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

[Richard Biener via llvm-dev]

On Sun, 12 Aug 2018, Kim Gräsman wrote:

> I did some more extensive testing and found that all GCCs older than 7
> trigger the warning, but only if CMAKE_BUILD_TYPE=Release (which I
> guess indicates optimizations are enabled).
> 
> There's a patch up for disabling the warning here:
> https://reviews.llvm.org/D50607.
> 
> 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.

The warning code is simply broken - do not trust it in any way to
decide whether GCC will later optimize sth.  But removing it wasn't
met with approval because we do not have anything better and it
did point out issues in the past.

Richard.

> While writing this I realized that LH_Mouse's double static_cast also
> silences the warning, and I think that's a nicer workaround, so I put
> up a patch for that too, here:
> https://reviews.llvm.org/D50608
> 
> Let me know what you think,
> - Kim
> On Fri, Aug 10, 2018 at 11:30 PM Kim Gräsman <kim.grasman at gmail.com> wrote:
> >
> > On Fri, Aug 10, 2018 at 6:08 PM, Sam McCall <sam.mccall at gmail.com> wrote:
> > > json::Value in JSON.h is a discriminated union.
> > > The storage is a char array of appropriate type and alignment. The storage
> > > holds one object at a time, it's initialized (and for nontrivial types,
> > > destroyed) at the right times to ensure this. The cast is only to the type
> > > of object that's already there, there's no magic here.
> > >
> > >
> > > On Fri, Aug 10, 2018, 17:52 Andrew Haley <aph at redhat.com> wrote:
> > >>
> > >> Not exactly.  You can cast a pointer to a pointer to some character
> > >> type or the type of the object stored in memory.  It does not matter
> > >> whether you use an intermediate type or not.  Having not seen the test
> > >> case, I can't tell whether this rule is followed.
> >
> > Yes, if Andrew's explanation agrees with the standard, then this code
> > should be benign (if it breaks any of these rules, that's a bug in and
> > of itself).
> >
> > So maybe we're back to figuring out how to silence GCC's warning machinery :)
> >
> > - Kim
> 
> 

-- 
Richard Biener <rguenther at suse.de>
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