populate instcombine's initial worklist more carefully, causing

it to visit instructions from the start of the function to the
end of the function in the first path.  This greatly speeds up
some pathological cases (e.g. PR5150). 

llvm-svn: 83814

llvm/lib/Transforms/Scalar/InstructionCombining.cpp

@@ -101,6 +101,20 @@ namespace {
         Add(I);
     }
     
+    /// AddInitialGroup - Add the specified batch of stuff in reverse order.
+    /// which should only be done when the worklist is empty and when the group
+    /// has no duplicates.
+    void AddInitialGroup(Instruction *const *List, unsigned NumEntries) {
+      assert(Worklist.empty() && "Worklist must be empty to add initial group");
+      Worklist.reserve(NumEntries+16);
+      DEBUG(errs() << "IC: ADDING: " << NumEntries << " instrs to worklist\n");
+      for (; NumEntries; --NumEntries) {
+        Instruction *I = List[NumEntries-1];
+        WorklistMap.insert(std::make_pair(I, Worklist.size()));
+        Worklist.push_back(I);
+      }
+    }
+    
     // Remove - remove I from the worklist if it exists.
     void Remove(Instruction *I) {
       DenseMap<Instruction*, unsigned>::iterator It = WorklistMap.find(I);
@@ -12663,6 +12677,9 @@ static void AddReachableCodeToWorklist(BasicBlock *BB,
                                        const TargetData *TD) {
   SmallVector<BasicBlock*, 256> Worklist;
   Worklist.push_back(BB);
+  
+  std::vector<Instruction*> InstrsForInstCombineWorklist;
+  InstrsForInstCombineWorklist.reserve(128);
 
   while (!Worklist.empty()) {
     BB = Worklist.back();
@@ -12709,7 +12726,7 @@ static void AddReachableCodeToWorklist(BasicBlock *BB,
         DBI_Prev = 0;
       }
 
-      IC.Worklist.Add(Inst);
+      InstrsForInstCombineWorklist.push_back(Inst);
     }
 
     // Recursively visit successors.  If this is a branch or switch on a
@@ -12741,6 +12758,14 @@ static void AddReachableCodeToWorklist(BasicBlock *BB,
     for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
       Worklist.push_back(TI->getSuccessor(i));
   }
+  
+  // Once we've found all of the instructions to add to instcombine's worklist,
+  // add them in reverse order.  This way instcombine will visit from the top
+  // of the function down.  This jives well with the way that it adds all uses
+  // of instructions to the worklist after doing a transformation, thus avoiding
+  // some N^2 behavior in pathological cases.
+  IC.Worklist.AddInitialGroup(&InstrsForInstCombineWorklist[0],
+                              InstrsForInstCombineWorklist.size());
 }
 
 bool InstCombiner::DoOneIteration(Function &F, unsigned Iteration) {