strrchr.s

嵌入式系统设计与实例开发源码

/*
 * arch/alpha/lib/strrchr.S
 * Contributed by Richard Henderson ([e-mail])
 *
 * Return the address of the last occurrance of a given character
 * within a null-terminated string, or null if it is not found.
 */

#include <alpha/regdef.h>

	.set noreorder
	.set noat

	.align 3
	.ent strrchr
	.globl strrchr
strrchr:
	.frame sp, 0, ra
	.prologue 0

	zapnot	a1, 1, a1	# e0    : zero extend our test character
	mov	zero, t6	# .. e1 : t6 is last match aligned addr
	sll	a1, 8, t5	# e0    : replicate our test character
	mov	zero, t8	# .. e1 : t8 is last match byte compare mask
	or	t5, a1, a1	# e0    :
	ldq_u   t0, 0(a0)	# .. e1 : load first quadword
	sll	a1, 16, t5	# e0    :
	andnot  a0, 7, v0	# .. e1 : align source addr
	or	t5, a1, a1	# e0    :
	lda	t4, -1		# .. e1 : build garbage mask
	sll	a1, 32, t5	# e0    :
	cmpbge  zero, t0, t1	# .. e1 : bits set iff byte == zero
	mskqh	t4, a0, t4	# e0    :
	or	t5, a1, a1	# .. e1 : character replication complete
	xor	t0, a1, t2	# e0    : make bytes == c zero
	cmpbge	zero, t4, t4	# .. e1 : bits set iff byte is garbage
	cmpbge  zero, t2, t3	# e0    : bits set iff byte == c
	andnot	t1, t4, t1	# .. e1 : clear garbage from null test
	andnot	t3, t4, t3	# e0    : clear garbage from char test
	bne	t1, $eos	# .. e1 : did we already hit the terminator?

	/* Character search main loop */
$loop:
	ldq	t0, 8(v0)	# e0    : load next quadword
	cmovne	t3, v0, t6	# .. e1 : save previous comparisons match
	cmovne	t3, t3, t8	# e0    :
	addq	v0, 8, v0	# .. e1 :
	xor	t0, a1, t2	# e0    :
	cmpbge	zero, t0, t1	# .. e1 : bits set iff byte == zero
	cmpbge	zero, t2, t3	# e0    : bits set iff byte == c
	beq	t1, $loop	# .. e1 : if we havnt seen a null, loop

	/* Mask out character matches after terminator */
$eos:
	negq	t1, t4		# e0    : isolate first null byte match
	and	t1, t4, t4	# e1    :
	subq	t4, 1, t5	# e0    : build a mask of the bytes upto...
	or	t4, t5, t4	# e1    : ... and including the null

	and	t3, t4, t3	# e0    : mask out char matches after null
	cmovne	t3, t3, t8	# .. e1 : save it, if match found
	cmovne	t3, v0, t6	# e0    :

	/* Locate the address of the last matched character */

	/* Retain the early exit for the ev4 -- the ev5 mispredict penalty
	   is 5 cycles -- the same as just falling through.  */
	beq	t8, $retnull	# .. e1 :

	and	t8, 0xf0, t2	# e0    : binary search for the high bit set
	cmovne	t2, t2, t8	# .. e1 (zdb)
	cmovne	t2, 4, t2	# e0    :
	and	t8, 0xcc, t1	# .. e1 :
	cmovne	t1, t1, t8	# e0    :
	cmovne	t1, 2, t1	# .. e1 :
	and	t8, 0xaa, t0	# e0    :
	cmovne	t0, 1, t0	# .. e1 (zdb)
	addq	t2, t1, t1	# e0    :
	addq	t6, t0, v0	# .. e1 : add our aligned base ptr to the mix
	addq	v0, t1, v0	# e0    :
	ret			# .. e1 :

$retnull:
	mov	zero, v0	# e0    :
	ret			# .. e1 :

	.end strrchr