// -*- C++ -*- header.

// Copyright (C) 2008, 2009, 2010 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library 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.

// This library 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.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.

/** @file atomic
 *  This is a Standard C++ Library header.
 */

// Based on "C++ Atomic Types and Operations" by Hans Boehm and Lawrence Crowl.
// http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2427.html

#ifndef _GLIBCXX_ATOMIC
#define _GLIBCXX_ATOMIC 1

#pragma GCC system_header

#ifndef __GXX_EXPERIMENTAL_CXX0X__
# include <bits/c++0x_warning.h>
#endif

#include <bits/atomic_base.h>
#include <cstddef>

_GLIBCXX_BEGIN_NAMESPACE(std)

  /**
   * @addtogroup atomics
   * @{
   */

  /// kill_dependency
  template<typename _Tp>
    inline _Tp
    kill_dependency(_Tp __y)
    {
      _Tp ret(__y);
      return ret;
    }

  inline memory_order
  __calculate_memory_order(memory_order __m)
  {
    const bool __cond1 = __m == memory_order_release;
    const bool __cond2 = __m == memory_order_acq_rel;
    memory_order __mo1(__cond1 ? memory_order_relaxed : __m);
    memory_order __mo2(__cond2 ? memory_order_acquire : __mo1);
    return __mo2;
  }

  //
  // Three nested namespaces for atomic implementation details.
  //
  // The nested namespace inlined into std:: is determined by the value
  // of the _GLIBCXX_ATOMIC_PROPERTY macro and the resulting
  // ATOMIC_*_LOCK_FREE macros. See file atomic_base.h.
  //
  // 0 == __atomic0 == Never lock-free
  // 1 == __atomic1 == Best available, sometimes lock-free
  // 2 == __atomic2 == Always lock-free
#include <bits/atomic_0.h>
#include <bits/atomic_2.h>

  /// atomic
  /// 29.4.3, Generic atomic type, primary class template.
  template<typename _Tp>
    struct atomic
    {
    private:
      _Tp _M_i;

    public:
      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(_Tp __i) : _M_i(__i) { }

      operator _Tp() const;

      _Tp
      operator=(_Tp __i) { store(__i); return __i; }

      bool
      is_lock_free() const volatile;

      void
      store(_Tp, memory_order = memory_order_seq_cst) volatile;

      _Tp
      load(memory_order = memory_order_seq_cst) const volatile;

      _Tp
      exchange(_Tp __i, memory_order = memory_order_seq_cst) volatile;

      bool
      compare_exchange_weak(_Tp&, _Tp, memory_order, memory_order) volatile;

      bool
      compare_exchange_strong(_Tp&, _Tp, memory_order, memory_order) volatile;

      bool
      compare_exchange_weak(_Tp&, _Tp,
			    memory_order = memory_order_seq_cst) volatile;

      bool
      compare_exchange_strong(_Tp&, _Tp,
			      memory_order = memory_order_seq_cst) volatile;
    };


  /// Partial specialization for pointer types.
  template<typename _Tp>
    struct atomic<_Tp*> : atomic_address
    {
      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(_Tp* __v) : atomic_address(__v) { }

      void
      store(_Tp* __v, memory_order __m = memory_order_seq_cst)
      { atomic_address::store(__v, __m); }

      _Tp*
      load(memory_order __m = memory_order_seq_cst) const
      { return static_cast<_Tp*>(atomic_address::load(__m)); }

      _Tp*
      exchange(_Tp* __v, memory_order __m = memory_order_seq_cst)
      { return static_cast<_Tp*>(atomic_address::exchange(__v, __m)); }

      bool
      compare_exchange_weak(_Tp*&, _Tp*, memory_order, memory_order);

      bool
      compare_exchange_strong(_Tp*&, _Tp*, memory_order, memory_order);

      bool
      compare_exchange_weak(_Tp*&, _Tp*, memory_order = memory_order_seq_cst);

      bool
      compare_exchange_strong(_Tp*&, _Tp*, memory_order = memory_order_seq_cst);

      _Tp*
      fetch_add(ptrdiff_t, memory_order = memory_order_seq_cst);

      _Tp*
      fetch_sub(ptrdiff_t, memory_order = memory_order_seq_cst);

      operator _Tp*() const
      { return load(); }

      _Tp*
      operator=(_Tp* __v)
      {
	store(__v);
	return __v;
      }

      _Tp*
      operator++(int) { return fetch_add(1); }

      _Tp*
      operator--(int) { return fetch_sub(1); }

      _Tp*
      operator++() { return fetch_add(1) + 1; }

      _Tp*
      operator--() { return fetch_sub(1) - 1; }

      _Tp*
      operator+=(ptrdiff_t __d)
      { return fetch_add(__d) + __d; }

      _Tp*
      operator-=(ptrdiff_t __d)
      { return fetch_sub(__d) - __d; }
    };


