[Bug target/102239] powerpc suboptimal boolean test of contiguous bits

["segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102239

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to luoxhu from comment #2)
> (In reply to Segher Boessenkool from comment #1)
> > Confirmed.
> > 
> > So the relevant insn
> > 
> > (parallel [(set (reg:CC 123)
> >                 (compare:CC (and:DI (reg:DI 124)
> >                                     (const_int 25769803776 [0x600000000]))
> >                             (const_int 0 [0])))
> >            (clobber (scratch:DI))])
> > 
> > is matched by *and<mode>3_2insn but not by any pattern that ends up as just
> > one insn.  Not *and<mode>3_mask_dot, because that doesn't do a shift first,
> > is just an AND and there are no machine insns to do that; but there is no
> > pattern for what we can do.
> > 
> > *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.
> 
> Seems different with what you describe, in combine, it was combined to
> anddi3_2insn_dot:
> 
> (insn 9 8 10 2 (parallel [
>             (set (reg:CC 122)
>                 (compare:CC (and:DI (reg:DI 123)
>                         (const_int 25769803776 [0x600000000]))
>                     (const_int 0 [0])))
>             (clobber (scratch:DI))
>         ]) "pr102239.c":3:6 210 {*anddi3_2insn_dot}
>      (expr_list:REG_DEAD (reg:DI 123)
>         (nil)))
> (jump_insn 10 9 11 2 (set (pc)
>         (if_then_else (eq (reg:CC 122)
>                 (const_int 0 [0]))
>             (label_ref 15)
>             (pc))) "pr102239.c":3:6 868 {*cbranch}
>      (expr_list:REG_DEAD (reg:CC 122)
>         (int_list:REG_BR_PROB 536870916 (nil)))
> 
> 
> 
> Then in pr102239.c.302r.split2, it is split by "*and<mode>3_2insn_dot" to
> rotldi3_mask+lshrdi3_dot:
> 
> Splitting with gen_split_80 (rs6000.md:3721)
> 
> (insn 32 8 33 2 (set (reg:DI 3 3 [124])
>         (and:DI (ashift:DI (reg:DI 3 3 [123])
>                 (const_int 29 [0x1d]))
>             (const_int -4611686018427387904 [0xc000000000000000])))
> "pr102239.c":3:6 236 {*rotldi3_mask}
>      (nil))
> (insn 33 32 10 2 (parallel [
>             (set (reg:CC 100 0 [122])
>                 (compare:CC (lshiftrt:DI (reg:DI 3 3 [124])
>                         (const_int 29 [0x1d]))
>                     (const_int 0 [0])))
>             (clobber (reg:DI 3 3 [124]))
>         ]) "pr102239.c":3:6 281 {*lshrdi3_dot}
>      (nil))
> 
> 
> Why this difference happens?

What difference?  It is split by the same pattern that matched it, and that
is the 2insn pattern.  I'm not sure what problem you see?

> 0x600000000 is not a valid mask for anddi3_2insn_dot:

It should be though!

>  "(<MODE>mode == Pmode || UINTVAL (operands[2]) <= 0x7fffffff)
>    && rs6000_is_valid_2insn_and (operands[2], <MODE>mode)
>    && !(rs6000_is_valid_and_mask (operands[2], <MODE>mode)
> 	|| logical_const_operand (operands[2], <MODE>mode))"
> 
> 
> (gdb) p UINTVAL (operands[2]) <= 0x7fffffff
> $84 = false

But we have Pmode here so all is fine.

> (gdb) p rs6000_is_valid_2insn_and (operands[2], E_DImode)
> $85 = true
> (gdb) p logical_const_operand (operands[2], E_DImode)
> $86 = false
> (gdb) p rs6000_is_valid_and_mask (operands[2], E_DImode)
> $87 = false
> (gdb) p Pmode
> $88 = DImode

Yes, and <MODE>mode is DImode as well.

My point is that we do not have any pattern that can recognise this that
will end up as just one machine insn.  anddi3_2insn_dot split to two
machine insns, and it should: its name indicates we want that!