[llvm-dev] GCC 5 and -Wstrict-aliasing in JSON.h

[Richard Biener via llvm-dev]

On Tue, 14 Aug 2018, Andrew Haley wrote:

> On 08/14/2018 11:55 AM, Kim Gräsman wrote:
> > On Tue, Aug 14, 2018 at 11:51 AM Andrew Haley <aph at redhat.com> wrote:
> >>
> >> On 08/12/2018 02:19 PM, Kim Gräsman wrote:
> >>> I still feel a little uncomfortable, because I think Jonathan makes an
> >>> excellent point -- if GCC thinks there's a strict-aliasing violation
> >>> (whether the standard agrees or not) and classifies this as undefined
> >>> behavior, we might invoke the optimizers wrath. The warning is a nice
> >>> hint that this could happen.
> >>
> >> Indeed.  And I'm not convinced that the pointer cast is necessary anyway:
> >> if the type passed in is a union, why not simply take the union member of
> >> the appropriate type?
> > 
> > As it turns out, Union is not a union ¯\_(ツ)_/¯. (I thought it was, up
> > until this point.)
> > 
> > It's a template producing a char array suitably aligned for any number
> > of mutually exclusive types, much like
> > https://en.cppreference.com/w/cpp/types/aligned_union.
> > 
> > I can't tell if that makes the waters less dangerous, but there's an
> > invariant for this code that it only reinterpret_casts out Ts from the
> > char buffer that it has previously put in there using placement new.
> 
> So I think GCC is wrong here. The intent of aligned_union is clearly that
> it is a POD-type suitable for any of its declared types.

If you look at the implementation of the strict-aliasing warning
you will immediately notice that it's stupid and the only way to "fix"
it is to remove it.

Properly warning for strict-aliasing violations isn't possible - you'd
need to conservatively detect must-aliases and track dynamic types.
If you'd detect a violation the compiler should better not exploit it.
You may have noticed we do not "miscompile"

int foo (int *p)
{
  *p = 1;
  *(float *)p = 0.0;
  return *p;
}

since quite some time but correctly zero *p and return zero.  We
do not warn for the above though because the access uses a pointer.
We only warn when we see a declared type of the storage in the very
same GENERIC expression(!).

> So what exactly is the type Union?  It has a field called buffer.

GCC 6+ contain fixes for sth that later C++ standards appearantly
need, namely that copy-assignment of aggregates with embedded
storage copies the dynamic type of said storage.  Otherwise placement
new should work just fine.

Richard.