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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 Lesser General Public License as published by
dnl the Free Software Foundation; either version 2.1 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 Lesser General Public
dnl License for more details.
dnl You should have received a copy of the GNU Lesser 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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