[LLVMdev] Bug in visitSIGN_EXTEND in DAGCombiner.cpp?

Relph, Richard Richard.Relph at amd.com
Mon Mar 11 08:34:44 PDT 2013

On Mar 8, 2013, at 2:29 PM, Nadav Rotem <nrotem at apple.com<mailto:nrotem at apple.com>>

Hi Richard,

visitSIGN_EXTEND() in DAGCombiner.cpp generates an ISD::SELECT even if VT is a vector, which causes ExpandSELECT() to assert during legalization.
I think what's required is to have visitSIGN_EXTEND generate a VSELECT if VT is a vector…

ISD::SELECT should be used for cases where the selector is a scalar, even if the operands are vector. If you found a case where SELECT is used with a vector operand then this is a bug.

I did… It originates from an icmp ne <2x i8>, zero initializer followed by a sext of the result 2x i1 to 2x i8. When we visit the SIGN_EXTEND, we generate the ISD::SELECT even though the selector and both operands are vectors.

We should probably add an assert in SelectionDAG::getNode().

I've tried a local change that cures this particular assert, but uncovers another assert later, so I'm a bit uncertain if I'm heading off in the wrong direction.

I think that replacing SELECT with VSELECT is the right thing to do.

VT.isVector() ? ISD::VSELECT : ISD::SELECT in lieu of the fixed ISD::SELECT seems to do the trick for me. Except that it sometimes hits the assert in ExpandVSELECT...

The subsequent assert comes in ExpandVSELECT, which expects (for no good reason as far as I can tell) that the "mask" (really the 'selector') and the operands will be the same width, rather than trying to make them the same width (in our problem cases, the mask is an i32 produced from getSetCCResultType() and the ops are i8 or i16 - all vectors of the same length, of course.)

If I remember correctly the trick of expanding VSELECT into a sequence of XOR/AND/OR only works if the mask is known to be all-one or all-zero.

Well, that's not a problem since the mask was generated from a sign extend from an i1… ;-) The problem comes about because the i32 mask and i8 or i16 operands are different widths and expandVSELECT is not equipped to deal with that case.

I don't really want to fix that, as the generated XOR/AND/AND/OR sequence isn't desirable at all for our target, which has an instruction that I think does precisely VSELECT…

It turns out that for i8/i16 types, we'd have to promote them to i32 types to take advantage of our instruction… so the sequence isn't as bad as I thought at first.


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