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Annotation of OpenXM_contrib/gmp/mpn/alpha/cntlz.asm, Revision 1.1

1.1     ! maekawa     1: dnl  Alpha auxiliary for longlong.h's count_leading_zeros
        !             2:
        !             3: dnl  Copyright (C) 1997, 2000 Free Software Foundation, Inc.
        !             4:
        !             5: dnl  This file is part of the GNU MP Library.
        !             6:
        !             7: dnl  The GNU MP Library is free software; you can redistribute it and/or modify
        !             8: dnl  it under the terms of the GNU Lesser General Public License as published by
        !             9: dnl  the Free Software Foundation; either version 2.1 of the License, or (at your
        !            10: dnl  option) any later version.
        !            11:
        !            12: dnl  The GNU MP Library is distributed in the hope that it will be useful, but
        !            13: dnl  WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
        !            14: dnl  or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
        !            15: dnl  License for more details.
        !            16:
        !            17: dnl  You should have received a copy of the GNU Lesser General Public License
        !            18: dnl  along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
        !            19: dnl  the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
        !            20: dnl  MA 02111-1307, USA.
        !            21:
        !            22: include(`../config.m4')
        !            23:
        !            24: dnl  DISCUSSION:
        !            25:
        !            26: dnl  Other methods have been tried, and using a 128-entry table actually trims
        !            27: dnl  about 10% of the execution time (on a 21164) when the table is in the L1
        !            28: dnl  cache.  But under non-benchmarking conditions, the table will hardly be in
        !            29: dnl  the L1 cache.  Tricky bit-fiddling methods with multiplies and magic tables
        !            30: dnl  are also possible, but they require many more instructions than the current
        !            31: dnl  code.  (But for count_trailing_zeros, such tricks are beneficial.)
        !            32: dnl  Finally, converting to floating-point and extracting the exponent is much
        !            33: dnl  slower.
        !            34:
        !            35: ASM_START()
        !            36: PROLOGUE(MPN(count_leading_zeros))
        !            37:        bis     r31,63,r0               C initialize partial result count
        !            38:
        !            39:        srl     r16,32,r1               C shift down 32 steps -> r1
        !            40:        cmovne  r1,r1,r16               C select r1 if non-zero
        !            41:        cmovne  r1,31,r0                C if r1 is nonzero choose smaller count
        !            42:
        !            43:        srl     r16,16,r1               C shift down 16 steps -> r1
        !            44:        subq    r0,16,r2                C generate new partial result count
        !            45:        cmovne  r1,r1,r16               C choose new r1 if non-zero
        !            46:        cmovne  r1,r2,r0                C choose new count if r1 was non-zero
        !            47:
        !            48:        srl     r16,8,r1
        !            49:        subq    r0,8,r2
        !            50:        cmovne  r1,r1,r16
        !            51:        cmovne  r1,r2,r0
        !            52:
        !            53:        srl     r16,4,r1
        !            54:        subq    r0,4,r2
        !            55:        cmovne  r1,r1,r16
        !            56:        cmovne  r1,r2,r0
        !            57:
        !            58:        srl     r16,2,r1
        !            59:        subq    r0,2,r2
        !            60:        cmovne  r1,r1,r16
        !            61:        cmovne  r1,r2,r0
        !            62:
        !            63:        srl     r16,1,r1                C extract bit 1
        !            64:        subq    r0,r1,r0                C subtract it from partial result
        !            65:
        !            66:        ret     r31,(r26),1
        !            67: EPILOGUE(MPN(count_leading_zeros))
        !            68: ASM_END()