dnl  Alpha ev67 mpn_gcd_1 -- Nx1 greatest common divisor.

dnl  Copyright 2003, 2004 Free Software Foundation, Inc.

dnl  This file is part of the GNU MP Library.
dnl
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dnl  modify it under the terms of the GNU Lesser General Public License as
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dnl  Lesser General Public License for more details.
dnl
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include(`../config.m4')


C ev67: 3.4 cycles/bitpair for 1x1 part


C mp_limb_t mpn_gcd_1 (mp_srcptr xp, mp_size_t xsize, mp_limb_t y);
C
C In the 1x1 part, the algorithm is to change x,y to abs(x-y),min(x,y) and
C strip trailing zeros from abs(x-y) to maintain x and y both odd.
C
C The trailing zeros are calculated from just x-y, since in twos-complement
C there's the same number of trailing zeros on d or -d.  This means the cttz
C runs in parallel with abs(x-y).
C
C The loop takes 5 cycles, and at 0.68 iterations per bit for two N-bit
C operands with this algorithm gives the measured 3.4 c/l.
C
C The slottings shown are for SVR4 style systems, Unicos differs in the
C initial gp setup and the LEA.
C
C Enhancement:
C
C On the jsr, !lituse_jsr! (when available) would allow the linker to relax
C it to a bsr, but probably only in a static binary.  Plain "jsr foo" gives
C the right object code for relaxation, and ought to be available
C everywhere, but we prefer to schedule the GOT ldq (LEA) back earlier, for
C the usual case of running in a shared library.
C
C bsr could perhaps be used explicitly anyway.  We should be able to assume
C modexact is in the same module as us (ie. shared library or mainline).
C Would there be any worries about the size of the displacement?  Could
C always put modexact and gcd_1 in the same .o to be certain.

ASM_START()
PROLOGUE(mpn_gcd_1, gp)

	C r16	xp
	C r17	size
	C r18	y

	C ldah				C l
	C lda				C u

	ldq	r0, 0(r16)		C L   x = xp[0]
	lda	r30, -32(r30)		C u   alloc stack

	LEA(  r27, mpn_modexact_1c_odd)	C L   modexact addr, ldq (gp)
	stq	r10, 16(r30)		C L   save r10
	cttz	r18, r10		C U0  y twos
	cmpeq	r17, 1, r5		C u   test size==1

	stq	r9, 8(r30)		C L   save r9
	clr	r19			C u   zero c for modexact
	unop
	unop

	cttz	r0, r6			C U0  x twos
	stq	r26, 0(r30)		C L   save ra

	srl	r18, r10, r18		C U   y odd

	mov	r18, r9			C l   hold y across call

	cmpult	r6, r10, r2		C u   test x_twos < y_twos

	cmovne	r2, r6, r10		C l   common_twos = min(x_twos,y_twos)
	bne	r5, L(one)		C U   no modexact if size==1
	jsr	r26, (r27), mpn_modexact_1c_odd   C L0

	LDGP(	r29, 0(r26))		C u,l ldah,lda
	cttz	r0, r6			C U0  new x twos
	ldq	r26, 0(r30)		C L   restore ra

L(one):
	mov	r9, r1			C u   y
	ldq	r9, 8(r30)		C L   restore r9
	mov	r10, r2			C u   common twos
	ldq	r10, 16(r30)		C L   restore r10

	lda	r30, 32(r30)		C l   free stack
	beq	r0, L(done)		C U   return y if x%y==0

	srl	r0, r6, r0		C U   x odd
	unop

	ALIGN(16)
L(top):
	C r0	x
	C r1	y
	C r2	common twos, for use at end

	subq	r0, r1, r7		C l0  d = x - y
	cmpult	r0, r1, r16		C u0  test x >= y

	subq	r1, r0, r4		C l0  new_x = y - x
	cttz	r7, r8			C U0  d twos

	cmoveq	r16, r7, r4		C l0  new_x = d if x>=y
	cmovne	r16, r0, r1		C u0  y = x if x<y
	unop				C l   \ force cmoveq into l0
	unop				C u   /

	C				C cmoveq2 L0, cmovne2 U0

	srl	r4, r8, r0		C U0  x = new_x >> twos
	bne	r7, L(top)		C U1  stop when d==0


L(done):
	sll	r1, r2, r0		C U0  return y << common_twos
	ret	r31, (r26), 1		C L0

EPILOGUE()
ASM_END()