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8179004: Add an efficient implementation of the "count trailing zeros" operation

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Added count_trailing_zeros function, with platform-dependent implementations.

Reviewed-by: dholmes, cjplummer
This commit is contained in:
Kim Barrett 2017-05-05 16:20:42 -04:00
parent 7b8b238986
commit 48d21bc1c7
4 changed files with 244 additions and 0 deletions
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41
hotspot/src/os_cpu/solaris_sparc/vm/count_trailing_zeros_solaris_sparc.hpp Normal file
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/*
* Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#ifndef OS_CPU_SOLARIS_SPARC_VM_COUNTTRAILINGZEROS_HPP
#define OS_CPU_SOLARIS_SPARC_VM_COUNTTRAILINGZEROS_HPP
#include "utilities/globalDefinitions.hpp"
inline unsigned count_trailing_zeros(uintx x) {
assert(x != 0, "precondition");
// Reduce to mask with ones in all positions below the least
// significant set bit of x, and zeros otherwise.
uintx rx = (x - 1) & ~x; // sub x, 1, rx; andn rx, x, rx;
// Then count the set bits in the reduction.
uintx result;
__asm__(" popc %1, %0\n\t" : "=r" (result) : "r" (rx));
return result;
}
#endif // include guard

37
hotspot/src/os_cpu/solaris_x86/vm/count_trailing_zeros_solaris_x86.hpp Normal file
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/*
* Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#ifndef OS_CPU_SOLARIS_X86_VM_COUNTTRAILINGZEROS_HPP
#define OS_CPU_SOLARIS_X86_VM_COUNTTRAILINGZEROS_HPP
#include "utilities/globalDefinitions.hpp"
inline unsigned count_trailing_zeros(uintx x) {
assert(x != 0, "precondition");
uintx result;
__asm__(" rep bsfq %1, %0" : "=r" (result) : "rm" (x));
return result;
}
#endif // include guard

108
hotspot/src/share/vm/utilities/count_trailing_zeros.hpp Normal file
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/*
* Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#ifndef SHARE_VM_UTILITIES_COUNTTRAILINGZEROS_HPP
#define SHARE_VM_UTILITIES_COUNTTRAILINGZEROS_HPP
#include "utilities/debug.hpp"
#include "utilities/globalDefinitions.hpp"
// unsigned count_trailing_zeros(uintx x)
// Return the number of trailing zeros in x, e.g. the zero-based index
// of the least significant set bit in x.
// Precondition: x != 0.
// Dispatch on toolchain to select implementation.
/*****************************************************************************
* GCC and compatible (including Clang)
*****************************************************************************/
#if defined(TARGET_COMPILER_gcc)
inline unsigned count_trailing_zeros(uintx x) {
STATIC_ASSERT(sizeof(unsigned long) == sizeof(uintx));
assert(x != 0, "precondition");
return __builtin_ctzl(x);
}
/*****************************************************************************
* Microsoft Visual Studio
*****************************************************************************/
#elif defined(TARGET_COMPILER_visCPP)
#include <intrin.h>
#ifdef _LP64
#pragma intrinsic(_BitScanForward64)
#else
#pragma intrinsic(_BitScanForward)
#endif
inline unsigned count_trailing_zeros(uintx x) {
assert(x != 0, "precondition");
unsigned long index;
#ifdef _LP64
_BitScanForward64(&index, x);
#else
_BitScanForward(&index, x);
#endif
return index;
}
/*****************************************************************************
* IBM XL C/C++
*****************************************************************************/
#elif defined(TARGET_COMPILER_xlc)
#include <builtins.h>
inline unsigned count_trailing_zeros(uintx x) {
assert(x != 0, "precondition");
#ifdef _LP64
return __cnttz8(x);
#else
return __cnttz4(x);
#endif
}
/*****************************************************************************
* Oracle Studio
*****************************************************************************/
#elif defined(TARGET_COMPILER_sparcWorks)
// No compiler built-in / intrinsic, so use inline assembler.
#include "utilities/macros.hpp"
#include OS_CPU_HEADER(count_trailing_zeros)
/*****************************************************************************
* Unknown toolchain
*****************************************************************************/
#else
#error Unknown TARGET_COMPILER
#endif // Toolchain dispatch
#endif // include guard

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hotspot/test/native/utilities/test_count_trailing_zeros.cpp Normal file
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/*
* Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*
*/
#include "precompiled.hpp"
#include "utilities/count_trailing_zeros.hpp"
#include "utilities/globalDefinitions.hpp"
#include "unittest.hpp"
TEST(count_trailing_zeros, one_or_two_set_bits) {
unsigned i = 0; // Position of a set bit.
for (uintx ix = 1; ix != 0; ix <<= 1, ++i) {
unsigned j = 0; // Position of a set bit.
for (uintx jx = 1; jx != 0; jx <<= 1, ++j) {
uintx value = ix | jx;
EXPECT_EQ(MIN2(i, j), count_trailing_zeros(value))
<< "value = " << value;
}
}
}
TEST(count_trailing_zeros, all_ones_followed_by_all_zeros) {
unsigned i = BitsPerWord - 1; // Index of most significant set bit.
uintx value = ~(uintx)0;
for ( ; value != 0; value >>= 1, --i) {
EXPECT_EQ(0u, count_trailing_zeros(value))
<< "value = " << value;
}
}
TEST(count_trailing_zeros, all_zeros_followed_by_all_ones) {
unsigned i = 0; // Index of least significant set bit.
uintx value = ~(uintx)0;
for ( ; value != 0; value <<= 1, ++i) {
EXPECT_EQ(i, count_trailing_zeros(value))
<< "value = " << value;
}
}