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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@83790 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Transforms/Scalar/InstructionCombining.cpp b/lib/Transforms/Scalar/InstructionCombining.cpp
index 06a5660f72ec8b8af6e3f45274d30f897074af4c..9d883543c800373594cd2a516febb272fea47be2 100644 (file)
--- a/lib/Transforms/Scalar/InstructionCombining.cpp
+++ b/lib/Transforms/Scalar/InstructionCombining.cpp
@@ -12681,6 +12681,9 @@ 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();
@@ -12727,7 +12730,7 @@ 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
@@ -12759,6 +12762,16 @@ 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.
+  for (std::vector<Instruction*>::reverse_iterator
+       I = InstrsForInstCombineWorklist.rbegin(),
+       E = InstrsForInstCombineWorklist.rend(); I != E; ++I)
+    IC.Worklist.Add(*I);
 }
 
 bool InstCombiner::DoOneIteration(Function &F, unsigned Iteration) {