On Mon, Aug 2, 2021 at 12:51 PM Joseph Myers <joseph_at_[hidden]> wrote:
>
> On Mon, 2 Aug 2021, Aaron Ballman via Liaison wrote:
>
> > I spoke with Lu Hongjiu (Intel's rep on psABI matters) internally
> > about psABI changes and his determination was that no psABI changes
> > were needed to support _BitInt because they aligned with existing
> > calling conventions.
>
> That's not correct. It's a new type, not mentioned in the existing psABI
> documents, so it needs to be added to that document, just like other new
> types.
>
> > "With _BitInt(N) types align with existing calling conventions. They
> > have the same size and alignment as the smallest basic type that can
> > contain them. Types that are larger than __int64_t are conceptually
> > treated as struct of register size chunks. The number of chunks is the
> > smallest number that can contain the type. there is no need for psABI
> > change."
>
> The above description is a starting point for what goes in psABI
> documents, but it's not sufficient by itself. For example: are bits
> beyond the width of the _BitInt value but within the width of the smallest
> basic type containing it required (for argument passing and return, or for
> that matter for objects of the type in memory) to be
> sign-or-zero-extended, or may they take unspecified values? What about
> bits between the width of the smallest basic type containing it and the
> register-sized container? And likewise for the high word of
> larger-than-register _BitInt values?
>
> These are exactly the sort of issues that need to be considered in psABI
> groups for each architecture. The well-known
> https://bugs.llvm.org/show_bug.cgi?id=12207 and
> https://bugs.llvm.org/show_bug.cgi?id=44228 concerning sign and zero
> extension of narrower-than-register types illustrate the importance of
> getting such issues specified explicitly in psABI documents.

Thank you for this, Joseph! I've passed along this information to HJ
to hopefully restart the discussions.

~Aaron

>
> --
> Joseph S. Myers
> joseph_at_[hidden]