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

[Andrew Haley via llvm-dev]

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. <https://www.redhat.com>
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