Adding Freeze instruction and fix loop unswitch

[David Majnemer via llvm-commits]

Will we need to change the shufflevector <
http://llvm.org/docs/LangRef.html#shufflevector-instruction> semantics? It
defines undef to be meaningful for the second vector.

On Tue, Jan 24, 2017 at 10:37 AM, Friedman, Eli via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> 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
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
> Foundation Collaborative Project
>
>
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