Adding Freeze instruction and fix loop unswitch

[Friedman, Eli via llvm-commits]

On 1/23/2017 4:15 AM, Nuno Lopes via llvm-commits wrote:
> Hi,
>
> Following our talk at the LLVM dev meeting regarding poison, we would 
> like to introduce the freeze instruction in the LLVM IR so that we can 
> fix a longstanding bug in loop unswitch.
> In the talk I mentioned a bigger proposal with several orthogonal 
> changes. We are now splitting that proposal in smaller pieces, and 
> this set of patches is the first piece.  These pieces are independent 
> and can be reviewed independently.
> Juneyoung Lee implemented the whole thing; I'm just the messenger.
>
> A few details about the problem:
>
> Loop unswitch does the following transformation:
>   while (...) {
>     if (C) { A }
>     else   { B }
>   }
>
> into:
>   if (C) {
>     while (...) { A }
>   } else {
>     while (...) { B }
>   }
>
>
> This transformation is incorrect if C may be poison/undef, since 
> branching on poison/undef is UB. If, for example, the loop never 
> executed, the transformed code would be branching on C while the 
> original was not, and so we introduced UB (if C is undef/poison).
>
> This has been shown to lead to end-to-end miscompilations when loop 
> unswitch is combined with other passes, like GVN:
>  - https://llvm.org/bugs/show_bug.cgi?id=27506
>  - https://llvm.org/bugs/show_bug.cgi?id=31652
>
>
> We propose a simple change, which is to transform the code above into:
>   C' = freeze(c)
>   if (C') {
>     while (...) { A }
>   } else {
>     while (...) { B }
>   }
>
>
> Freeze always returns a non-poison/undef value. When given a 
> poison/undef value, it returns a non-deterministic value that is fixed 
> for all uses.
>
>
> We've split the changes in 5 patches:
>  - Add Freeze instruction to IR: https://reviews.llvm.org/D29011
>  - InstCombine/InstSimplify support for freeze: 
> https://reviews.llvm.org/D29013
>  - SelectionDAG support for freeze: https://reviews.llvm.org/D29014
>  - Fix wrong code emission by loop unswitch: 
> https://reviews.llvm.org/D29015
>  - Optimize placement of freeze in loop unswitch: 
> https://reviews.llvm.org/D29016

I'm not seeing any patch in this series with the necessary changes to 
LangRef?

-Eli

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