THREADS AND LOCKS

Nonatomic Treatment of

double

 and

long

17.4

After a variable is created, every thread must perform an

assign

 or

load

 action

on that variable before performing a

use

 or

store

 action on that variable. (Less

formally: a new variable is created only in main memory and is not initially in

any thread's working memory.)

Provided that all the constraints above and below are obeyed, a

load

 or

store

action may be issued at any time by any thread on any variable, at the whim of the

implementation.

There are also certain constraints on the

read

 and

write

 actions performed by

main memory:

For every

load

 action performed by any thread

T

 on its working copy of a

variable

V

, there must be a corresponding preceding

read

 action by the main

memory on the master copy of

V

, and the

load

 action must put into the work 

ing copy the data transmitted by the corresponding

read

 action.

For every

store

 action performed by any thread

T

 on its working copy of a

variable

V

, there must be a corresponding following

write

 action by the main

memory on the master copy of

V

, and the

write

 action must put into the master

copy the data transmitted by the corresponding

store

 action.

Let action

A

 be a

load

 or

store

 by thread

T

 on variable

V

, and let action

P

 be

the corresponding

read

 or

write

 by the main memory on variable

V

. Similarly,

let action

B

be some other

load

 or

store

 by thread

T

 on that same variable

V

,

and let action

Q

 be the corresponding

read

 or

write

 by the main memory on

variable

V

. If

A

 precedes

B

, then

P

 must precede

Q

. (Less formally: actions on

the master copy of any given variable on behalf of a thread are performed by

the main memory in exactly the order that the thread requested.)

Note that this last rule applies

only

 to actions by a thread on the

same

 variable.

However, there is a more stringent rule for

volatile

 variables ( 17.7).

17.4   Nonatomic Treatment of

double

 and

long

If a

double

 or

long

 variable is not declared

volatile

, then for the purposes of

load

,

store

,

read

, and

write

 actions they are treated as if they were two variables of

32 bits each: wherever the rules require one of these actions, two such actions are

performed, one for each 32 bit half. The manner in which the 64 bits of a

double

or

long

 variable are encoded into two 32 bit quantities is implementation depen 

dent.

This matters only because a

read

 or

write

 of a

double

 or

long

 variable may

be handled by an actual main memory as two 32 bit

read

 or

write

 actions that may

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