On 09/02/17 19:49, Bob Paddock wrote:
On Thu, Feb 9, 2017 at 1:13 PM, David Brown <da...@westcontrol.com> wrote:
Note also that it is only control
of the memory access that is needed for code correctness - moving
instruction execution affects timing, but not the results.
As I'm sure you are aware even if the code if mathematically correct,
giving the correct answers, failing to meet timing goals can be
catastrophic in Real Time Systems.
Yes.
So the only solution is to use different parts, such as ARM, with its
Instruction Synchronization Barrier (ISB) [that can waste thousands of
cycles]?
No. That won't help. The compiler can re-arrange "harmless" execution
and calculations around an inline assembly instruction with the "ISB"
opcode (or any similar opcode), even if the inline assembly is marked
"volatile". Given "isb(); x = a/b; isb(); return x;" the compiler is
free to order the division before or after either of these isb() calls,
if the compiler knows that the isb() function/macro/inline assembly does
not affect the results of the calculation.
In C, the only things that can be ordered are /visible/ effects. Those
are volatile memory accesses, file I/O, program start/stop, and calling
external code with unknown effect (since that external code could have
visible effects). C11 adds some atomic access and synchronisation
functions, and C implementations can add more - gcc adds volatile inline
assembly. Other things - non-volatile memory accesses, and
calculations, can be shuffled around at will, including back and forth
across volatile accesses.
The only way I know of to force control of the order of execution is to
make "visible" dependencies on the results or the perquisites to the
calculations. (And if the calculation does not have any results, it is
not actually needed at all as far as C is concerned.) You have to use
the techniques I gave in my earlier posts here - they are as convenient,
safe, and efficient as it gets. But it does mean that there is /no/
"general" execution barrier, in the same way that "asm ("":::"memory")"
is a general memory barrier.
If someone knows differently, I'd be happy to be corrected here.
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