John Fine wrote: > Andrew Haley wrote: >> You are allowed to take a pointer, copy it as a void *, and copy it >> back to the same type and dereference it. If the compiler removes >> the copy step, then the compiler is broken. If your code accesses >> the object via an incompatible type, then your code is broken. > > I think the difference between that and what I'm doing is the extra > level of indirection. > > I write a Datatype* indirectly through a Datatype**. Then I read a > void* indirectly through a void** that happens to be the same pointer as > that Datatype**. > Next I do the reverse (write through void** to read through Datatype**). By my reckoning this should still work. Writing through the void** will be fine. Casting &(void*) to Datatype** is the bit that won't work. But you don't need to do that... > You're saying I can copy a Datatype* to a void* then back to a Datatype* > and use it. That wouldn't be a violation of strict aliasing, because it > never dereferences two different type pointers to the same address. OK, I now understand. > I know my code is "broken". I didn't write it and I have no practical > way to rewrite it. I'm looking for a local work around to the problem. > > I've looked at the generated assembler code and the copy step is > optimized out when compiled without -fno-strict-alias and isn't > optimized out when compiled with -fno-strict-alias. OK, but you still haven't said what "the copy step" is. We obviously don't want to start having a no-strict-alias attribute for pointers. I think it's pointless because it would only apply to new code, and the whole point of no-strict-alias is to make old code work. Andrew.
