https://codereview.chromium.org/213943002/diff/20001/src/arm64/simulator-arm64.h
File src/arm64/simulator-arm64.h (right):
https://codereview.chromium.org/213943002/diff/20001/src/arm64/simulator-arm64.h#newcode179
src/arm64/simulator-arm64.h:179: } reg = { new_value };
On 2014/03/28 11:46:04, Sven Panne wrote:
On 2014/03/27 19:58:33, Fritz wrote:
> I'm specifically trying to type-alias here in a safe way. [...]
I don't think that such a thing is possible: IIRC, the only defined
way of using
a union is accessing its fields consistently, i.e. read only out of
the field
you have last written into, everything else is totally undefined (at
least this
was the definition for C). Or in other words: The only safe way to use
a union
is to save space, not to do some funny casting/conversion tricks.
It seems to be allowed in C99 TC3 (dated the September 2007-09-07). See
section 6.5.2.3, page 73: "If the member used to access the contents of
a union object is not the same as the member last used to store a value
in the object, the appropriate part of the object representation of the
value is reinterpreted as an object representation in the new type
[...]"
I don't think it was allowed in TC2, but I don't have it to hand.
Has this changed in C++? If yes, could somebody point me to the right
section of
the spec? It would be good to know, because I'm quite sure we already
had
trouble in this area.
I don't think it's allowed in C++98 but I can't find a clear bit of the
specification that says that. However, it does have a rather surprising
note on reinterpret_cast suggesting that reinterpret_cast<T&>(x) can be
used to safely do type-punning. I'd not seen this before, and every
resource I've seen until now has said that this is _not_ safe. This
(draft) standard is dated 2005-10-19 and I'm not sure where I got it
from.
I still think that the only _portable_ way to do it is to use memcpy;
that's safe in every version of the standards, but the rules are rather
more relaxed than I previously thought.
https://codereview.chromium.org/213943002/
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