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.

-- 
Joseph S. Myers
joseph_at_[hidden]