From 30cf54bf3072be942847400c1669bcd63aab039e Mon Sep 17 00:00:00 2001 From: Adhemerval Zanella Date: Tue, 10 Jan 2023 18:00:58 -0300 Subject: string: Improve generic strcmp It follows the strategy: - Align the first input to word boundary using byte operations. - If second input is also word aligned, read a word per time, check for null (using has_zero), and check final words using byte operation. - If second input is not word aligned, loop by aligning the source, and merging the result of two reads. Similar to aligned case, check for null with has_zero, and check final words using byte operation. Checked on x86_64-linux-gnu, i686-linux-gnu, powerpc64-linux-gnu, and powerpc-linux-gnu by removing the arch-specific assembly implementation and disabling multi-arch (it covers both LE and BE for 64 and 32 bits). Co-authored-by: Richard Henderson Reviewed-by: Noah Goldstein --- string/strcmp.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 94 insertions(+), 16 deletions(-) (limited to 'string') diff --git a/string/strcmp.c b/string/strcmp.c index 053f5a8d2b..11ec8bac81 100644 --- a/string/strcmp.c +++ b/string/strcmp.c @@ -15,33 +15,111 @@ License along with the GNU C Library; if not, see . */ +#include +#include +#include +#include #include +#include -#undef strcmp - -#ifndef STRCMP -# define STRCMP strcmp +#ifdef STRCMP +# define strcmp STRCMP #endif +static inline int +final_cmp (const op_t w1, const op_t w2) +{ + unsigned int idx = index_first_zero_ne (w1, w2); + return extractbyte (w1, idx) - extractbyte (w2, idx); +} + +/* Aligned loop: if a difference is found, exit to compare the bytes. Else + if a zero is found we have equal strings. */ +static inline int +strcmp_aligned_loop (const op_t *x1, const op_t *x2, op_t w1) +{ + op_t w2 = *x2++; + + while (w1 == w2) + { + if (has_zero (w1)) + return 0; + w1 = *x1++; + w2 = *x2++; + } + + return final_cmp (w1, w2); +} + +/* Unaligned loop: align the first partial of P2, with 0xff for the rest of + the bytes so that we can also apply the has_zero test to see if we have + already reached EOS. If we have, then we can simply fall through to the + final comparison. */ +static inline int +strcmp_unaligned_loop (const op_t *x1, const op_t *x2, op_t w1, uintptr_t ofs) +{ + op_t w2a = *x2++; + uintptr_t sh_1 = ofs * CHAR_BIT; + uintptr_t sh_2 = sizeof(op_t) * CHAR_BIT - sh_1; + + op_t w2 = MERGE (w2a, sh_1, (op_t)-1, sh_2); + if (!has_zero (w2)) + { + op_t w2b; + + /* Unaligned loop. The invariant is that W2B, which is "ahead" of W1, + does not contain end-of-string. Therefore it is safe (and necessary) + to read another word from each while we do not have a difference. */ + while (1) + { + w2b = *x2++; + w2 = MERGE (w2a, sh_1, w2b, sh_2); + if (w1 != w2) + return final_cmp (w1, w2); + if (has_zero (w2b)) + break; + w1 = *x1++; + w2a = w2b; + } + + /* Zero found in the second partial of P2. If we had EOS in the aligned + word, we have equality. */ + if (has_zero (w1)) + return 0; + + /* Load the final word of P1 and align the final partial of P2. */ + w1 = *x1++; + w2 = MERGE (w2b, sh_1, 0, sh_2); + } + + return final_cmp (w1, w2); +} + /* Compare S1 and S2, returning less than, equal to or greater than zero if S1 is lexicographically less than, equal to or greater than S2. */ int -STRCMP (const char *p1, const char *p2) +strcmp (const char *p1, const char *p2) { - const unsigned char *s1 = (const unsigned char *) p1; - const unsigned char *s2 = (const unsigned char *) p2; - unsigned char c1, c2; - - do + /* Handle the unaligned bytes of p1 first. */ + uintptr_t n = -(uintptr_t)p1 % sizeof(op_t); + for (int i = 0; i < n; ++i) { - c1 = (unsigned char) *s1++; - c2 = (unsigned char) *s2++; - if (c1 == '\0') - return c1 - c2; + unsigned char c1 = *p1++; + unsigned char c2 = *p2++; + int diff = c1 - c2; + if (c1 == '\0' || diff != 0) + return diff; } - while (c1 == c2); - return c1 - c2; + /* P1 is now aligned to op_t. P2 may or may not be. */ + const op_t *x1 = (const op_t *) p1; + op_t w1 = *x1++; + uintptr_t ofs = (uintptr_t) p2 % sizeof(op_t); + return ofs == 0 + ? strcmp_aligned_loop (x1, (const op_t *)p2, w1) + : strcmp_unaligned_loop (x1, (const op_t *)(p2 - ofs), w1, ofs); } +#ifndef STRCMP libc_hidden_builtin_def (strcmp) +#endif -- cgit 1.4.1