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

[mgrang]

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

[fhahn]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

9551

I might be missing something but could we still end up with multiple clusters where a.Prob == b.Prob && a.Low == b.Low ? Then the order of those nodes would still be non-deterministic, no?

[mgrang]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

9551

@fhahn Thanks! I am not sure if we two clusters can have the same Low. How about I add another predicate a.High < b.High?

The fallback option is to simply change this to use std::stable_sort but I wanted to avoid this if we can simply get by adding stronger predicates.

[hans]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

9551

The CaseClusters can't overlap, so having the same Low should not be possible.

I like this fix. Please expand the "Order cases by probability" comment to explain what's going on here. It would also be nice to have a test for this.

[fhahn]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

9551

The CaseClusters can't overlap, so having the same Low should not be possible

Ok great, thanks for confirming

[mgrang]

Added comment to explain what's going on.

[mgrang]

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

9551

I believe writing individual test cases for such scenarios is difficult as well as an overkill.

Once all failures uncovered by shuffling std::sort containers are fixed, I will push patches to change all std:sort to llvm::sort, which will shuffle containers by default in a +asserts build. Once that is done, the existing tests will automatically become test cases to uncover such non-determinism.

[mgrang]

Fixed another instance of non-deterministic ordering causing failure in CodeGen/X86/switch.ll.

lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

if (Small.Low == Small.High && Big.Low == Big.High &&
       }
     }
   }
 
   if (TM.getOptLevel() != CodeGenOpt::None) {
     // Order cases by probability so the most likely case will be checked first.
     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
     // without without changing the order of probabilities.
     for (CaseClusterIt I = W.LastCluster; I > W.FirstCluster; ) {
       --I;
       if (I->Prob > W.LastCluster->Prob)
         break;