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

[Sam McCall]

Author of the problematic code here. Thanks everyone, and sorry to have
caused difficulty!

Obviously if there really is something illegal here we should fix it in
LLVM, but it looks like this warning is a false positive (anyone disagree?)

Still if there's a simple source-level workaround, or we can suppress the
warning with a #pragma, I'd be happy to do that. GCC 4.9.3 is a supported
compiler for LLVM and the more configurations we build cleanly in, the
better.

If this is a useful direction, could someone with an affected environment
send me a small patch? I don't have the right setup to verify myself.

Cheers, Sam

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

[Sam McCall]

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  wrote:

> On 08/10/2018 05:30 AM, Liu Hao wrote:
> > Only an lvalue of a pointer to (possibly CV-qualified) `void` or a
> > pointer to a character type (in C) / any of `char`, `unsigned char` or
> > `std::byte` (in C++) can alias objects.
>
> Yes.
>
> > That is to say, in order to eliminate the aliasing problem an
> > intermediate lvalue pointer is required.
>
> 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.
>
> What you can't do is store as a double, cast the double pointer to a
> character pointer, cast that pointer to some other pointer type, and
> read from memory.  GCC won't give you a warning for that, but it's
> still undefined.
>
> JSON.h seems to hope that if you cast a pointer to T to a pointer to
> some union type, magic will happen.  It won't work, unless the object
> stored in the memory at that address was stored as the union type.
>
> Do not lie to the compiler or it will get its revenge.
>
> --
> Andrew Haley
> Java Platform Lead Engineer
> Red Hat UK Ltd. 
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>