Remove possible loop creation in DAGCombiner

[Matt Arsenault]

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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> 
> -- 
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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