xnu-4903.241.1.tar.gz

[apple/xnu.git] / osfmk / arm / strncmp.s

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.text
.syntax unified
.code 32
.globl _strncmp
// int strncmp(const char *s1, const char *s2, size_t n);
//
// Returns zero if the two NUL-terminated strings s1 and s2 are equal up to
// n characters.  Otherwise, returns the difference between the first two
// characters that do not match, interpreted as unsigned integers.

#define ESTABLISH_FRAME        \
	push   {r4-r7,lr}         ;\
	add     r7,     sp, #12   ;\
	push   {r8,r10}
#define CLEAR_FRAME            \
	pop    {r8,r10}            ;\
	pop    {r4-r7,lr}

.align 3
.long 0, 0x01010101
_strncmp:
//  If n < 16, jump straight to the byte-by-byte comparison loop.
	cmp     r2,         #16
	blo     L_byteCompareLoop
//  Load a character from each string and advance the pointers.  If the loaded
//  characters are unequal or NUL, return their difference.
0:	ldrb    r3,    [r0],#1
	ldrb    ip,    [r1],#1
	sub     r2,         #1
	cmp     r3,         #1
	cmphs   r3,         ip
	bne     L_earlyReturn
//  If the address of the next character from s1 does not have word alignment,
//  continue with the character-by-character comparison.  Otherwise, fall
//  through into the word-by-word comparison path.
	tst     r0,         #3
	bne     0b
	
//  We have not encountered a NUL or a mismatch, and s1 has word alignment.
//  Establish a frame, since we're going to need additional registers anyway.
	ESTABLISH_FRAME
	ldr     lr,    (_strncmp-4)

//  Word align s2, and place the remainder in r10.  Compute the right- and
//  left-shifts to extract each word that we will compare to the other source
//  from the aligned words that we load:
//
//      aligned s2        to be loaded on next iteration
//      |   "true" s2     |
//      v   v             v
//      +---+---+---+---+ +---+---+---+---+
//      | 0 | 1 | 2 | 3 | | 4 | 5 | 6 | 7 |
//      +---+---+---+---+ +---+---+---+---+
//          ^-----------------^
//          to be compared on next iteration
	and     r10,    r1, #3
	bic     r1,     r1, #3
	mov     r10,        r10, lsl #3
	rsb     r6,     r10,#32
	
//  Subtract the number of bytes of the initial word load from s2 that will 
//  actually be used from n.
	sub     r2,     r2, r6, lsr #3
	
//  Load the first aligned word of s2.  OR 0x01 into any bytes that preceed the
//  "true s2", to prevent our check for NUL from generating a false positive.
//  Then check for NUL, and jump to the byte-by-byte comparison loop after
//  unwinding the pointers if we enounter one.
	ldr     r8,    [r1],#4
	orr     r8,     r8, lr, lsr r6
	sub     r3,     r8, lr
	bic     r3,     r3, r8
	tst     r3,         lr, lsl #7
	mov     r5,         r8, lsr r10
	bne     L_unwindLoopPreload
	
.align 3
L_wordCompareLoop:
//  If n < 4, abort the word compare loop before we load any more data.
	subs    r2,     r2, #4
	blo     L_nIsLessThanFour
//  Load the next aligned word of s2 and check if it contains any NUL bytes.
//  Load the next aligned word of s1, and extract the corresponding bytes from
//  the two words of s2 loaded in this and the previous iteration of the loop.
//  Compare these two words.
//  If no NUL or mismatched words have been encountered, continue the loop.
	ldr     r8,    [r1],#4
#if defined _ARM_ARCH_6
    uqsub8  r3,     lr, r8
    tst     r3,         r3
    ldr     ip,    [r0],#4
#else
	sub     r3,     r8, lr
	bic     r3,     r3, r8
	ldr     ip,    [r0],#4
	tst     r3,         lr, lsl #7
#endif
	orr     r4,     r5, r8, lsl r6
	cmpeq   ip,         r4
	mov     r5,         r8, lsr r10
	beq     L_wordCompareLoop

//  Either we have encountered a NUL, or we have found a mismatch between s1
//  and s2.  Unwind the pointers and use a byte-by-byte comparison loop.
	sub     r0,     r0, #4
	sub     r1,     r1, #4
L_nIsLessThanFour:
	add     r2,     r2, #4
L_unwindLoopPreload:
	sub     r1,     r1, r6, lsr #3
	add     r2,     r2, r6, lsr #3
	CLEAR_FRAME
	
L_byteCompareLoop:
//  If n-- == 0, we have exhausted the allowed number of comparisons, and need
//  to return zero without additional loads.
	subs    r2,     r2, #1
	movlo   r0,         #0
	bxlo    lr
//  Load a character from each string and advance the pointers.  If the loaded
//  characters are unequal or NUL, return their difference.
	ldrb    r3,    [r0],#1
	ldrb    ip,    [r1],#1
	cmp     r3,         #1
	cmpcs   r3,         ip
	beq     L_byteCompareLoop
	
L_earlyReturn:
//  Return the difference between the last two characters loaded.
	sub     r0,     r3, ip
	bx      lr