[PATCH] D40541: [SelectionDAG] Make sorting predicate stronger to remove non-deterministic ordering

[Mandeep Singh Grang via Phabricator via llvm-commits]

mgrang created this revision.
Herald added subscribers: kristof.beyls, aemerson.

Recommitting this with the correct sorting predicate. The Low field of Clusters is a ConstantInt and
cannot be directly compared. So we needed to invoke slt (signed less than) to compare correctly.

This fixes failures in the following tests uncovered by https://reviews.llvm.org/D39245:

LLVM :: CodeGen/ARM/ifcvt3.ll
LLVM :: CodeGen/ARM/switch-minsize.ll
LLVM :: CodeGen/X86/switch.ll
LLVM :: CodeGen/X86/switch-bt.ll 
LLVM :: CodeGen/X86/switch-density.ll


Repository:
  rL LLVM

https://reviews.llvm.org/D40541

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp


Index: lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -9350,10 +9350,12 @@
 
   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 +9544,15 @@
   }
 
   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->getValue().slt(b.Low->getValue());
     });
 
     // Rearrange the case blocks so that the last one falls through if possible


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