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 <jcmvbkbc@gmail.com>

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 */