From ac62dce5e5ff8e02682e2129e8fecc211c707551 Mon Sep 17 00:00:00 2001 From: Richard Sandiford Date: Thu, 17 Dec 2020 00:15:01 +0000 Subject: [PATCH] Add a class that multiplexes two pointer types This patch adds a pointer_mux class that provides similar functionality to: union { T1 *a; T2 *b; }; ... bool is_b_rather_than_a; except that the is_b_rather_than_a tag is stored in the low bit of the pointer. See the comments in the patch for a comparison between the two approaches and why this one can be more efficient. I've tried to microoptimise the class a fair bit, since a later patch uses it extensively in order to keep the sizes of data structures down. gcc/ * mux-utils.h: New file. --- gcc/mux-utils.h | 251 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 251 insertions(+) create mode 100644 gcc/mux-utils.h diff --git a/gcc/mux-utils.h b/gcc/mux-utils.h new file mode 100644 index 00000000000..b026a9fa4c1 --- /dev/null +++ b/gcc/mux-utils.h @@ -0,0 +1,251 @@ +// Multiplexer utilities +// Copyright (C) 2020 Free Software Foundation, Inc. +// +// This file is part of GCC. +// +// GCC is free software; you can redistribute it and/or modify it under +// the terms of the GNU General Public License as published by the Free +// Software Foundation; either version 3, or (at your option) any later +// version. +// +// GCC 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 +// for more details. +// +// You should have received a copy of the GNU General Public License +// along with GCC; see the file COPYING3. If not see +// . + +#ifndef GCC_MUX_UTILS_H +#define GCC_MUX_UTILS_H 1 + +// A class that stores a choice "A or B", where A has type T1 * and B has +// type T2 *. Both T1 and T2 must have an alignment greater than 1, since +// the low bit is used to identify B over A. T1 and T2 can be the same. +// +// A can be a null pointer but B cannot. +// +// Barring the requirement that B must be nonnull, using the class is +// equivalent to using: +// +// union { T1 *A; T2 *B; }; +// +// and having a separate tag bit to indicate which alternative is active. +// However, using this class can have two advantages over a union: +// +// - It avoides the need to find somewhere to store the tag bit. +// +// - The compiler is aware that B cannot be null, which can make checks +// of the form: +// +// if (auto *B = mux.dyn_cast ()) +// +// more efficient. With a union-based representation, the dyn_cast +// check could fail either because MUX is an A or because MUX is a +// null B, both of which require a run-time test. With a pointer_mux, +// only a check for MUX being A is needed. +template +class pointer_mux +{ +public: + // Return an A pointer with the given value. + static pointer_mux first (T1 *); + + // Return a B pointer with the given (nonnull) value. + static pointer_mux second (T2 *); + + pointer_mux () = default; + + // Create a null A pointer. + pointer_mux (std::nullptr_t) : m_ptr (nullptr) {} + + // Create an A or B pointer with the given value. This is only valid + // if T1 and T2 are distinct and if T can be resolved to exactly one + // of them. + template::value + != std::is_convertible::value>::type> + pointer_mux (T *ptr); + + // Return true unless the pointer is a null A pointer. + explicit operator bool () const { return m_ptr; } + + // Assign A and B pointers respectively. + void set_first (T1 *ptr) { *this = first (ptr); } + void set_second (T2 *ptr) { *this = second (ptr); } + + // Return true if the pointer is an A pointer. + bool is_first () const { return !(uintptr_t (m_ptr) & 1); } + + // Return true if the pointer is a B pointer. + bool is_second () const { return uintptr_t (m_ptr) & 1; } + + // Return the contents of the pointer, given that it is known to be + // an A pointer. + T1 *known_first () const { return reinterpret_cast (m_ptr); } + + // Return the contents of the pointer, given that it is known to be + // a B pointer. + T2 *known_second () const { return reinterpret_cast (m_ptr - 1); } + + // If the pointer is an A pointer, return its contents, otherwise + // return null. Thus a null return can mean that the pointer is + // either a null A pointer or a B pointer. + // + // If all A pointers are nonnull, it is more efficient to use: + // + // if (ptr.is_first ()) + // ...use ptr.known_first ()... + // + // over: + // + // if (T1 *a = ptr.first_or_null ()) + // ...use a... + T1 *first_or_null () const; + + // If the pointer is a B pointer, return its contents, otherwise + // return null. Using: + // + // if (T1 *b = ptr.second_or_null ()) + // ...use b... + // + // should be at least as efficient as: + // + // if (ptr.is_second ()) + // ...use ptr.known_second ()... + T2 *second_or_null () const; + + // Return true if the pointer is a T. + // + // This is only valid if T1 and T2 are distinct and if T can be + // resolved to exactly one of them. The condition is checked using + // a static assertion rather than SFINAE because it gives a clearer + // error message. + template + bool is_a () const; + + // Assert that the pointer is a T and return it as such. See is_a + // for the restrictions on T. + template + T as_a () const; + + // If the pointer is a T, return it as such, otherwise return null. + // See is_a for the restrictions on T. + template + T dyn_cast () const; + +private: + pointer_mux (char *ptr) : m_ptr (ptr) {} + + // The pointer value for A pointers, or the pointer value + 1 for B pointers. + // Using a pointer rather than a uintptr_t tells the compiler that second () + // can never return null, and that second_or_null () is only null if + // is_first (). + char *m_ptr; +}; + +template +inline pointer_mux +pointer_mux::first (T1 *ptr) +{ + gcc_checking_assert (!(uintptr_t (ptr) & 1)); + return reinterpret_cast (ptr); +} + +template +inline pointer_mux +pointer_mux::second (T2 *ptr) +{ + gcc_checking_assert (ptr && !(uintptr_t (ptr) & 1)); + return reinterpret_cast (ptr) + 1; +} + +template +template +inline pointer_mux::pointer_mux (T *ptr) + : m_ptr (reinterpret_cast (ptr)) +{ + if (std::is_convertible::value) + { + gcc_checking_assert (m_ptr); + m_ptr += 1; + } +} + +template +inline T1 * +pointer_mux::first_or_null () const +{ + return is_first () ? known_first () : nullptr; +} + +template +inline T2 * +pointer_mux::second_or_null () const +{ + // Micro optimization that's effective as of GCC 11: compute the value + // of the second pointer as an integer and test that, so that the integer + // result can be reused as the pointer and so that all computation can + // happen before a branch on null. This reduces the number of branches + // needed for loops. + return (uintptr_t (m_ptr) - 1) & 1 ? nullptr : known_second (); +} + +template +template +inline bool +pointer_mux::is_a () const +{ + static_assert (std::is_convertible::value + != std::is_convertible::value, + "Ambiguous pointer type"); + if (std::is_convertible::value) + return is_second (); + else + return is_first (); +} + +template +template +inline T +pointer_mux::as_a () const +{ + static_assert (std::is_convertible::value + != std::is_convertible::value, + "Ambiguous pointer type"); + if (std::is_convertible::value) + { + gcc_checking_assert (is_second ()); + return reinterpret_cast (m_ptr - 1); + } + else + { + gcc_checking_assert (is_first ()); + return reinterpret_cast (m_ptr); + } +} + +template +template +inline T +pointer_mux::dyn_cast () const +{ + static_assert (std::is_convertible::value + != std::is_convertible::value, + "Ambiguous pointer type"); + if (std::is_convertible::value) + { + if (is_second ()) + return reinterpret_cast (m_ptr - 1); + } + else + { + if (is_first ()) + return reinterpret_cast (m_ptr); + } + return nullptr; +} + +#endif -- 2.30.2