Read optimizations on shared memory - c

Question

Suppose you have a function that make several read access to a shared variable whose access is atomic. All in running in the same process. Imagine them as threads of a process or as a sw running on bare metal platform with no MMU.
As a requirement you must ensure that the value of that read is consistent for all the length of the function so the code must not re-read the memory location and have to put in a local variable or on a register. How can we ensure that this behaviour is respected?
As an example...
shared is the only shared variable
extern uint32_t a, b, shared;
void useless_function()
{
__ASM volatile ("":::"memory");
uint32_t value = shared;
a = value *2;
b = value << 3;
}
Can value be optimized out by direct readings of shared variable in some contexts? If yes, how can I be sure this cannot happen?

Answer 1

As a requirement you must ensure that the value of that read is consistent for all the length of the function so the code must not re-read the memory location and have to put in a local variable or on a register. How can we ensure that this behaviour is respected?
You can do that with READ_ONCE macro from Linux kernel:
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
* READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
* particular ordering. One way to make the compiler aware of ordering is to
* put the two invocations of READ_ONCE or WRITE_ONCE in different C
* statements.
*
* These two macros will also work on aggregate data types like structs or
* unions. If the size of the accessed data type exceeds the word size of
* the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
* fall back to memcpy(). There's at least two memcpy()s: one for the
* __builtin_memcpy() and then one for the macro doing the copy of variable
* - '__u' allocated on the stack.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
* and (2) Ensuring that the compiler does not fold, spindle, or otherwise
* mutilate accesses that either do not require ordering or that interact
* with an explicit memory barrier or atomic instruction that provides the
* required ordering.
*/
E.g.:
uint32_t value = READ_ONCE(shared);
READ_ONCE macro essentially casts the object you read to be volatile because the compiler cannot emit extra reads or writes for volatile objects.
The above is equivalent to:
uint32_t value = *(uint32_t volatile*)&shared;
Alternatively:
uint32_t value;
memcpy(&value, &shared, sizeof value);
memcpy breaks the dependency between shared and value, so that the compiler cannot re-load shared instead of loading value.

Answer 2

In the example given you are not using the variable value in the function at all. So it will definitely be optimised.
Also, as mentioned in comments, in a multitasking system, the value of shared can be changed within the function.
What I need is that shared is read only once and it local value keeped for all function length and not re-evaluated
I would suggest something like this below.
extern uint32_t a, b, shared;
void useless_function()
{
__ASM volatile ("":::"memory");
uint32_t value = shared;
a = value*2;
b = value << 3;
}
Here shared is read only once in the function. It will be read again on next call of the function.