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Re: C++ standards question


On Mon, Feb 4, 2013 at 4:31 PM, leon zadorin <leonleon77@gmail.com> wrote:
> On Monday, 4 February 2013, Ian Lance Taylor wrote:
>>
>> On Sun, Feb 3, 2013 at 3:24 PM, John Fine <johnsfine@verizon.net> wrote:
>> >
>> > The optimizer changes that to the equivalent of
>> >
>> > *p = (expression of *p ) ? '-' : *p;
>> >
>> > Is that a valid optimization?
>>
>> Not these days, and current GCC should not do it.
>>
>> This is called a speculative store.
>>
> ...
>>
>> Speculative stores are
>> disallowed by the C++11 memory model.
>
>
> Just curious, given the:
> 1) zero-overhead design principles of c++ standard (e.g. if one does not
> want to use a given feature, then one does not pay overhead of using such a
> feature).

I'm afraid that is a goal rather than a principle.  Even exceptions
cost something even if you don't use them.

> 2) one is not forced to use multithreaded programming in ones (c++) code.
> 3) one can build GCC with single-threaded memory model (e.g. disable threads
> during GCC config, etc.)
>
> Would GCC still use speculative store optimazations when a single-threaded
> yet still -std=c++11 (i.e. with the use of rvalue refs and other c++11
> features) code is built by an instance of GCC which was configured without
> support for "threaded-code"?

I suppose it could in principle, but in practice the answer is no.

Ian


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