stxcpy.S

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * arch/alpha/lib/stxcpy.S
 * Contributed by Richard Henderson ([email protected])
 *
 * Copy a null-terminated string from SRC to DST.
 *
 * This is an internal routine used by strcpy, stpcpy, and strcat.
 * As such, it uses special linkage conventions to make implementation
 * of these public functions more efficient.
 *
 * On input:
 *	t9 = return address
 *	a0 = DST
 *	a1 = SRC
 *
 * On output:
 *	t12 = bitmask (with one bit set) indicating the last byte written
 *	a0  = unaligned address of the last *word* written
 *
 * Furthermore, v0, a3-a5, t11, and t12 are untouched.
 */

#include <asm/regdef.h>

	.set noat
	.set noreorder

	.text

/* There is a problem with either gdb (as of 4.16) or gas (as of 2.7) that
   doesn't like putting the entry point for a procedure somewhere in the
   middle of the procedure descriptor.  Work around this by putting the
   aligned copy in its own procedure descriptor */

	.ent stxcpy_aligned
	.align 3
stxcpy_aligned:
	.frame sp, 0, t9
	.prologue 0

	/* On entry to this basic block:
	   t0 == the first destination word for masking back in
	   t1 == the first source word.  */

	/* Create the 1st output word and detect 0's in the 1st input word.  */
	lda	t2, -1		# e1    : build a mask against false zero
	mskqh	t2, a1, t2	# e0    :   detection in the src word
	mskqh	t1, a1, t3	# e0    :
	ornot	t1, t2, t2	# .. e1 :
	mskql	t0, a1, t0	# e0    : assemble the first output word
	cmpbge	zero, t2, t8	# .. e1 : bits set iff null found
	or	t0, t3, t1	# e0    :
	bne	t8, $a_eos	# .. e1 :

	/* On entry to this basic block:
	   t0 == the first destination word for masking back in
	   t1 == a source word not containing a null.  */

$a_loop:
	stq_u	t1, 0(a0)	# e0    :
	addq	a0, 8, a0	# .. e1 :
	ldq_u	t1, 0(a1)	# e0    :
	addq	a1, 8, a1	# .. e1 :
	cmpbge	zero, t1, t8	# e0 (stall)
	beq	t8, $a_loop	# .. e1 (zdb)

	/* Take care of the final (partial) word store.
	   On entry to this basic block we have:
	   t1 == the source word containing the null
	   t8 == the cmpbge mask that found it.  */
$a_eos:
	negq	t8, t6		# e0    : find low bit set
	and	t8, t6, t12	# e1 (stall)

	/* For the sake of the cache, don't read a destination word
	   if we're not going to need it.  */
	and	t12, 0x80, t6	# e0    :
	bne	t6, 1f		# .. e1 (zdb)

	/* We're doing a partial word store and so need to combine
	   our source and original destination words.  */
	ldq_u	t0, 0(a0)	# e0    :
	subq	t12, 1, t6	# .. e1 :
	zapnot	t1, t6, t1	# e0    : clear src bytes >= null
	or	t12, t6, t8	# .. e1 :
	zap	t0, t8, t0	# e0    : clear dst bytes <= null
	or	t0, t1, t1	# e1    :

1:	stq_u	t1, 0(a0)	# e0    :
	ret	(t9)		# .. e1 :

	.end stxcpy_aligned

	.align 3
	.ent __stxcpy
	.globl __stxcpy
__stxcpy:
	.frame sp, 0, t9
	.prologue 0

	/* Are source and destination co-aligned?  */
	xor	a0, a1, t0	# e0    :
	unop			#       :
	and	t0, 7, t0	# e0    :
	bne	t0, $unaligned	# .. e1 :

	/* We are co-aligned; take care of a partial first word.  */
	ldq_u	t1, 0(a1)	# e0    : load first src word
	and	a0, 7, t0	# .. e1 : take care not to load a word ...
	addq	a1, 8, a1		# e0    :
	beq	t0, stxcpy_aligned	# .. e1 : ... if we wont need it
	ldq_u	t0, 0(a0)	# e0    :
	br	stxcpy_aligned	# .. e1 :


/* The source and destination are not co-aligned.  Align the destination
   and cope.  We have to be very careful about not reading too much and
   causing a SEGV.  */

	.align 3
$u_head:
	/* We know just enough now to be able to assemble the first
	   full source word.  We can still find a zero at the end of it
	   that prevents us from outputting the whole thing.

	   On entry to this basic block:
	   t0 == the first dest word, for masking back in, if needed else 0
	   t1 == the low bits of the first source word
	   t6 == bytemask that is -1 in dest word bytes */

	ldq_u	t2, 8(a1)	# e0    :
	addq	a1, 8, a1	# .. e1 :

	extql	t1, a1, t1	# e0    :
	extqh	t2, a1, t4	# e0    :
	mskql	t0, a0, t0	# e0    :
	or	t1, t4, t1	# .. e1 :
	mskqh	t1, a0, t1	# e0    :
	or	t0, t1, t1	# e1    :

	or	t1, t6, t6	# e0    :
	cmpbge	zero, t6, t8	# .. e1 :
	lda	t6, -1		# e0    : for masking just below
	bne	t8, $u_final	# .. e1 :

	mskql	t6, a1, t6		# e0    : mask out the bits we have
	or	t6, t2, t2		# e1    :   already extracted before
	cmpbge	zero, t2, t8		# e0    :   testing eos
	bne	t8, $u_late_head_exit	# .. e1 (zdb)

	/* Finally, we've got all the stupid leading edge cases taken care
	   of and we can set up to enter the main loop.  */

	stq_u	t1, 0(a0)	# e0    : store first output word
	addq	a0, 8, a0	# .. e1 :
	extql	t2, a1, t0	# e0    : position ho-bits of lo word
	ldq_u	t2, 8(a1)	# .. e1 : read next high-order source word
	addq	a1, 8, a1	# e0    :
	cmpbge	zero, t2, t8	# .. e1 :
	nop			# e0    :
	bne	t8, $u_eos	# .. e1 :

	/* Unaligned copy main loop.  In order to avoid reading too much,
	   the loop is structured to detect zeros in aligned source words.
	   This has, unfortunately, effectively pulled half of a loop
	   iteration out into the head and half into the tail, but it does
	   prevent nastiness from accumulating in the very thing we want
	   to run as fast as possible.