  /// Explicit specialization for void*
  template<>
    struct atomic<void*> : public atomic_address
    {
      typedef void* 			__integral_type;
      typedef atomic_address 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for bool.
  template<>
    struct atomic<bool> : public atomic_bool
    {
      typedef bool 			__integral_type;
      typedef atomic_bool 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for char.
  template<>
    struct atomic<char> : public atomic_char
    {
      typedef char 			__integral_type;
      typedef atomic_char 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for signed char.
  template<>
    struct atomic<signed char> : public atomic_schar
    {
      typedef signed char 		__integral_type;
      typedef atomic_schar 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for unsigned char.
  template<>
    struct atomic<unsigned char> : public atomic_uchar
    {
      typedef unsigned char 		__integral_type;
      typedef atomic_uchar 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for short.
  template<>
    struct atomic<short> : public atomic_short
    {
      typedef short 			__integral_type;
      typedef atomic_short 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for unsigned short.
  template<>
    struct atomic<unsigned short> : public atomic_ushort
    {
      typedef unsigned short 	      	__integral_type;
      typedef atomic_ushort 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for int.
  template<>
    struct atomic<int> : atomic_int
    {
      typedef int 			__integral_type;
      typedef atomic_int 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for unsigned int.
  template<>
    struct atomic<unsigned int> : public atomic_uint
    {
      typedef unsigned int		__integral_type;
      typedef atomic_uint 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for long.
  template<>
    struct atomic<long> : public atomic_long
    {
      typedef long 			__integral_type;
      typedef atomic_long 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for unsigned long.
  template<>
    struct atomic<unsigned long> : public atomic_ulong
    {
      typedef unsigned long 		__integral_type;
      typedef atomic_ulong 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for long long.
  template<>
    struct atomic<long long> : public atomic_llong
    {
      typedef long long 		__integral_type;
      typedef atomic_llong 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for unsigned long long.
  template<>
    struct atomic<unsigned long long> : public atomic_ullong
    {
      typedef unsigned long long       	__integral_type;
      typedef atomic_ullong 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for wchar_t.
  template<>
    struct atomic<wchar_t> : public atomic_wchar_t
    {
      typedef wchar_t 			__integral_type;
      typedef atomic_wchar_t 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for char16_t.
  template<>
    struct atomic<char16_t> : public atomic_char16_t
    {
      typedef char16_t 			__integral_type;
      typedef atomic_char16_t 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  /// Explicit specialization for char32_t.
  template<>
    struct atomic<char32_t> : public atomic_char32_t
    {
      typedef char32_t 			__integral_type;
      typedef atomic_char32_t 		__base_type;

      atomic() = default;
      ~atomic() = default;
      atomic(const atomic&) = delete;
      atomic& operator=(const atomic&) volatile = delete;

      atomic(__integral_type __i) : __base_type(__i) { }

      using __base_type::operator __integral_type;
      using __base_type::operator=;
    };

  template<typename _Tp>
    bool
    atomic<_Tp*>::compare_exchange_weak(_Tp*& __r, _Tp* __v, memory_order __m1,
					memory_order __m2)
    {
      void** __vr = reinterpret_cast<void**>(&__r);
      void* __vv = static_cast<void*>(__v);
      return atomic_address::compare_exchange_weak(*__vr, __vv, __m1, __m2);
    }

  template<typename _Tp>
    bool
    atomic<_Tp*>::compare_exchange_strong(_Tp*& __r, _Tp* __v,
					  memory_order __m1,
					  memory_order __m2)
    {
      void** __vr = reinterpret_cast<void**>(&__r);
      void* __vv = static_cast<void*>(__v);
      return atomic_address::compare_exchange_strong(*__vr, __vv, __m1, __m2);
    }

  template<typename _Tp>
    bool
    atomic<_Tp*>::compare_exchange_weak(_Tp*& __r, _Tp* __v,
					memory_order __m)
    {
      return compare_exchange_weak(__r, __v, __m,
				   __calculate_memory_order(__m));
    }

  template<typename _Tp>
    bool
    atomic<_Tp*>::compare_exchange_strong(_Tp*& __r, _Tp* __v,
					memory_order __m)
    {
      return compare_exchange_strong(__r, __v, __m,
				     __calculate_memory_order(__m));
    }

  template<typename _Tp>
    _Tp*
    atomic<_Tp*>::fetch_add(ptrdiff_t __d, memory_order __m)
    {
      void* __p = atomic_fetch_add_explicit(this, sizeof(_Tp) * __d, __m);
      return static_cast<_Tp*>(__p);
    }

  template<typename _Tp>
    _Tp*
    atomic<_Tp*>::fetch_sub(ptrdiff_t __d, memory_order __m)
    {
      void* __p = atomic_fetch_sub_explicit(this, sizeof(_Tp) * __d, __m);
      return static_cast<_Tp*>(__p);
    }

