On Sun, Feb 3, 2013 at 3:24 PM, John Fine <johnsfine@xxxxxxxxxxx> wrote: > I have a construct that the optimizer in VS2010 64-bit changes to something > that doesn't work. > > I want to know whether this is a bug in the compiler or in the code being > compiled. > > I would have no idea where to look in the standards document for this > answer. I know it is off-topic (a C++ not gcc question) but this is where > the experts are. > > My function takes a parameter char* p > One of the things the function does is test and conditionally modifies a > char pointed to through P. Something like: > > if ( expression of *p ) *p = '-'; > > 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. Oh, wait, you were asking about Visual Studio (at least, I am guessing that that is what you mean by VS2010). Speculative stores are disallowed by the C++11 memory model. So when VS2010 supports C++11, it will no longer perform this optimization. That said, I am told that Visual Studio still does not support C99 more than ten years later, so who knows if it will ever support C++11. Ian
