changeset 46437:cda23e690843

8179004: Add an efficient implementation of the "count trailing zeros" operation Summary: Added count_trailing_zeros function, with platform-dependent implementations. Reviewed-by: dholmes, cjplummer
author kbarrett
date Fri, 05 May 2017 16:20:42 -0400
parents 755e01cd0b9d
children 6de560f6c1ad
files hotspot/src/os_cpu/solaris_sparc/vm/count_trailing_zeros_solaris_sparc.hpp hotspot/src/os_cpu/solaris_x86/vm/count_trailing_zeros_solaris_x86.hpp hotspot/src/share/vm/utilities/count_trailing_zeros.hpp hotspot/test/native/utilities/test_count_trailing_zeros.cpp
diffstat 4 files changed, 244 insertions(+), 0 deletions(-) [+]
line wrap: on
line diff
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/os_cpu/solaris_sparc/vm/count_trailing_zeros_solaris_sparc.hpp	Fri May 05 16:20:42 2017 -0400
@@ -0,0 +1,41 @@
+/*
+ * 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
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/os_cpu/solaris_x86/vm/count_trailing_zeros_solaris_x86.hpp	Fri May 05 16:20:42 2017 -0400
@@ -0,0 +1,37 @@
+/*
+ * 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
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/share/vm/utilities/count_trailing_zeros.hpp	Fri May 05 16:20:42 2017 -0400
@@ -0,0 +1,108 @@
+/*
+ * 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
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/test/native/utilities/test_count_trailing_zeros.cpp	Fri May 05 16:20:42 2017 -0400
@@ -0,0 +1,58 @@
+/*
+ * 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;
+  }
+}