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dnl Alpha auxiliary for longlong.h's count_leading_zeros
dnl Copyright (C) 1997, 2000 Free Software Foundation, Inc.
dnl This file is part of the GNU MP Library.
dnl The GNU MP Library is free software; you can redistribute it and/or modify
dnl it under the terms of the GNU Library General Public License as published by
dnl the Free Software Foundation; either version 2 of the License, or (at your
dnl option) any later version.
dnl The GNU MP Library is distributed in the hope that it will be useful, but
dnl WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
dnl or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public
dnl License for more details.
dnl You should have received a copy of the GNU Library General Public License
dnl along with the GNU MP Library; see the file COPYING.LIB. If not, write to
dnl the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
dnl MA 02111-1307, USA.
include(`../config.m4')
dnl DISCUSSION:
dnl Other methods have been tried, and using a 128-entry table actually trims
dnl about 10% of the execution time (on a 21164) when the table is in the L1
dnl cache. But under non-benchmarking conditions, the table will hardly be in
dnl the L1 cache. Tricky bit-fiddling methods with multiplies and magic tables
dnl are also possible, but they require many more instructions than the current
dnl code. (But for count_trailing_zeros, such tricks are beneficial.)
dnl Finally, converting to floating-point and extracting the exponent is much
dnl slower.
ASM_START()
PROLOGUE(MPN(count_leading_zeros))
bis r31,63,r0 C initialize partial result count
srl r16,32,r1 C shift down 32 steps -> r1
cmovne r1,r1,r16 C select r1 if non-zero
cmovne r1,31,r0 C if r1 is nonzero choose smaller count
srl r16,16,r1 C shift down 16 steps -> r1
subq r0,16,r2 C generate new partial result count
cmovne r1,r1,r16 C choose new r1 if non-zero
cmovne r1,r2,r0 C choose new count if r1 was non-zero
srl r16,8,r1
subq r0,8,r2
cmovne r1,r1,r16
cmovne r1,r2,r0
srl r16,4,r1
subq r0,4,r2
cmovne r1,r1,r16
cmovne r1,r2,r0
srl r16,2,r1
subq r0,2,r2
cmovne r1,r1,r16
cmovne r1,r2,r0
srl r16,1,r1 C extract bit 1
subq r0,r1,r0 C subtract it from partial result
ret r31,(r26),1
EPILOGUE(MPN(count_leading_zeros))
ASM_END()
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