[llvm] r318680 - [SelectionDAG] Make sorting predicate stronger to remove non-deterministic ordering

[Hans Wennborg via llvm-commits]

The buildbots are sad:
http://bb.pgr.jp/builders/test-llvm-i686-linux-RA/builds/1330/

On Mon, Nov 20, 2017 at 10:46 AM, Mandeep Singh Grang via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: mgrang
> Date: Mon Nov 20 10:46:11 2017
> New Revision: 318680
>
> URL: http://llvm.org/viewvc/llvm-project?rev=318680&view=rev
> Log:
> [SelectionDAG] Make sorting predicate stronger to remove non-deterministic ordering
>
> Summary:
> This fixes failures in the following tests uncovered by D39245:
>         LLVM :: CodeGen/ARM/ifcvt3.ll
>         LLVM :: CodeGen/ARM/switch-minsize.ll
>         LLVM :: CodeGen/X86/switch.ll
>
> Reviewers: hans, efriedma
>
> Reviewed By: hans
>
> Subscribers: fhahn, aemerson, kristof.beyls, llvm-commits
>
> Differential Revision: https://reviews.llvm.org/D39995
>
> Modified:
>     llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
>
> Modified: llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp?rev=318680&r1=318679&r2=318680&view=diff
> ==============================================================================
> --- llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp (original)
> +++ llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp Mon Nov 20 10:46:11 2017
> @@ -9350,10 +9350,12 @@ bool SelectionDAGBuilder::buildBitTests(
>
>    BitTestInfo BTI;
>    std::sort(CBV.begin(), CBV.end(), [](const CaseBits &a, const CaseBits &b) {
> -    // Sort by probability first, number of bits second.
> +    // Sort by probability first, number of bits second, bit mask third.
>      if (a.ExtraProb != b.ExtraProb)
>        return a.ExtraProb > b.ExtraProb;
> -    return a.Bits > b.Bits;
> +    if (a.Bits != b.Bits)
> +      return a.Bits > b.Bits;
> +    return a.Mask < b.Mask;
>    });
>
>    for (auto &CB : CBV) {
> @@ -9542,10 +9544,13 @@ void SelectionDAGBuilder::lowerWorkItem(
>    }
>
>    if (TM.getOptLevel() != CodeGenOpt::None) {
> -    // Order cases by probability so the most likely case will be checked first.
> +    // Here, we order cases by probability so the most likely case will be
> +    // checked first. However, two clusters can have the same probability in
> +    // which case their relative ordering is non-deterministic. So we use Low
> +    // as a tie-breaker as clusters are guaranteed to never overlap.
>      std::sort(W.FirstCluster, W.LastCluster + 1,
>                [](const CaseCluster &a, const CaseCluster &b) {
> -      return a.Prob > b.Prob;
> +      return a.Prob != b.Prob ? a.Prob > b.Prob : a.Low < b.Low;
>      });
>
>      // Rearrange the case blocks so that the last one falls through if possible
>
>
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