  // Convenience function definitions, atomic_flag.
  inline bool
  atomic_flag_test_and_set_explicit(atomic_flag* __a, memory_order __m)
  { return __a->test_and_set(__m); }

  inline void
  atomic_flag_clear_explicit(atomic_flag* __a, memory_order __m)
  { return __a->clear(__m); }


  // Convenience function definitions, atomic_address.
  inline bool
  atomic_is_lock_free(const atomic_address* __a)
  { return __a->is_lock_free(); }

  inline void
  atomic_store(atomic_address* __a, void* __v)
  { __a->store(__v); }

  inline void
  atomic_store_explicit(atomic_address* __a, void* __v, memory_order __m)
  { __a->store(__v, __m); }

  inline void*
  atomic_load(const atomic_address* __a)
  { return __a->load(); }

  inline void*
  atomic_load_explicit(const atomic_address* __a, memory_order __m)
  { return __a->load(__m); }

  inline void*
  atomic_exchange(atomic_address* __a, void* __v)
  { return __a->exchange(__v); }

  inline void*
  atomic_exchange_explicit(atomic_address* __a, void* __v, memory_order __m)
  { return __a->exchange(__v, __m); }

  inline bool
  atomic_compare_exchange_weak(atomic_address* __a, void** __v1, void* __v2)
  {
    return __a->compare_exchange_weak(*__v1, __v2, memory_order_seq_cst,
				      memory_order_seq_cst);
  }

  inline bool
  atomic_compare_exchange_strong(atomic_address* __a,
			       void** __v1, void* __v2)
  {
    return __a->compare_exchange_strong(*__v1, __v2, memory_order_seq_cst,
				      memory_order_seq_cst);
  }

  inline bool
  atomic_compare_exchange_weak_explicit(atomic_address* __a,
					void** __v1, void* __v2,
					memory_order __m1, memory_order __m2)
  { return __a->compare_exchange_weak(*__v1, __v2, __m1, __m2); }

  inline bool
  atomic_compare_exchange_strong_explicit(atomic_address* __a,
					  void** __v1, void* __v2,
					  memory_order __m1, memory_order __m2)
  { return __a->compare_exchange_strong(*__v1, __v2, __m1, __m2); }

  inline void*
  atomic_fetch_add_explicit(atomic_address* __a, ptrdiff_t __d,
			    memory_order __m)
  { return __a->fetch_add(__d, __m); }

  inline void*
  atomic_fetch_add(atomic_address* __a, ptrdiff_t __d)
  { return __a->fetch_add(__d); }

  inline void*
  atomic_fetch_sub_explicit(atomic_address* __a, ptrdiff_t __d,
			    memory_order __m)
  { return __a->fetch_sub(__d, __m); }

  inline void*
  atomic_fetch_sub(atomic_address* __a, ptrdiff_t __d)
  { return __a->fetch_sub(__d); }


  // Convenience function definitions, atomic_bool.
  inline bool
  atomic_is_lock_free(const atomic_bool* __a)
  { return __a->is_lock_free(); }

  inline void
  atomic_store(atomic_bool* __a, bool __i)
  { __a->store(__i); }

  inline void
  atomic_store_explicit(atomic_bool* __a, bool __i, memory_order __m)
  { __a->store(__i, __m); }

  inline bool
  atomic_load(const atomic_bool* __a)
  { return __a->load(); }

  inline bool
  atomic_load_explicit(const atomic_bool* __a, memory_order __m)
  { return __a->load(__m); }

  inline bool
  atomic_exchange(atomic_bool* __a, bool __i)
  { return __a->exchange(__i); }

  inline bool
  atomic_exchange_explicit(atomic_bool* __a, bool __i, memory_order __m)
  { return __a->exchange(__i, __m); }

  inline bool
  atomic_compare_exchange_weak(atomic_bool* __a, bool* __i1, bool __i2)
  {
    return __a->compare_exchange_weak(*__i1, __i2, memory_order_seq_cst,
				      memory_order_seq_cst);
  }

  inline bool
  atomic_compare_exchange_strong(atomic_bool* __a, bool* __i1, bool __i2)
  {
    return __a->compare_exchange_strong(*__i1, __i2, memory_order_seq_cst,
					memory_order_seq_cst);
  }

  inline bool
  atomic_compare_exchange_weak_explicit(atomic_bool* __a, bool* __i1,
					bool __i2, memory_order __m1,
					memory_order __m2)
  { return __a->compare_exchange_weak(*__i1, __i2, __m1, __m2); }

  inline bool
  atomic_compare_exchange_strong_explicit(atomic_bool* __a,
					  bool* __i1, bool __i2,
					  memory_order __m1, memory_order __m2)
  { return __a->compare_exchange_strong(*__i1, __i2, __m1, __m2); }


