Remove possible loop creation in DAGCombiner

[deadal nix]

Ok so you'll face the same issue line 11422 when selecting between 32 bits
pointer size.

Wouldn't it be possible to add a pattern in the td files to handle that
case precisely ?

2014-09-27 11:56 GMT-07:00 Matt Arsenault <arsenm2 at gmail.com>:

>
> On Sep 26, 2014, at 1:48 AM, deadal nix <deadalnix at gmail.com> wrote:
>
> Which target ? I can try to figure out a solution. As per submitted test
> cases, this do not affect X86 or X86-64 int that very scenario, the 32 bit
> select is generated properly.
>
>
> Not an in tree target, AMDIL
>
>
> 2014-09-25 13:57 GMT-07:00 Matt Arsenault <arsenm2 at gmail.com>:
>
>>
>> On Sep 25, 2014, at 1:42 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>>
>> > ----- Original Message -----
>> >> From: "deadal nix" <deadalnix at gmail.com>
>> >> To: "llvm-commits" <llvm-commits at cs.uiuc.edu>
>> >> Sent: Thursday, September 25, 2014 12:09:07 AM
>> >> Subject: Remove possible loop creation in DAGCombiner
>> >>
>> >>
>> >>
>> >>
>> >> DAGCombiner can create loop in the DAG when you sext the result of a
>> >> setcc, depending on the target.
>> >>
>> >> This diff remove the call to DAG.getSExtOrTrunc that create the loop.
>> >> This call used to be necessary for x86 backend, but it doesn't look
>> >> necessary anymore. I've added tests to ensure it doesn't.
>> >
>> > -        EVT SelectVT = getSetCCResultType(VT);
>> > -        return DAG.getSelect(DL, VT,
>> > -                             DAG.getSExtOrTrunc(SetCC, DL, SelectVT),
>> > -                             NegOne, DAG.getConstant(0, VT));
>> > -
>> > +        return DAG.getSelect(DL, VT, SetCC,
>> > +                          NegOne, DAG.getConstant(0, VT));
>> >       }
>> >
>> > It seems clear to me that creating an SIGN_EXTEND inside of
>> visitSIGN_EXTEND could easily lead to a loop, and we should avoid that.
>> However, getSExtOrTrunc might also have been providing a TRUNC, and if the
>> SIGN_EXTEND really would be needed, we may just want to abort the
>> transformation all together. For one thing, I don't think that it can be
>> different unless VT != N0.getOperand(0).getValueType().
>>
>> I do need the trunc behavior, for the case of selecting between 32-bit
>> values based on a 64-bit comparison on a target which has the width of the
>> setcc type change depending on the width of the compared operands
>>
>> >
>> > In short, you're proposing to change target-independent code, and I'd
>> be more conservative. Why don't you just disable the transformation when
>> SelectVT.bitsGT(SetCC.getValueType()) is true (which I believe is the
>> condition under which we'd get the SIGN_EXTEND).
>>
>> I agree, and what I was planning on trying when I got back to fixing that
>> patch.
>>
>> >
>> > -Hal
>> >
>> >>
>> >>
>> >>
>> >> _______________________________________________
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>> >> llvm-commits at cs.uiuc.edu
>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>> >>
>> >
>> > --
>> > Hal Finkel
>> > Assistant Computational Scientist
>> > Leadership Computing Facility
>> > Argonne National Laboratory
>> > _______________________________________________
>> > llvm-commits mailing list
>> > llvm-commits at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>>
>>
>
>
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