Re: [PATCH] c++, v2: Allow indeterminate unsigned char or std::byte in bit_cast - P1272R4

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Dec 02, 2021 at 03:32:58PM -0500, Jason Merrill wrote:
> > So IMHO we need the patch I've posted (with the testcases slightly adjusted
> > not to do those extra copies afterwards for now),
> > and try to deal with the lvalue-to-rvalue conversion of indeterminate later,
> > and once done, perhaps in a copy of those testcases readd those extra copies
> > afterwards and verify it is rejected.
> 
> Makes sense, except:
> 
> > +	      gcc_assert (end == 1 || end == 2);
> 
> This seems to assert that the value representation of a bit-field can't span
> more than two bytes, which is wrong for, say,
> 
> struct A
> {
>   int : 1;
>   unsigned __int128 c : 128; // value representation spans 17 bytes
> };

Sure, for arbitrary bitfields yes.  But, the patch only deals with
unsigned char and std::byte TYPE_MAIN_VARIANT (DECL_BIT_FIELD_TYPE (fld))
bitfields (others don't have the type listed in the standard and so should
error right away, which is done by keeping the mask bits uncleared).

For PCC_BITFIELD_TYPE_MATTERS I think end must be 1 even, if a bit-field
would cross the byte boundary, it would be allocated in the next byte.
But !PCC_BITFIELD_TYPE_MATTERS targets can cross the boundary, but as it
can't be more than BITS_PER_UNIT bits (oversized bitfields are with warning
downsized to the underlying's type precision), it can occupy at most 2
bytes.

	Jakub