[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [avr-gcc-list] Avr-libc-user-manual: "Problems with reordering code"

From: David Brown
Subject: Re: [avr-gcc-list] Avr-libc-user-manual: "Problems with reordering code"
Date: Thu, 9 Feb 2017 21:20:40 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 09/02/17 19:49, Bob Paddock wrote:
On Thu, Feb 9, 2017 at 1:13 PM, David Brown <address@hidden> 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.


So the only solution is to use different parts, such as ARM, with its
Instruction Synchronization Barrier (ISB) [that can waste thousands of

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.

reply via email to

[Prev in Thread] Current Thread [Next in Thread]