[LLVMdev] Deriving undefined behavior from nsw/inbounds/poison for scalar evolution (original) (raw)

Sanjoy Das sanjoy at playingwithpointers.com
Wed Jul 1 00:11:11 PDT 2015

To make sure that I understand correctly, are you suggesting making the nuw (and similar) flags part of the key when looking up an SCEV in the UniqueSCEVs member variable on ScalarEvolution? That way, an

Yes.

instruction with nuw and one without will not map to the same SCEV unless the nuw can be inferred in some other way. Sounds good to me,

Yes.

I'm happy to go with either one of that or inferring UB from poison.

Just to be clear, it is not clear to me which approach is better. I was only offering a potential alternative.

Adding the flags to the key/identity does break the idea that mathematically equivalent expressions should cancel if subtracted. Andrew Trick wrote something about that previously:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20141215/249390.html

It depends on what sort of semantics you assign to an overflowing scev expression. If such an overflowing expression is supposed to produce poison, then I think you can cancel "(a +nsw b) - (a + b)" since "poison - " == "poison" --> can be replaced with "0".

Another way to look at this is that it should always be legal for the compiler to drop to do a simplification.

-- Sanjoy

More information about the llvm-dev mailing list