[PATCH] D30538: Add documentation for -fno-strict-aliasing

[Daniel Berlin via Phabricator via cfe-commits]

dberlin added a comment.

In https://reviews.llvm.org/D30538#690699, @hans wrote:

> +1 for documenting this, but I have to leave it to the language lawyers for how to fomulate it.
>
> > Enables/disables the strict aliasing assumption, which assumes that objects of different types do not share the same location in memory.
>
> I think it needs to say incompatible types at least (and `char` and `unsigned char` are also special). And isn't it really about pointers -- the compiler assumes that when dereferncing two pointers of incompatible types, those pointers do not refer to the same memory?
>
> > clang does not allow "type-punning" by writing and reading from different union members
>
> I thought clang does allow type-punning through unions, as long as it's in a single function, but that it fails when things get more complicated.


The limitation is really "union accesses must be visibly through a union, and if you try to trick the compiler, you will lose".
That is what we meant to allow, it just is still broken *anyway*

> +dannyb who enjoys this stuff ;)

You are so mean :)


https://reviews.llvm.org/D30538