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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102989] Implement C2x's n2763 (_BitInt)
Date: Wed, 24 May 2023 12:46:25 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102989

--- Comment #44 from rguenther at suse dot de <rguenther at suse dot de> ---
On Wed, 24 May 2023, jakub at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102989
>=20
> Jakub Jelinek <jakub at gcc dot gnu.org> changed:
>=20
>            What    |Removed                     |Added
> -------------------------------------------------------------------------=
---
>   Attachment #55141|0                           |1
>         is obsolete|                            |
>=20
> --- Comment #43 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
> Created attachment 55148
>   --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D55148&action=3Dedit
> gcc14-bitint-wip.patch
>=20
> Another update.  This version can emit _BitInt(N) values in non-automatic
> variable initializers, handles passing/returning _BitInt(N) and for N <=
=3D 64
> (i.e. what fits into a single limb) from what I can see handling it in GI=
MPLE
> passes and and even expansion/RTL seems to work.
> Now, as discussed earlier, for N > GET_MODE_PRECISION (limb_mode) I think=
 we
> want to lower it in some pass in between IPA and vectorization.  For N wh=
ich
> fits into DImode if limb is 32-bit (currently no target does that as we h=
ave
> just x86-64 support) or which fits into TImode for 64-bit if TImode is
> supported, I guess we want to map arithmetics
> to TImode arithmetics, for say 2-4x larger emit code for arithmetics (exc=
ept
> perhaps multiplication/division) inline as straight line code and for even
> larger as loops.
> In the last case, a question is if we could use e.g. TARGET_MEM_REF for t=
he
> variable offset in those loops on the vars even when they aren't
> TREE_ADDRESSABLE (but would force them into memory during expansion).

Note you should use TARGET_MEM_REF only when it describes the actual
addressing mode you want to use.  Otherwise just synthesize ARRAY_REFs
like ARRAY_REF <VIEW_CONVERT_EXPR (limb[]) <limbs>, index> with
an appropriate VLA libm[] array type.

I'd do the lowering right before pass_complete_unrolli and generally
emit loopy form (another pass placement required in the -Og pipeline).=