WidenVectorOperand() bug?

Jonas Paulsson via llvm-commits llvm-commits at lists.llvm.org
Wed Jan 18 06:13:01 PST 2017


I wanted to add a custom handling of ISD::SIGN_EXTEND in 
ReplaceNodeResults(), so that
a sext of v2i16->v2i32 would be done the same way as a v4i16 -> v4i32. 
First, I added

setOperationAction(ISD::SIGN_EXTEND, MVT::v2i32, Custom);

This all worked out well after a while...

To my surprise though, another test case then failed, with a sext v2i32 
-> v2i64. The fail
is caused by the sext end up in LowerOperation(), where it isn't handled 
and so an assert

This is quite weird, I think - to have the SIGN_EXTEND of *another type* 
than given with setOperationAction() all of a sudden get custom lowered!

What happens is that in WidenVectorOperand(), this code:

// See if the target wants to custom widen this node.
   if (CustomLowerNode(N, N->getOperand(OpNo).getValueType(), false))
     return false;

checks for custom lowering a sext of v2i32, while N is actually a sext 

I can't imagine this to be right so I am guessing that usually the 
operands and results have the same value type, so this usually doesn't 
matter, and so this is in fact a bug?

On the other hand, with this patch plenty of regression tests in the X86 
backend fail, so please help me out. (If this is the way the code must 
behave, I suppose then LowerOperation() must then return 'Op' unchanged 
when the sext we don't want to custom handle appears...?)


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