Harlequin standard atomic operations.

Files
file	hqatomic.h
	Macros implementing atomic operations for multi-thread variable access.
file	hqspin.h
	Spinlocks implemented using atomic operations.

Macros
#define	HQ_ATOMIC_SUPPORTED   1
#define	HqAtomicIncrement(ptr_, before_)
#define	HqAtomicDecrement(ptr_, after_)
#define	HqAtomicAdd(ptr_, value_, after_)
#define	HqAtomicAdd(ptr_, value_, after_)
#define	HqAtomicMaximum(ptr_, value_, after_)
#define	HqAtomicMinimum(ptr_, value_, after_)
#define	HqAtomicCAS(ptr_, compareto_, swapfor_, swapped_)
#define	HqAtomicCASPointer(ptr_, compareto_, swapfor_, swapped_, type_)
#define	yield_processor()   (void)SwitchToThread()
	Yield the processor to another thread, if there is one runnable.
#define	spinlock_pointer(addr_, locked_, count_)
	Lock a pointer using atomic operations. More...
#define	spintrylock_pointer(addr_, locked_, didlock_)
	Try to lock a pointer using atomic operations. More...
#define	spinunlock_pointer(addr_, unlocked_)
	Unlock a pointer using atomic operations. More...
#define	spin_pointer_without_lock(ptr_, uptr_)
	Get the value of a potentially spinlocked pointer in its unlocked state. More...
#define	spinlock_counter(addr_, count_)
	Lock a counter semaphore using atomic operations. More...
#define	spinunlock_counter(addr_)
	Unlock a semaphore counter using atomic operations. More...

Typedefs
typedef long	hq_atomic_counter_t
typedef void *	spinlock_void_ptr
	Aliased void pointer, to quieten GCC warnings and prevent harmful optimisations.

Enumerations
enum	{ HQSPIN_YIELD_NEVER = 0 , HQSPIN_YIELD_ALWAYS = 1 }
	Constants for yields per spinlock cycle. More...

Detailed Description

We provide a set of minimal atomic operations, which we can support effectively across multiple compilers and processor architectures, using either compiler intrinsics or in-line assembly. The minimal set of operations provided are:

• HqAtomicIncrement() - Increment a signed integral value.
• HqAtomicDecrement() - Decrement a signed integral value.
• HqAtomicCAS() - Compare and swap two signed integral values.
• HqAtomicCASPointer() - Compare and swap two pointers.

In principle, all other atomic operations can be built on top of CAS (compare and swap). However, it's just a bit too clunky having to implement some simple operations on top of CAS.

A couple of additional atomic integer operations are also supplied:

• HqAtomicAdd() - Atomically add two values.
• HqAtomicMaximum() - Atomically set the maximum of two values.
• HqAtomicMinimum() - Atomically set the minimum of two values.

The equivalent prototypes for the operations implemented are:

typedef ... hq_atomic_counter_t ;

An integer type suitable for the atomic increment and decrement.

HqAtomicIncrement(hq_atomic_counter_t *ptr, hq_atomic_counter_t& before) ;

Atomically increment the contents of *ptr, and return the value before it was incremented in before.

HqAtomicDecrement(hq_atomic_counter_t *ptr, hq_atomic_counter_t& after) ;

Atomically decrement the contents of *ptr, and return the value after it was decremented in after.

HqAtomicCAS(hq_atomic_counter_t *ptr, hq_atomic_counter_t compare,
            hq_atomic_counter_t swap, HqBool& swapped) ;

Atomically compare the value of *ptr with compare, and if they match, swap the contents of *ptr for swap. Store a boolean in swapped indicating if the swap was performed.

HqAtomicCASPointer(type **ptr, type *compare, type *swap,
                   HqBool& swapped, type) ;

HqAtomicAdd(hq_atomic_counter_t *ptr, hq_atomic_counter_t value,
            hq_atomic_counter_t& after) ;

Atomically set *ptr to the sum of *ptr and value, returning the value after the addition in after.

HqAtomicMaximum(hq_atomic_counter_t *ptr, hq_atomic_counter_t value,
                hq_atomic_counter_t& after) ;

Atomically set *ptr to the maximum of *ptr and value, returning the maximum value in after.

HqAtomicMinimum(hq_atomic_counter_t *ptr, hq_atomic_counter_t value,
                hq_atomic_counter_t& after) ;

Atomically set *ptr to the minimum of *ptr and value, returning the minimum value in after.

Macro Definition Documentation

HQ_ATOMIC_SUPPORTED

#define HQ_ATOMIC_SUPPORTED   1

Major compiler versions that support atomic operations are defined first. If the define HQ_ATOMIC_SUPPORTED is not defined, then the architecture specific assembly versions may be used.

