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// Multiplexer utilities
// Copyright (C) 2020-2021 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
// <http://www.gnu.org/licenses/>.
#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<T2 *> ())
//
// 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<typename T1, typename T2 = T1>
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<typename T,
typename Enable = typename
std::enable_if<std::is_convertible<T *, T1 *>::value
!= std::is_convertible<T *, T2 *>::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<T1 *> (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<T2 *> (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<typename T>
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<typename T>
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<typename T>
T dyn_cast () const;
private:
pointer_mux (char *ptr) : m_ptr (ptr) {}
// Points to the first byte of an object for A pointers or the second
// byte of an object 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<typename T1, typename T2>
inline pointer_mux<T1, T2>
pointer_mux<T1, T2>::first (T1 *ptr)
{
gcc_checking_assert (!(uintptr_t (ptr) & 1));
return reinterpret_cast<char *> (ptr);
}
template<typename T1, typename T2>
inline pointer_mux<T1, T2>
pointer_mux<T1, T2>::second (T2 *ptr)
{
gcc_checking_assert (ptr && !(uintptr_t (ptr) & 1));
return reinterpret_cast<char *> (ptr) + 1;
}
template<typename T1, typename T2>
template<typename T, typename Enable>
inline pointer_mux<T1, T2>::pointer_mux (T *ptr)
: m_ptr (reinterpret_cast<char *> (ptr))
{
if (std::is_convertible<T *, T2 *>::value)
{
gcc_checking_assert (m_ptr);
m_ptr += 1;
}
}
template<typename T1, typename T2>
inline T1 *
pointer_mux<T1, T2>::first_or_null () const
{
return is_first () ? known_first () : nullptr;
}
template<typename T1, typename T2>
inline T2 *
pointer_mux<T1, T2>::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<typename T1, typename T2>
template<typename T>
inline bool
pointer_mux<T1, T2>::is_a () const
{
static_assert (std::is_convertible<T1 *, T>::value
!= std::is_convertible<T2 *, T>::value,
"Ambiguous pointer type");
if (std::is_convertible<T2 *, T>::value)
return is_second ();
else
return is_first ();
}
template<typename T1, typename T2>
template<typename T>
inline T
pointer_mux<T1, T2>::as_a () const
{
static_assert (std::is_convertible<T1 *, T>::value
!= std::is_convertible<T2 *, T>::value,
"Ambiguous pointer type");
if (std::is_convertible<T2 *, T>::value)
{
gcc_checking_assert (is_second ());
return reinterpret_cast<T> (m_ptr - 1);
}
else
{
gcc_checking_assert (is_first ());
return reinterpret_cast<T> (m_ptr);
}
}
template<typename T1, typename T2>
template<typename T>
inline T
pointer_mux<T1, T2>::dyn_cast () const
{
static_assert (std::is_convertible<T1 *, T>::value
!= std::is_convertible<T2 *, T>::value,
"Ambiguous pointer type");
if (std::is_convertible<T2 *, T>::value)
{
if (is_second ())
return reinterpret_cast<T> (m_ptr - 1);
}
else
{
if (is_first ())
return reinterpret_cast<T> (m_ptr);
}
return nullptr;
}
#endif