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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/97437] builtins subcarry and addcarry still not generate
 the right code. Not get optimized to immediate value
Date: Thu, 15 Oct 2020 14:00:48 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97437

--- Comment #7 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Segher Boessenkool from comment #5)
> Trying 7 -> 9:
>     7: r97:SI=3D0x2a
>     9: {flags:CCC=3Dcmp(r97:SI+r98:SI,r97:SI);r99:SI=3Dr97:SI+r98:SI;}
>       REG_DEAD r98:SI
>       REG_DEAD r97:SI
> Failed to match this instruction:
> (parallel [
>         (set (reg:CC 17 flags)
>             (compare:CC (reg:SI 98 [ *b_12(D) ])
>                 (const_int -42 [0xffffffffffffffd6])))
>         (set (reg:SI 99)
>             (plus:SI (reg:SI 98 [ *b_12(D) ])
>                 (const_int 42 [0x2a])))
>     ])
>=20
> On rs6000 we have four special variants for the immediate add-with-carry
> insn patterns: imm 0, imm -1, imm pos, imm neg.  All of these have
> different canonical RTL.

The i386 backend has various patterns for that too, the problem is with that
CCmode vs. CCCmode that was used originally here (otherwise sure, I can add
more).
These patterns ensure only CF is valid and so need to use CCCmode which ens=
ures
that users will only query CF flag and nothing else.=