8h, wtmp
+ cmeq vhas_nul.16b, vdata.16b, 0
+ lsl shift, srcin, 2
+ and vhas_nul.16b, vhas_nul.16b, vrepmask.16b
+ addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */
+ fmov synd, dend
+ lsr synd, synd, shift
+ cbz synd, L(loop)
+
+ rbit synd, synd
+ clz result, synd
+ lsr result, result, 2
ret
- /* The inner loop processes 32 bytes per iteration and uses the fast
- NUL check. If we encounter non-ASCII characters, use a second
- loop with the accurate NUL check. */
- .p2align 4
-L(main_loop_entry):
- bic src, srcin, 15
- sub src, src, 16
-L(main_loop):
- ldp data1, data2, [src, 32]!
-L(page_cross_entry):
- sub tmp1, data1, zeroones
- sub tmp3, data2, zeroones
- orr tmp2, tmp1, tmp3
- tst tmp2, zeroones, lsl 7
- bne 1f
- ldp data1, data2, [src, 16]
- sub tmp1, data1, zeroones
- sub tmp3, data2, zeroones
- orr tmp2, tmp1, tmp3
- tst tmp2, zeroones, lsl 7
- beq L(main_loop)
- add src, src, 16
-1:
- /* The fast check failed, so do the slower, accurate NUL check. */
- orr tmp2, data1, REP8_7f
- orr tmp4, data2, REP8_7f
- bics has_nul1, tmp1, tmp2
- bic has_nul2, tmp3, tmp4
- ccmp has_nul2, 0, 0, eq
- beq L(nonascii_loop)
-
- /* Enter with C = has_nul1 == 0. */
-L(tail):
-#ifdef __AARCH64EB__
- /* For big-endian, carry propagation (if the final byte in the
- string is 0x01) means we cannot use has_nul1/2 directly. The
- easiest way to get the correct byte is to byte-swap the data
- and calculate the syndrome a second time. */
- csel data1, data1, data2, cc
- rev data1, data1
- sub tmp1, data1, zeroones
- orr tmp2, data1, REP8_7f
- bic has_nul1, tmp1, tmp2
-#else
- csel has_nul1, has_nul1, has_nul2, cc
+ .p2align 5
+L(loop):
+ ldr data, [src, 16]!
+ cmeq vhas_nul.16b, vdata.16b, 0
+ umaxp vend.16b, vhas_nul.16b, vhas_nul.16b
+ fmov synd, dend
+ cbz synd, L(loop)
+
+ and vhas_nul.16b, vhas_nul.16b, vrepmask.16b
+ addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */
+ sub result, src, srcin
+ fmov synd, dend
+#ifndef __AARCH64EB__
+ rbit synd, synd
#endif
- sub len, src, srcin
- rev has_nul1, has_nul1
- add tmp2, len, 8
- clz tmp1, has_nul1
- csel len, len, tmp2, cc
- add len, len, tmp1, lsr 3
+ clz tmp, synd
+ add result, result, tmp, lsr 2
ret
-L(nonascii_loop):
- ldp data1, data2, [src, 16]!
- sub tmp1, data1, zeroones
- orr tmp2, data1, REP8_7f
- sub tmp3, data2, zeroones
- orr tmp4, data2, REP8_7f
- bics has_nul1, tmp1, tmp2
- bic has_nul2, tmp3, tmp4
- ccmp has_nul2, 0, 0, eq
- bne L(tail)
- ldp data1, data2, [src, 16]!
- sub tmp1, data1, zeroones
- orr tmp2, data1, REP8_7f
- sub tmp3, data2, zeroones
- orr tmp4, data2, REP8_7f
- bics has_nul1, tmp1, tmp2
- bic has_nul2, tmp3, tmp4
- ccmp has_nul2, 0, 0, eq
- beq L(nonascii_loop)
- b L(tail)
-
- /* Load 16 bytes from [srcin & ~15] and force the bytes that precede
- srcin to 0x7f, so we ignore any NUL bytes before the string.
- Then continue in the aligned loop. */
-L(page_cross):
- bic src, srcin, 15
- ldp data1, data2, [src]
- lsl tmp1, srcin, 3
- mov tmp4, -1
-#ifdef __AARCH64EB__
- /* Big-endian. Early bytes are at MSB. */
- lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */
-#else
- /* Little-endian. Early bytes are at LSB. */
- lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */
-#endif
- orr tmp1, tmp1, REP8_80
- orn data1, data1, tmp1
- orn tmp2, data2, tmp1
- tst srcin, 8
- csel data1, data1, tmp4, eq
- csel data2, data2, tmp2, eq
- b L(page_cross_entry)
END (STRLEN)
weak_alias (STRLEN, strlen)
libc_hidden_builtin_def (strlen)
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