	   On entry to this basic block:
	   t0 == the shifted high-order bits from the previous source word
	   t2 == the unshifted current source word

	   We further know that t2 does not contain a null terminator.  */

	.align 3
$u_loop:
	extqh	t2, a1, t1	# e0    : extract high bits for current word
	addq	a1, 8, a1	# .. e1 :
	extql	t2, a1, t3	# e0    : extract low bits for next time
	addq	a0, 8, a0	# .. e1 :
	or	t0, t1, t1	# e0    : current dst word now complete
	ldq_u	t2, 0(a1)	# .. e1 : load high word for next time
	stq_u	t1, -8(a0)	# e0    : save the current word
	mov	t3, t0		# .. e1 :
	cmpbge	zero, t2, t8	# e0    : test new word for eos
	beq	t8, $u_loop	# .. e1 :

	/* We've found a zero somewhere in the source word we just read.
	   If it resides in the lower half, we have one (probably partial)
	   word to write out, and if it resides in the upper half, we
	   have one full and one partial word left to write out.

	   On entry to this basic block:
	   t0 == the shifted high-order bits from the previous source word
	   t2 == the unshifted current source word.  */
$u_eos:
	extqh	t2, a1, t1	# e0    :
	or	t0, t1, t1	# e1    : first (partial) source word complete

	cmpbge	zero, t1, t8	# e0    : is the null in this first bit?
	bne	t8, $u_final	# .. e1 (zdb)

$u_late_head_exit:
	stq_u	t1, 0(a0)	# e0    : the null was in the high-order bits
	addq	a0, 8, a0	# .. e1 :
	extql	t2, a1, t1	# e0    :
	cmpbge	zero, t1, t8	# .. e1 :

	/* Take care of a final (probably partial) result word.
	   On entry to this basic block:
	   t1 == assembled source word
	   t8 == cmpbge mask that found the null.  */
$u_final:
	negq	t8, t6		# e0    : isolate low bit set
	and	t6, t8, t12	# e1    :

	and	t12, 0x80, t6	# e0    : avoid dest word load if we can
	bne	t6, 1f		# .. e1 (zdb)

	ldq_u	t0, 0(a0)	# e0    :
	subq	t12, 1, t6	# .. e1 :
	or	t6, t12, t8	# e0    :
	zapnot	t1, t6, t1	# .. e1 : kill source bytes >= null
	zap	t0, t8, t0	# e0    : kill dest bytes <= null
	or	t0, t1, t1	# e1    :

1:	stq_u	t1, 0(a0)	# e0    :
	ret	(t9)		# .. e1 :

	/* Unaligned copy entry point.  */
	.align 3
$unaligned:

	ldq_u	t1, 0(a1)	# e0    : load first source word

	and	a0, 7, t4	# .. e1 : find dest misalignment
	and	a1, 7, t5	# e0    : find src misalignment

	/* Conditionally load the first destination word and a bytemask
	   with 0xff indicating that the destination byte is sacrosanct.  */

	mov	zero, t0	# .. e1 :
	mov	zero, t6	# e0    :
	beq	t4, 1f		# .. e1 :
	ldq_u	t0, 0(a0)	# e0    :
	lda	t6, -1		# .. e1 :
	mskql	t6, a0, t6	# e0    :
1:
	subq	a1, t4, a1	# .. e1 : sub dest misalignment from src addr

	/* If source misalignment is larger than dest misalignment, we need
	   extra startup checks to avoid SEGV.  */

	cmplt	t4, t5, t12	# e0    :
	beq	t12, $u_head	# .. e1 (zdb)

	lda	t2, -1		# e1    : mask out leading garbage in source
	mskqh	t2, t5, t2	# e0    :
	nop			# e0    :
	ornot	t1, t2, t3	# .. e1 :
	cmpbge	zero, t3, t8	# e0    : is there a zero?
	beq	t8, $u_head	# .. e1 (zdb)

	/* At this point we've found a zero in the first partial word of
	   the source.  We need to isolate the valid source data and mask
	   it into the original destination data.  (Incidentally, we know
	   that we'll need at least one byte of that original dest word.) */

	ldq_u	t0, 0(a0)	# e0    :

	negq	t8, t6		# .. e1 : build bitmask of bytes <= zero
	and	t6, t8, t12	# e0    :
	and	a1, 7, t5	# .. e1 :
	subq	t12, 1, t6	# e0    :
	or	t6, t12, t8	# e1    :
	srl	t12, t5, t12	# e0    : adjust final null return value

	zapnot	t2, t8, t2	# .. e1 : prepare source word; mirror changes
	and	t1, t2, t1	# e1    : to source validity mask
	extql	t2, a1, t2	# .. e0 :
	extql	t1, a1, t1	# e0    :

	andnot	t0, t2, t0	# .. e1 : zero place for source to reside
	or	t0, t1, t1	# e1    : and put it there
	stq_u	t1, 0(a0)	# .. e0 :
	ret	(t9)		# e1    :

	.end __stxcpy