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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/102989] Implement C2x's n2763 (_BitInt)
Date: Tue, 06 Jun 2023 07:13:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102989

--- Comment #63 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #62)
> What the patch including incremental one currently does is:
> 1) small _BitInt (on x86-64 N <=3D 64) - the BITINT_TYPEs are kept as is =
in
> the IL
>    and expanded, they always have non-BLKmode (QI/HI/SI/DI) and are handl=
ed
> like any
>    other INTEGER_TYPEs (except preserved in calls to ensure correct ABI
> passing)
> 2) middle _BitInt (on x86-64 N <=3D 128) - I keep in the IL just copy
> operations and
>    casts between them and INTEGER_TYPE (TImode in this case), actual
> arithmetics is
>    done on the INTEGER_TYPE
> 3) large _BitInt (on x86-64 that will be <=3D 255) - the intent was using
>    BIT_FIELD_REFs/BIT_INSERT_EXPR to make the IL simple and perform stuff=
 on
> the
>    up to 4 limbs in this case in straight line code

So these large _BitInt already have BLKmode?  If so I'd suggest to
initially handle them like the huge _BitInt code but "unrolled" and
iterate on the code-gen later - I can have a look once one can play
with the actual code and testcases.

> 4) huge _BitInt (on x86-64 N > 255) use loops, VAR_DECL destination,
> VCE+ARRAY_REF
>    on the sources,
>    dunno yet if I can get good code by making the VAR_DECL clobbered
> immediately after
>    I load a SSA_NAME from it (whether out of SSA/expansion could then ext=
end
> the
>    lifetime of the VAR_DECL, or if I should have some pass do that, or the
> bitint pass
>    figure out the last use and put clobber only after that, or even repla=
ce
> the SSA_NAME
>    uses with accesses to VAR_DECL
>=20
> Anyway, I think I'll work now on the add/sub with carry now and continue =
on
> _BitInt only after that.=