[uclibc-ng-devel] [PATCH v2] xtensa: fix strcmp

Loops with 'loop forever' annotation inside strcmp are actually meant to loop forever. Falling through the end of the first loop may result in equal strings being compared unequal, e.g.: #include <string.h> int main(void) { char a[4096] __attribute__((aligned(4))); char b[4096] __attribute__((aligned(4))); memset(a, ' ', 258 * 8); memset(b, ' ', 258 * 8); a[255 * 8] = 0; a[256 * 8] = 'a'; b[255 * 8] = 0; b[256 * 8] = 'b'; return !(strcmp(a, b) == 0); } Falling through the end of the second loop may result in unequal strings being compared as equal, e.g.: #include <string.h> int main(void) { char a[4096] __attribute__((aligned(4))); char b[4096] __attribute__((aligned(4))); memset(a, ' ', 514 * 6); memset(b, ' ', 514 * 6); a[514 * 6 + 0] = 'a'; a[514 * 6 + 1] = 0; b[514 * 6 + 0] = 'b'; b[514 * 6 + 1] = 0; return !(strcmp(a, b) != 0); } Use 0 as a loop counter to make 2^32 - 1 iterations which is enough to cover all addressable memory. While at it drop useless nop at the end of the first loop and use a11 for all loop counters. Signed-off-by: Max Filippov &lt;jcmvbkbc(a)gmail.com&gt; --- Changes v1->v2: - wrong test case for the second loop, fixed. libc/string/xtensa/strcmp.S | 13 +++++-------- 1 file changed, 5 insertions(+), 8 deletions(-) diff --git a/libc/string/xtensa/strcmp.S b/libc/string/xtensa/strcmp.S index 2dce590dbd42..a16da5da2e92 100644 --- a/libc/string/xtensa/strcmp.S +++ b/libc/string/xtensa/strcmp.S @@ -92,8 +92,8 @@ ENTRY (strcmp) /* (2 mod 4) alignment for loop instruction */ .Lunaligned: #if XCHAL_HAVE_LOOPS - _movi.n a8, 0 /* set up for the maximum loop count */ - loop a8, .Lretdiff /* loop forever (almost anyway) */ + movi a11, 0 /* set up for the maximum loop count */ + loop a11, .Lretdiff /* loop forever (almost anyway) */ #endif .Lnextbyte: l8ui a8, a2, 0 @@ -131,11 +131,10 @@ ENTRY (strcmp) #if XCHAL_HAVE_LOOPS .Laligned: .begin no-transform + movi a11, 0 l32r a4, .Lmask0 /* mask for byte 0 */ l32r a7, .Lmask4 - /* Loop forever. (a4 is more than than the maximum number - of iterations) */ - loop a4, .Laligned_done + loop a11, .Laligned_done /* Loop forever. */ /* First unrolled loop body. */ l32i a8, a2, 0 /* get word from s1 */ @@ -156,8 +155,6 @@ ENTRY (strcmp) addi a2, a2, 8 /* advance s1 pointer */ addi a3, a3, 8 /* advance s2 pointer */ .Laligned_done: - or a1, a1, a1 /* nop */ - .Lprobeq2: /* Adjust pointers to account for the loop unrolling. */ addi a2, a2, 4 @@ -198,7 +195,7 @@ ENTRY (strcmp) #if XCHAL_HAVE_LOOPS /* align (1 mod 4) */ - loop a4, .Leq /* loop forever (a4 is bigger than max iters) */ + loop a11, .Leq /* loop forever */ .end no-transform l32i a8, a2, 0 /* get word from s1 */ -- 2.1.4