Re: [boost] Proposed SG14 <system_error2> ready for feedback

Subject: Re: [boost] Proposed SG14 <system_error2> ready for feedback
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2018-03-05 23:39:43

>> Structure alignment would align any int to the pointer, so same space in
>> any case. Two CPU registers.
> 
> Perhaps, but the difference between value bits and padding bits is
> significant, and it seems pointless (and prone to compiler warnings
> about losing significant bits if you then try to assign the value back
> to its "real" type) to misrepresent its size.

Ah, I have a cunning piece of code to handle that. The problem was if
the original source was say a char padded by the compiler to an
intptr_t, and you erase that into an intptr_t. Seven bytes are therefore
being accessed as UB, and random data could appear in there. Both of
which are unhelpful.

So I have a bit of trampoline code which constexpr initialises a union
of the source type and the intptr_t which is initialised to all bits
zero. We then set the source type, and read back the intptr_t. This
ensures that the padding bits are all bits zero in the type erased form.

>> In private discussions with some on WG21, it is felt that status code
>> needs to always be two CPU registers exactly. So I'll be bumping that to
>> intptr_t on all platforms.
> 
> Why?  What benefit does this provide?

So the compiler could be given a hard guarantee that an `error` can
always be trivially returned. On x64 Itanium ABI, RAX+RDX is used to
return a 16 byte value. The MSVC ABI currently does not use that
mechanism on x64, it can only return up to 8 bytes trivially.

A function potentially returning `error` would be mangled to say that it
does so. The compiler therefore knows that one of the CPU flags (e.g. DF
or SF) means whether to interpret RAX+RDX as an `error` or as an
ordinary non-failure return value.

That's why it's important that `error` fit into two CPU registers.

`error` is now ready incidentally.
https://ned14.github.io/status-code/doc_error.html. It's looking very
promising. Be aware that https://wg21.link/P0709 currently results in a 404.

Niall

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