Re: How to use __attribute__((__may_alias__))

[Andrew Haley <aph at redhat dot com>]

John Fine wrote:

> I have a working version, using a union in a way that I think is
> incorrect, but this is the same concept I used last time I fixed one of
> these issues.
> 
> Everything I've tried so far with  __attribute__((__may_alias__)) has
> failed.  I'd really like to know how to make that work.
> 
> Working code:
> 
> union { DAT* pp; std::size_t dummy; } no_strict_alias;
> no_strict_alias.pp =foo();
> reinterpret_cast<DAT_PTR*>(&no_strict_alias.pp)->bar(x);
> 
> Notice that dummy is never used.  Notice that the reinterpret_cast is
> essentially unchanged (from the version that doesn't work).   The union
> is just doing what I would have expected the attribute to do,  marking
> pp as an object for which strict  aliasing optimizations should not be
> used.

No, this isn't guaranteed to work.  Your data must be *declared* as
union no_strict_alias.  Casting to a pointer to union no_strict_alias
isn't the same unless the only way you ever access your data is via
that cast.

Andrew.