diff options
Diffstat (limited to 'sysdeps/aarch64')
| -rw-r--r-- | sysdeps/aarch64/strlen.S | 230 |
1 files changed, 165 insertions, 65 deletions
diff --git a/sysdeps/aarch64/strlen.S b/sysdeps/aarch64/strlen.S index 54271b9..feb9e48 100644 --- a/sysdeps/aarch64/strlen.S +++ b/sysdeps/aarch64/strlen.S @@ -20,9 +20,13 @@ /* Assumptions: * - * ARMv8-a, AArch64 + * ARMv8-a, AArch64, unaligned accesses, min page size 4k. */ +/* 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 @@ -31,87 +35,183 @@ #define src x1 #define data1 x2 #define data2 x3 -#define data2a x4 -#define has_nul1 x5 -#define has_nul2 x6 -#define tmp1 x7 -#define tmp2 x8 -#define tmp3 x9 -#define tmp4 x10 -#define zeroones x11 -#define pos x12 +#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 - /* Start of critial section -- keep to one 64Byte cache line. */ +#ifdef TEST_PAGE_CROSS +# define MIN_PAGE_SIZE 15 +#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) - mov zeroones, #REP8_01 - bic src, srcin, #15 - ands tmp1, srcin, #15 - b.ne L(misaligned) - /* 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. */ - /* The inner loop deals with two Dwords at a time. This has a - slightly higher start-up cost, but we should win quite quickly, - especially on cores with a high number of issue slots per - cycle, as we get much better parallelism out of the operations. */ -L(loop): - ldp data1, data2, [src], #16 -L(realigned): + 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 + orr tmp2, data1, REP8_7f sub tmp3, data2, zeroones - orr tmp4, data2, #REP8_7f - bic has_nul1, tmp1, tmp2 - bics has_nul2, tmp3, tmp4 - ccmp has_nul1, #0, #0, eq /* NZCV = 0000 */ - b.eq L(loop) - /* End of critical section -- keep to one 64Byte cache line. */ + 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) - sub len, src, srcin - cbz has_nul1, L(nul_in_data2) -#ifdef __AARCH64EB__ - mov data2, data1 -#endif - sub len, len, #8 - mov has_nul2, has_nul1 -L(nul_in_data2): + /* 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 + 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_nul directly. 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. */ - rev data2, data2 - sub tmp1, data2, zeroones - orr tmp2, data2, #REP8_7f - bic has_nul2, tmp1, tmp2 + 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 #endif - sub len, len, #8 - rev has_nul2, has_nul2 - clz pos, has_nul2 - add len, len, pos, lsr #3 /* Bits to bytes. */ - RET - -L(misaligned): - cmp tmp1, #8 - neg tmp1, tmp1 - ldp data1, data2, [src], #16 - lsl tmp1, tmp1, #3 /* Bytes beyond alignment -> bits. */ - mov tmp2, #~0 + 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 + 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. */ - lsl tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */ + /* Big-endian. Early bytes are at MSB. */ + lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ #else /* Little-endian. Early bytes are at LSB. */ - lsr tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */ + lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ #endif - orr data1, data1, tmp2 - orr data2a, data2, tmp2 - csinv data1, data1, xzr, le - csel data2, data2, data2a, le - b L(realigned) + 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) libc_hidden_builtin_def (strlen) |