252 lines
7.4 KiB
C++
252 lines
7.4 KiB
C++
// Multiplexer utilities
|
|
// Copyright (C) 2020-2023 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
|