[PATCH] D102711: [GlobalOpt] Handle null check with global pointer variables

[Eli Friedman via Phabricator via llvm-commits]

efriedma added inline comments.


================
Comment at: llvm/lib/Transforms/IPO/GlobalOpt.cpp:979
       case ICmpInst::ICMP_ULT:
       case ICmpInst::ICMP_SLT:   // X < null -> always false
         LV = ConstantInt::getFalse(GV->getContext());
----------------
scui wrote:
> efriedma wrote:
> > Can we explicitly note here, for the signed compares, why it doesn't matter if the high bit of the global is set? The logic here isn't obvious.
> I'm not sure what comments I can add,  as I actually don't know the logic here for the folding of the signed compares.  The code has been like this from 2009 (the earliest I can found). Do you have any suggestions on how to handle all these signed compares?
We could probably just reject icmps with a signed predicate; frontends won't normally generate signed pointer comparisons anyway.

Alternatively, you could use some tortured logic here: the program doesn't actually capture the pointer, and you can't do anything useful just knowing the sign of the pointer. So just pretend the sign bit isn't set.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D102711/new/

https://reviews.llvm.org/D102711