HqAtomicAdd [1/2]

#define HqAtomicAdd	(		ptr_,
			value_,
			after_
	)

Value:

  MACRO_START                   \
  hq_atomic_counter_t _sum_, _value_ = (value_) ;                       \
  HqBool _done_ = FALSE ;                                               \
  do {                                                                  \
    hq_atomic_counter_t _prev_ = *(ptr_) ;                              \
    _sum_ = _prev_ + _value_ ;                                          \
    HqAtomicCAS(ptr_, _prev_, _sum_, _done_) ;                          \
  } while ( !_done_ ) {                                                 \
  after_ = _sum_ ;                                                      \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter
[in]	value_	A value to add to the atomic counter stored in *ptr_.
[out]	after_	The name of an atomic counter variable that will be set to the sum of *ptr_ and value_.

Atomically ensure that the value of *ptr_ is the sum of *ptr_, and value_, and return the sum in after_.

HqAtomicAdd [2/2]

#define HqAtomicAdd	(		ptr_,
			value_,
			after_
	)

Value:

  MACRO_START                   \
  after_ = _InterlockedExchangeAdd(ptr_, value_) + (value_) ;           \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter
[in]	value_	A value to add to the atomic counter stored in *ptr_.
[out]	after_	The name of an atomic counter variable that will be set to the sum of *ptr_ and value_.

Atomically ensure that the value of *ptr_ is the sum of *ptr_, and value_, and return the sum in after_.

HqAtomicCAS

#define HqAtomicCAS	(		ptr_,
			compareto_,
			swapfor_,
			swapped_
	)

Value:

  MACRO_START \
  hq_atomic_counter_t _compareto_ = (compareto_) ; \
  swapped_ = (_compareto_ == _InterlockedCompareExchange((ptr_), (swapfor_), _compareto_)) ; \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter
[in]	compareto_	A value to compare the atomic counter stored in *ptr_ with.
[in]	swapfor_	A value to swap the atomic counter stored in *ptr_ with.
[out]	swapped_	The name of a boolean variable that will be set to TRUE if the swap was made, FALSE if the swap was not made.

Atomically compare the value of *ptr_ with compareto_, and if they match, swap the contents of *ptr_ for swapfor_. Store a boolean in swapped_ indicating if the swap was performed.

HqAtomicCASPointer

#define HqAtomicCASPointer	(		ptr_,
			compareto_,
			swapfor_,
			swapped_,
			type_
	)

Value:

  MACRO_START \
  void *_compareto_ = (compareto_) ;                                    \
  swapped_ = (_compareto_ == _InterlockedCompareExchangePointer((ptr_), (swapfor_), _compareto_)) ; \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to a pointer
[in]	compareto_	A value to compare the pointer stored in *ptr_ with.
[in]	swapfor_	A value to swap the pointer stored in *ptr_ with.
[out]	swapped_	The name of a boolean variable that will be set to TRUE if the swap was made, FALSE if the swap was not made.
[in]	type_	The type of the pointer stored in *ptr_.

Atomically compare the value of *ptr_ with compareto_, and if they match, swap the contents of *ptr_ for swapfor_. Store a boolean in swapped_ indicating if the swap was performed. (This is the same operation as HqAtomicCAS(), but allows pointers to be compared and swapped.)

HqAtomicDecrement

#define HqAtomicDecrement	(		ptr_,
			after_
	)

Value:

  MACRO_START \
  after_ = _InterlockedDecrement(ptr_) ; \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter to decrement
[out]	after_	The name of an atomic counter variable that will be set to the value of the counter after this decrement.

Atomically decrement the contents of *ptr_, and return the value after it was decremented in after_.

HqAtomicIncrement

#define HqAtomicIncrement	(		ptr_,
			before_
	)

Value:

  MACRO_START \
  before_ = _InterlockedIncrement(ptr_) - 1 ; \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter to increment
[out]	before_	The name of an atomic counter variable that will be set to the value of the counter before this increment.

Atomically increment the contents of *ptr_, and return the value before it was incremented in before_.

HqAtomicMaximum

#define HqAtomicMaximum	(		ptr_,
			value_,
			after_
	)

Value:

  MACRO_START               \
  hq_atomic_counter_t _prev_, _value_ = (value_) ;                      \
  HqBool _done_ = FALSE ;                                               \
  while ( !_done_ && (_prev_ = *(ptr_)) < _value_ ) {                   \
    HqAtomicCAS(ptr_, _prev_, _value_, _done_) ;                        \
  }                                                                     \
  after_ = _done_ ? _value_ : _prev_ ;                                  \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter
[in]	value_	A value to compare against the atomic counter stored in *ptr_.
[out]	after_	The name of an atomic counter variable that will be set to the maximum of *ptr_ and value_.

Atomically ensure that the value of *ptr_ is the maximum of *ptr_, and value_, and return the maximum in after_.

HqAtomicMinimum

#define HqAtomicMinimum	(		ptr_,
			value_,
			after_
	)

Value:

  MACRO_START               \
  hq_atomic_counter_t _prev_, _value_ = (value_) ;                      \
  HqBool _done_ = FALSE ;                                               \
  while ( !_done_ && (_prev_ = *(ptr_)) > _value_ ) {                   \
    HqAtomicCAS(ptr_, _prev_, _value_, _done_) ;                        \
  }                                                                     \
  after_ = _done_ ? _value_ : _prev_ ;                                  \
MACRO_END

Parameters

[in,out]	ptr_	A pointer to an atomic counter
[in]	value_	A value to compare against the atomic counter stored in *ptr_.
[out]	after_	The name of an atomic counter variable that will be set to the minimum of *ptr_ and value_.

