Store/load atomicity on ARM Cortex-A9 MPCore

Question

Is it safe to assume that assigning and accessing 32 bit integers on an ARM Cortex-A9 MPCore implementations are atomic operations and that the assigned value is synchronized with all cores? Will the C compiler guarantee that

uint32_t *p;
*p = 4711;

and

uint32_t *p;
return *p;

are translated to atomic operations in assembler?

You could always disassemble the object files and check for yourself. — Some programmer dude
@JoachimPileborg, don't ever do that. The results might change from one build configuration to another. — sh1

Andy Ross Andy Ross · Accepted Answer · 2013-07-01T19:03:54

"Atomic" and "synchronized with all cores" are different requirements. All ARM cores in the market implement 32 bit operations to memory atomically (which is to say you can never see "part" of the word written without the rest). Not all of them are cache-coherent between cores, and the details (especially with the more exotic configurations like big.LITTLE) are complicated.

Use your OS synchronization primitives. That complexity is what they are designed to abstract.

Store/load atomicity on ARM Cortex-A9 MPCore

2 Answers