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From: "luoxhu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/102239] powerpc suboptimal boolean test of contiguous bits
Date: Mon, 29 Nov 2021 03:31:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102239

--- Comment #6 from luoxhu at gcc dot gnu.org ---
(In reply to Segher Boessenkool from comment #5)
> (In reply to luoxhu from comment #4)
> > Simply adjust the sequence of dot instruction could produce expected co=
de,
> > is this correct?
>=20
> No it isn't.  Sorry.

Sorry I don't understand what is wrong...

>=20
> > foo:
> > .LFB0:
> >         .cfi_startproc
> >         rldicr. 3,3,29,1
> >         beq 0,.L2
>=20
> This is fine, but only because it tests the EQ bit (not the LT or GT bits=
).
> So the generated RTL for this insn (the 2insn one) is not correct.

The generated RTL in pr102239.c.300r.split2 is:

(insn 32 8 33 2 (parallel [
            (set (reg:CC 100 0 [123])
                (compare:CC (and:DI (ashift:DI (reg:DI 3 3 [124])
                            (const_int 29 [0x1d]))
                        (const_int -4611686018427387904 [0xc000000000000000=
]))
                    (const_int 0 [0])))
            (clobber (reg:DI 3 3 [125]))
        ]) "pr102239.c":4:6 238 {*rotldi3_mask_dot}
     (nil))
(insn 33 32 10 2 (set (reg:DI 3 3 [125])
        (lshiftrt:DI (reg:DI 3 3 [125])
            (const_int 29 [0x1d]))) "pr102239.c":4:6 278 {lshrdi3}
     (nil))
(jump_insn 10 33 11 2 (set (pc)
        (if_then_else (eq (reg:CC 100 0 [123])
                (const_int 0 [0]))
            (label_ref 15)
            (pc))) "pr102239.c":4:6 868 {*cbranch}
     (int_list:REG_BR_PROB 536870916 (nil))
 -> 15)


rotldi3_mask_dot is what you mentioned in c#1, it is a shifted result and n=
ot
matter for comparing to 0:

> *rotl<mode>3_mask_dot cannot do this either; the base and the dot2 of that
> cannot be done, they return a shifted result, but that doesn't matter for
> comparing it to 0.  So we should add a specialised version.

What specialized version to add?=