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Add a class that multiplexes two pointer types
This patch adds a pointer_mux<T1, T2> 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.
This commit is contained in:
Richard Sandiford
parent
0dd4829643
commit
ac62dce5e5
251
gcc/mux-utils.h
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251
gcc/mux-utils.h
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// Multiplexer utilities
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// Copyright (C) 2020 Free Software Foundation, Inc.
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//
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// This file is part of GCC.
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//
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// GCC is free software; you can redistribute it and/or modify it under
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// the terms of the GNU General Public License as published by the Free
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// Software Foundation; either version 3, or (at your option) any later
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// version.
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//
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// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
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// WARRANTY; without even the implied warranty of MERCHANTABILITY or
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// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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// for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with GCC; see the file COPYING3. If not see
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// <http://www.gnu.org/licenses/>.
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#ifndef GCC_MUX_UTILS_H
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#define GCC_MUX_UTILS_H 1
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// A class that stores a choice "A or B", where A has type T1 * and B has
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// type T2 *. Both T1 and T2 must have an alignment greater than 1, since
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// the low bit is used to identify B over A. T1 and T2 can be the same.
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//
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// A can be a null pointer but B cannot.
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//
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// Barring the requirement that B must be nonnull, using the class is
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// equivalent to using:
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//
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// union { T1 *A; T2 *B; };
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//
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// and having a separate tag bit to indicate which alternative is active.
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// However, using this class can have two advantages over a union:
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//
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// - It avoides the need to find somewhere to store the tag bit.
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//
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// - The compiler is aware that B cannot be null, which can make checks
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// of the form:
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//
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// if (auto *B = mux.dyn_cast<T2 *> ())
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//
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// more efficient. With a union-based representation, the dyn_cast
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// check could fail either because MUX is an A or because MUX is a
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// null B, both of which require a run-time test. With a pointer_mux,
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// only a check for MUX being A is needed.
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template<typename T1, typename T2 = T1>
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class pointer_mux
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{
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public:
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// Return an A pointer with the given value.
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static pointer_mux first (T1 *);
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// Return a B pointer with the given (nonnull) value.
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static pointer_mux second (T2 *);
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pointer_mux () = default;
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// Create a null A pointer.
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pointer_mux (std::nullptr_t) : m_ptr (nullptr) {}
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// Create an A or B pointer with the given value. This is only valid
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// if T1 and T2 are distinct and if T can be resolved to exactly one
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// of them.
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template<typename T,
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typename Enable = typename
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std::enable_if<std::is_convertible<T *, T1 *>::value
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!= std::is_convertible<T *, T2 *>::value>::type>
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pointer_mux (T *ptr);
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// Return true unless the pointer is a null A pointer.
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explicit operator bool () const { return m_ptr; }
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// Assign A and B pointers respectively.
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void set_first (T1 *ptr) { *this = first (ptr); }
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void set_second (T2 *ptr) { *this = second (ptr); }
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// Return true if the pointer is an A pointer.
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bool is_first () const { return !(uintptr_t (m_ptr) & 1); }
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// Return true if the pointer is a B pointer.
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bool is_second () const { return uintptr_t (m_ptr) & 1; }
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// Return the contents of the pointer, given that it is known to be
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// an A pointer.
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T1 *known_first () const { return reinterpret_cast<T1 *> (m_ptr); }
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// Return the contents of the pointer, given that it is known to be
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// a B pointer.
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T2 *known_second () const { return reinterpret_cast<T2 *> (m_ptr - 1); }
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// If the pointer is an A pointer, return its contents, otherwise
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// return null. Thus a null return can mean that the pointer is
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// either a null A pointer or a B pointer.
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//
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// If all A pointers are nonnull, it is more efficient to use:
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//
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// if (ptr.is_first ())
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// ...use ptr.known_first ()...
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//
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// over:
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//
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// if (T1 *a = ptr.first_or_null ())
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// ...use a...
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T1 *first_or_null () const;
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// If the pointer is a B pointer, return its contents, otherwise
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// return null. Using:
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//
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// if (T1 *b = ptr.second_or_null ())
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// ...use b...
