Question about GCC 5.2.0 and expression reordering

[David Barto]

> On Jun 15, 2017, at 2:23 AM, David Brown <david@westcontrol.com> wrote:
> 
> On 14/06/17 18:56, Andrew Haley wrote:
>> On 14/06/17 17:40, David Barto wrote:
>>> Thanks for that. Can you point to the relevant portion of the standard that
>>> I can then quote back to my Boss who is quite concerned about this?
>> 
>> 1.9  Program execution  [intro.execution]
>> 
>> The semantic descriptions in this International Standard define a
>> parameterized nondeterministic abstract machine. This International
>> Standard places no requirement on the structure of conforming
>> implementations.  In particular, they need not copy or emulate the
>> structure of the abstract machine. Rather, conforming implementations
>> are required to emulate (only) the observable behavior of the abstract
>> machine as explained below.5
>> 
>> 5) This provision is sometimes called the “as-if” rule, because an
>> implementation is free to disregard any requirement of this
>> International Standard as long as the result is as if the requirement
>> had been obeyed, as far as can be determined from the observable
>> behavior of the program. For instance, an actual implementation need
>> not evaluate part of an expression if it can deduce that its value is
>> not used and that no side effects affecting the observable behavior of
>> the program are produced.
>> 
>>> And if there is such a flag to disable the “Optimization” performed here
>>> what would I enable?
>> 
>> I have no idea.
>> 
> 
> There is "-fsched-spec-load" and "-fsched-spec-load-dangerous" that
> might be relevant, but I did not think these were enabled by default.
> Maybe "-fno-sched-spec" ?  "-fhoist-adjecent-loads" ?
> 
> 
> If you need to avoid this optimisation in this one case, you can force
> the "f20type == URI" to have a side effect by using a cast to volatile
> (such as Linux's ACCESS_ONCE macro) on URI.
> 
> But gcc can do this sort of re-arrangement at any time, in any place.
> It is fine for the compiler to do something with undefined behaviour as
> long as the compiler knows what will actually happen (so it can add two
> ints that overflow, but it can't dereference a null pointer), and when
> the results of the undefined behaviour are not used.  In this particular
> case the compiler will want to schedule reading the URI as early as
> possible, so that it has made its way through caches and into the cpu
> for when it is needed.
> 

Fantastic! I will try these suggestions to see what the result is.

I knew that this list would be handy some day. Thanks for all your help and
support in getting a quick and accurate solution that I can present to my
boss.

	David


David Barto
barto@cambridgesemantics.com

Sometimes, my best code does nothing. Most of the rest of it has bugs.