[PATCH 1/2] libstdc++: Replace padding bits with a bit-field in __format::_Spec

[Jonathan Wakely]

On Thu, 1 Feb 2024, 17:23 Jonathan Wakely, <jwakely@redhat.com> wrote:

> On Thu, 1 Feb 2024 at 16:34, Andreas Schwab <schwab@suse.de> wrote:
> >
> > On Feb 01 2024, Jonathan Wakely wrote:
> >
> > > This will result in an ABI change for targets that use 1-byte alignment
> > > for all integral types, e.g. cris-elf.
> >
> > Or 2-byte alignment as on m68k.
>
> Ah yes.
>
> In fact it's a change for everybody, because previously it was 15 bits
> of bit-fields, 1 bit padding, 2 x 16-bit short, 16 bits padding,


(Except for targets with 1- or 2-byte alignment, which didn't have the
second padding)


then
> a char32_t, but now it's 15+17 bits of bit-fields, 2x 16-bit short,
> char32_t. So the shorts moved internally even if the size didn't
> change.
>
> I could make it 15+1 bit-fields, 2x16-bit short, 16 bit-field to avoid
> moving the shorts.
>
>   template<typename _CharT>
>     struct _Spec
>     {
>       _Align     _M_align : 2;
>       _Sign      _M_sign : 2;
>       unsigned   _M_alt : 1;
>       unsigned   _M_localized : 1;
>       unsigned   _M_zero_fill : 1;
>       _WidthPrec _M_width_kind : 2;
>       _WidthPrec _M_prec_kind : 2;
>       _Pres_type _M_type : 4;
>       unsigned _M_reserved : 1;
>       unsigned short _M_width;
>       unsigned short _M_prec;
>       unsigned _M_reserved2 : 16;
>       char32_t _M_fill = ' ';
>
> That would also address the PCC_BITFIELD_TYPE_MATTERS point H-P made.
>
>
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