linux/arch/sparc/lib/mul.S

/* $Id: mul.S,v 1.4 1996/09/30 02:22:32 davem Exp $
 * mul.S:       This routine was taken from glibc-1.09 and is covered
 *              by the GNU Library General Public License Version 2.
 */

/*
 * Signed multiply, from Appendix E of the Sparc Version 8
 * Architecture Manual.
 */

/*
 * Returns %o0 * %o1 in %o1%o0 (i.e., %o1 holds the upper 32 bits of
 * the 64-bit product).
 *
 * This code optimizes short (less than 13-bit) multiplies.
 */

	.globl .mul
.mul:
	mov	%o0, %y		! multiplier -> Y
	andncc	%o0, 0xfff, %g0	! test bits 12..31
	be	Lmul_shortway	! if zero, can do it the short way
	 andcc	%g0, %g0, %o4	! zero the partial product and clear N and V

	/*
	 * Long multiply.  32 steps, followed by a final shift step.
	 */
	mulscc	%o4, %o1, %o4	! 1
	mulscc	%o4, %o1, %o4	! 2
	mulscc	%o4, %o1, %o4	! 3
	mulscc	%o4, %o1, %o4	! 4
	mulscc	%o4, %o1, %o4	! 5
	mulscc	%o4, %o1, %o4	! 6
	mulscc	%o4, %o1, %o4	! 7
	mulscc	%o4, %o1, %o4	! 8
	mulscc	%o4, %o1, %o4	! 9
	mulscc	%o4, %o1, %o4	! 10
	mulscc	%o4, %o1, %o4	! 11
	mulscc	%o4, %o1, %o4	! 12
	mulscc	%o4, %o1, %o4	! 13
	mulscc	%o4, %o1, %o4	! 14
	mulscc	%o4, %o1, %o4	! 15
	mulscc	%o4, %o1, %o4	! 16
	mulscc	%o4, %o1, %o4	! 17
	mulscc	%o4, %o1, %o4	! 18
	mulscc	%o4, %o1, %o4	! 19
	mulscc	%o4, %o1, %o4	! 20
	mulscc	%o4, %o1, %o4	! 21
	mulscc	%o4, %o1, %o4	! 22
	mulscc	%o4, %o1, %o4	! 23
	mulscc	%o4, %o1, %o4	! 24
	mulscc	%o4, %o1, %o4	! 25
	mulscc	%o4, %o1, %o4	! 26
	mulscc	%o4, %o1, %o4	! 27
	mulscc	%o4, %o1, %o4	! 28
	mulscc	%o4, %o1, %o4	! 29
	mulscc	%o4, %o1, %o4	! 30
	mulscc	%o4, %o1, %o4	! 31
	mulscc	%o4, %o1, %o4	! 32
	mulscc	%o4, %g0, %o4	! final shift

	! If %o0 was negative, the result is
	!	(%o0 * %o1) + (%o1 << 32))
	! We fix that here.

#if 0
	tst	%o0
	bge	1f
	 rd	%y, %o0

	! %o0 was indeed negative; fix upper 32 bits of result by subtracting 
	! %o1 (i.e., return %o4 - %o1 in %o1).
	retl
	 sub	%o4, %o1, %o1

1:
	retl
	 mov	%o4, %o1
#else
	/* Faster code adapted from tege@sics.se's code for umul.S.  */
	sra	%o0, 31, %o2	! make mask from sign bit
	and	%o1, %o2, %o2	! %o2 = 0 or %o1, depending on sign of %o0
	rd	%y, %o0		! get lower half of product
	retl
	 sub	%o4, %o2, %o1	! subtract compensation 
				!  and put upper half in place
#endif

Lmul_shortway:
	/*
	 * Short multiply.  12 steps, followed by a final shift step.
	 * The resulting bits are off by 12 and (32-12) = 20 bit positions,
	 * but there is no problem with %o0 being negative (unlike above).
	 */
	mulscc	%o4, %o1, %o4	! 1
	mulscc	%o4, %o1, %o4	! 2
	mulscc	%o4, %o1, %o4	! 3
	mulscc	%o4, %o1, %o4	! 4
	mulscc	%o4, %o1, %o4	! 5
	mulscc	%o4, %o1, %o4	! 6
	mulscc	%o4, %o1, %o4	! 7
	mulscc	%o4, %o1, %o4	! 8
	mulscc	%o4, %o1, %o4	! 9
	mulscc	%o4, %o1, %o4	! 10
	mulscc	%o4, %o1, %o4	! 11
	mulscc	%o4, %o1, %o4	! 12
	mulscc	%o4, %g0, %o4	! final shift

	/*
	 *  %o4 has 20 of the bits that should be in the low part of the
	 * result; %y has the bottom 12 (as %y's top 12).  That is:
	 *
	 *	  %o4		    %y
	 * +----------------+----------------+
	 * | -12- |   -20-  | -12- |   -20-  |
	 * +------(---------+------)---------+
	 *  --hi-- ----low-part----
	 *
	 * The upper 12 bits of %o4 should be sign-extended to form the
	 * high part of the product (i.e., highpart = %o4 >> 20).
	 */

	rd	%y, %o5
	sll	%o4, 12, %o0	! shift middle bits left 12
	srl	%o5, 20, %o5	! shift low bits right 20, zero fill at left
	or	%o5, %o0, %o0	! construct low part of result
	retl
	 sra	%o4, 20, %o1	! ... and extract high part of result

	.globl	.mul_patch
.mul_patch:
	smul	%o0, %o1, %o0
	retl
	 rd	%y, %o1
	nop