SILCombiner: fix a non-determinism in a peephole optimization for load instructions.

Author: eeckstein

lib/SILOptimizer/SILCombiner/SILCombinerMiscVisitors.cpp

@@ -477,11 +477,18 @@ SILInstruction *SILCombiner::visitLoadInst(LoadInst *LI) {
   // Given a load with multiple struct_extracts/tuple_extracts and no other
   // uses, canonicalize the load into several (struct_element_addr (load))
   // pairs.
-  using ProjInstPairTy = std::pair<Projection, SingleValueInstruction *>;
+
+  struct ProjInstPair {
+    Projection P;
+    SingleValueInstruction *I;
+
+    // When sorting, just look at the projection and ignore the instruction.
+    bool operator<(const ProjInstPair &RHS) const { return P < RHS.P; }
+  };
 
   // Go through the loads uses and add any users that are projections to the
   // projection list.
-  llvm::SmallVector<ProjInstPairTy, 8> Projections;
+  llvm::SmallVector<ProjInstPair, 8> Projections;
   for (auto *UI : getNonDebugUses(LI)) {
     auto *User = UI->getUser();
 
@@ -503,8 +510,8 @@ SILInstruction *SILCombiner::visitLoadInst(LoadInst *LI) {
   Projection *LastProj = nullptr;
   LoadInst *LastNewLoad = nullptr;
   for (auto &Pair : Projections) {
-    auto &Proj = Pair.first;
-    auto *Inst = Pair.second;
+    auto &Proj = Pair.P;
+    auto *Inst = Pair.I;
 
     // If this projection is the same as the last projection we processed, just
     // replace all uses of the projection with the load we created previously.