On 11/08/2010 05:18 PM, Ian Lance Taylor wrote: > Francis Moreau <francis.moro@xxxxxxxxx> writes: > >> Ian Lance Taylor <iant@xxxxxxxxxx> writes: >> >>> "Segher Boessenkool" <segher@xxxxxxxxxxxxxxxxxxx> writes: >>> >>>>> BTW, I tried to compile the last example, and we agreed that it invokes >>>>> undefined behaviour. >>>> >>>> I did no such thing. With -fno-strict-aliasing, the code is well-defined. >>> >>> I think I would say this slightly differently. The code is still >>> undefined according to the C/C++ language standards. When you use >>> -fno-strict-aliasing, gcc is promising to avoid using a class of >>> optimizations based on those portions of the language standards. >>> Another way to say this is that gcc is promising to not optimize based >>> on the fact that memory locations are accessed using different types. >>> That is, the code is still incorrect; gcc is just avoiding a class of >>> optimizations which are known to cause trouble with real programs. >>> >>> I also am having trouble understanding what the OP is asking for here. >> >> Clarification on GCC's strict aliasing rules. >> >> But I think I got it now, although I still don't know which exactly >> undefined behaviours are defined by GCC's -fno-strict-aliasing. > > In the definitions that I use, I would say that no undefined behaviours > are defined. > > The -fno-strict-aliasing option disables a class of optimizations. > These are optimizations which look at two pointer references and > consider whether the two pointers might be pointing at the same memory > location. When -fno-strict-aliasing is in effect, these optimizations > do not consider the types of the pointers. When -fstrict-aliasing is in > effect, these optimizations do consider the types of the pointers. > > That's a moderately precise definition, but you'll note that it does not > really describe the effects on the generated code, nor does it describe > the types of source code that are supported. I think it may be worth pointing out that we really don't want to define a C-ish programming language of our own that defines a bunch of things Standard C leaves undefined. -fno-strict-aliasing is a concession to some incorrect programs, that's all. Andrew.