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//
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// should be at least as efficient as:
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//
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// if (ptr.is_second ())
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// ...use ptr.known_second ()...
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T2 *second_or_null () const;
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// Return true if the pointer is a T.
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//
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// This is only valid if T1 and T2 are distinct and if T can be
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// resolved to exactly one of them. The condition is checked using
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// a static assertion rather than SFINAE because it gives a clearer
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// error message.
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template<typename T>
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bool is_a () const;
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// Assert that the pointer is a T and return it as such. See is_a
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// for the restrictions on T.
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template<typename T>
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T as_a () const;
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// If the pointer is a T, return it as such, otherwise return null.
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// See is_a for the restrictions on T.
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template<typename T>
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T dyn_cast () const;
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private:
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pointer_mux (char *ptr) : m_ptr (ptr) {}
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// The pointer value for A pointers, or the pointer value + 1 for B pointers.
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// Using a pointer rather than a uintptr_t tells the compiler that second ()
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// can never return null, and that second_or_null () is only null if
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// is_first ().
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char *m_ptr;
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};
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template<typename T1, typename T2>
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inline pointer_mux<T1, T2>
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pointer_mux<T1, T2>::first (T1 *ptr)
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{
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gcc_checking_assert (!(uintptr_t (ptr) & 1));
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return reinterpret_cast<char *> (ptr);
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}
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template<typename T1, typename T2>
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inline pointer_mux<T1, T2>
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pointer_mux<T1, T2>::second (T2 *ptr)
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{
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gcc_checking_assert (ptr && !(uintptr_t (ptr) & 1));
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return reinterpret_cast<char *> (ptr) + 1;
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}
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template<typename T1, typename T2>
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template<typename T, typename Enable>
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inline pointer_mux<T1, T2>::pointer_mux (T *ptr)
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: m_ptr (reinterpret_cast<char *> (ptr))
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{
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if (std::is_convertible<T *, T2 *>::value)
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{
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gcc_checking_assert (m_ptr);
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m_ptr += 1;
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}
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}
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template<typename T1, typename T2>
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inline T1 *
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pointer_mux<T1, T2>::first_or_null () const
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{
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return is_first () ? known_first () : nullptr;
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}
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template<typename T1, typename T2>
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inline T2 *
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pointer_mux<T1, T2>::second_or_null () const
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{
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// Micro optimization that's effective as of GCC 11: compute the value
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// of the second pointer as an integer and test that, so that the integer
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// result can be reused as the pointer and so that all computation can
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// happen before a branch on null. This reduces the number of branches
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// needed for loops.
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return (uintptr_t (m_ptr) - 1) & 1 ? nullptr : known_second ();
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}
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template<typename T1, typename T2>
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template<typename T>
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inline bool
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pointer_mux<T1, T2>::is_a () const
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{
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static_assert (std::is_convertible<T1 *, T>::value
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!= std::is_convertible<T2 *, T>::value,
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"Ambiguous pointer type");
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if (std::is_convertible<T2 *, T>::value)
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return is_second ();
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else
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return is_first ();
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}
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template<typename T1, typename T2>
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template<typename T>
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inline T
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pointer_mux<T1, T2>::as_a () const
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{
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static_assert (std::is_convertible<T1 *, T>::value
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!= std::is_convertible<T2 *, T>::value,
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"Ambiguous pointer type");
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if (std::is_convertible<T2 *, T>::value)
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{
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gcc_checking_assert (is_second ());
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return reinterpret_cast<T> (m_ptr - 1);
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}
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else
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{
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gcc_checking_assert (is_first ());
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return reinterpret_cast<T> (m_ptr);
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}
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}
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template<typename T1, typename T2>
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template<typename T>
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inline T
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pointer_mux<T1, T2>::dyn_cast () const
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{
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static_assert (std::is_convertible<T1 *, T>::value
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!= std::is_convertible<T2 *, T>::value,
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"Ambiguous pointer type");
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if (std::is_convertible<T2 *, T>::value)
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{
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if (is_second ())
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return reinterpret_cast<T> (m_ptr - 1);
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}
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else
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{
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if (is_first ())
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return reinterpret_cast<T> (m_ptr);
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}
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return nullptr;
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}
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#endif
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