  // Free standing functions. Template argument should be constricted
  // to intergral types as specified in the standard.
  template<typename _ITp>
    inline void
    atomic_store_explicit(__atomic_base<_ITp>* __a, _ITp __i, memory_order __m)
    { __a->store(__i, __m); }

  template<typename _ITp>
    inline _ITp
    atomic_load_explicit(const __atomic_base<_ITp>* __a, memory_order __m)
    { return __a->load(__m); }

  template<typename _ITp>
    inline _ITp
    atomic_exchange_explicit(__atomic_base<_ITp>* __a, _ITp __i,
			     memory_order __m)
    { return __a->exchange(__i, __m); }

  template<typename _ITp>
    inline bool
    atomic_compare_exchange_weak_explicit(__atomic_base<_ITp>* __a,
					  _ITp* __i1, _ITp __i2,
					  memory_order __m1, memory_order __m2)
    { return __a->compare_exchange_weak(*__i1, __i2, __m1, __m2); }

  template<typename _ITp>
    inline bool
    atomic_compare_exchange_strong_explicit(__atomic_base<_ITp>* __a,
					    _ITp* __i1, _ITp __i2,
					    memory_order __m1,
					    memory_order __m2)
    { return __a->compare_exchange_strong(*__i1, __i2, __m1, __m2); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_add_explicit(__atomic_base<_ITp>* __a, _ITp __i,
			      memory_order __m)
    { return __a->fetch_add(__i, __m); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_sub_explicit(__atomic_base<_ITp>* __a, _ITp __i,
			      memory_order __m)
    { return __a->fetch_sub(__i, __m); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_and_explicit(__atomic_base<_ITp>* __a, _ITp __i,
			      memory_order __m)
    { return __a->fetch_and(__i, __m); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_or_explicit(__atomic_base<_ITp>* __a, _ITp __i,
			     memory_order __m)
    { return __a->fetch_or(__i, __m); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_xor_explicit(__atomic_base<_ITp>* __a, _ITp __i,
			      memory_order __m)
    { return __a->fetch_xor(__i, __m); }

  template<typename _ITp>
    inline bool
    atomic_is_lock_free(const __atomic_base<_ITp>* __a)
    { return __a->is_lock_free(); }

  template<typename _ITp>
    inline void
    atomic_store(__atomic_base<_ITp>* __a, _ITp __i)
    { atomic_store_explicit(__a, __i, memory_order_seq_cst); }

  template<typename _ITp>
    inline _ITp
    atomic_load(const __atomic_base<_ITp>* __a)
    { return atomic_load_explicit(__a, memory_order_seq_cst); }

  template<typename _ITp>
    inline _ITp
    atomic_exchange(__atomic_base<_ITp>* __a, _ITp __i)
    { return atomic_exchange_explicit(__a, __i, memory_order_seq_cst); }

  template<typename _ITp>
    inline bool
    atomic_compare_exchange_weak(__atomic_base<_ITp>* __a,
				 _ITp* __i1, _ITp __i2)
    {
      return atomic_compare_exchange_weak_explicit(__a, __i1, __i2,
						   memory_order_seq_cst,
						   memory_order_seq_cst);
    }

  template<typename _ITp>
    inline bool
    atomic_compare_exchange_strong(__atomic_base<_ITp>* __a,
				   _ITp* __i1, _ITp __i2)
    {
      return atomic_compare_exchange_strong_explicit(__a, __i1, __i2,
						     memory_order_seq_cst,
						     memory_order_seq_cst);
    }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_add(__atomic_base<_ITp>* __a, _ITp __i)
    { return atomic_fetch_add_explicit(__a, __i, memory_order_seq_cst); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_sub(__atomic_base<_ITp>* __a, _ITp __i)
    { return atomic_fetch_sub_explicit(__a, __i, memory_order_seq_cst); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_and(__atomic_base<_ITp>* __a, _ITp __i)
    { return atomic_fetch_and_explicit(__a, __i, memory_order_seq_cst); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_or(__atomic_base<_ITp>* __a, _ITp __i)
    { return atomic_fetch_or_explicit(__a, __i, memory_order_seq_cst); }

  template<typename _ITp>
    inline _ITp
    atomic_fetch_xor(__atomic_base<_ITp>* __a, _ITp __i)
    { return atomic_fetch_xor_explicit(__a, __i, memory_order_seq_cst); }

  // @} group atomics

_GLIBCXX_END_NAMESPACE

#endif