Atomically ensure that the value of *ptr_ is the minimum of *ptr_, and value_, and return the minimum in after_.

spin_pointer_without_lock

#define spin_pointer_without_lock	(		ptr_,
			uptr_
	)

Value:

  MACRO_START              \
  HQASSERT(sizeof(*(ptr_)) > 1 && sizeof(*(ptr_)) == sizeof(*(uptr_)),  \
           "Spinlock pointer type alignment invalid") ;                 \
  uptr_ = (void *)((intptr_t)(ptr_) & ~(intptr_t)1) ;                   \
MACRO_END

Get the value of a potentially spinlocked pointer in its unlocked state.

Parameters

[in]	ptr_	The pointer that may be spinlocked.
[out]	uptr_	The pointer without the spinlock mark.

Some data structure algorithms can use atomic compare and swap directly to update pointers. The pointers may also be subject to spinlocks for data structure mutation. spin_pointer_without_lock() gets the value of a pointer without the spinlock mark. This can be used to prepare pointer values so that atomic updates can be done when a spinlock is unlocked.

spinlock_counter

#define spinlock_counter	(		addr_,
			count_
	)

Lock a counter semaphore using atomic operations.

Parameters

[in,out]	addr_	The address of the counter to lock.
[in]	count_	Number of spin cycles before each processor yield.

Semaphore counters are locked when non-zero, unlocked when zero. spinlock_counter() will wait until a counter is zero, and lock it using the value 1. The semaphore counter may be test-locked by using an atomic increment, testing if the value before increment was zero, and decrementing the semaphore to release it.

spinlock_pointer

#define spinlock_pointer	(		addr_,
			locked_,
			count_
	)

Lock a pointer using atomic operations.

Parameters

[in,out]	addr_	The address of the pointer to lock.
[out]	locked_	The pointer value to dereference in the locked section.
[in]	count_	Number of spin cycles before each processor yield.

spinlock_pointer() uses the lowest bit as lock mark, so is only valid for halfword or greater aligned pointers. It modifies the stored value of the pointer in memory, and loads a dereferencable version of the pointer into a variable. Within the locked section, code must use the dereferencable version of the pointer, and not re-load the original pointer from memory.

spintrylock_pointer

#define spintrylock_pointer	(		addr_,
			locked_,
			didlock_
	)

Try to lock a pointer using atomic operations.

Parameters

[in,out]	addr_	The address of the pointer to lock.
[out]	locked_	The pointer value to dereference in the locked section, only set if didlock_ is TRUE.
[out]	didlock_	A Boolean to indicate if managed to lock.

This is like spinlock_pointer(), except that it doesn't spin if it can't get the lock, it just returns.

spinunlock_counter

#define spinunlock_counter

(

addr_

)

Value:

  MACRO_START                           \
  hq_atomic_counter_t _after_ ;                                         \
  HqAtomicDecrement((addr_), _after_) ;                                 \
  HQASSERT(_after_ >= 0, "Counter semaphore was not locked") ;          \
MACRO_END

Unlock a semaphore counter using atomic operations.

Parameters

[out]

addr_

The address of the counter to unlock.

spinunlock_pointer

#define spinunlock_pointer	(		addr_,
			unlocked_
	)

Value:

  MACRO_START                \
  HqBool _didcas_ ;                                                     \
  HQASSERT(sizeof(**(addr_)) > 1, "Spinlock pointer type alignment invalid") ; \
  HQASSERT(((intptr_t)*(addr_) & 1) != 0, "Pointer is not currently locked") ; \
  /* Need a release barrier to prevent compiler optimising over the unlock. */ \
  HqAtomicCASPointer(addr_, *(addr_), unlocked_, _didcas_, spinlock_void_ptr) ; \
  HQASSERT(_didcas_, "Failed to unlock spinlock") ;                     \
  UNUSED_VARIABLE(HqBool, _didcas_) ;                                   \
MACRO_END

Unlock a pointer using atomic operations.

Parameters

[out]	addr_	The address of the pointer to unlock.
[in]	unlocked_	The new value of the unlocked pointer.

spinunlock_pointer() unlocks a pointer that was locked using spinlock_pointer(). The lowest bit of the pointer is used as lock mark, so this is only valid for halfword or greater aligned pointers. The new value of the unlocked pointer will usually be the dereferencable pointer value saved by spinlock_pointer(), but it may also be a different pointer of the correct type. This can be used to safely replace objects, by locking the only pointer reference to the object, but unlocking with NULL or a different object pointer.

Typedef Documentation

hq_atomic_counter_t

hq_atomic_counter_t

An integer type suitable for the atomic increment and decrement.

Enumeration Type Documentation

anonymous enum

anonymous enum

Constants for yields per spinlock cycle.

Enumerator
HQSPIN_YIELD_NEVER	Practically never yield processor.
HQSPIN_YIELD_ALWAYS	Yield processor every spinlock cycle.