Two changes:

1. Implement InstCombine/deadcode.ll by not adding instructions in unreachable
   blocks (due to constants in conditional branches/switches) to the worklist.
   This causes them to be deleted before instcombine starts up, leading to
   better optimization.

2. In the prepass over instructions, do trivial constprop/dce as we go.  This
   has the effect of improving the effectiveness of #1.  In addition, it
   *significantly* speeds up instcombine on test cases with large amounts of
   constant folding code (for example, that produced by code specialization
   or partial evaluation).  In one example, it speeds up instcombine from
   0.0589s to 0.0224s with a release build (a 2.6x speedup).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28215 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/InstructionCombining.cpp

@@ -48,7 +48,6 @@
 #include "llvm/Support/InstVisitor.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/PatternMatch.h"
-#include "llvm/ADT/DepthFirstIterator.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/STLExtras.h"
 #include <algorithm>
@@ -7318,17 +7317,83 @@ static bool TryToSinkInstruction(Instruction *I, BasicBlock *DestBlock) {
   return true;
 }
 
+
+/// AddReachableCodeToWorklist - Walk the function in depth-first order, adding
+/// all reachable code to the worklist.
+///
+/// This has a couple of tricks to make the code faster and more powerful.  In
+/// particular, we constant fold and DCE instructions as we go, to avoid adding
+/// them to the worklist (this significantly speeds up instcombine on code where
+/// many instructions are dead or constant).  Additionally, if we find a branch
+/// whose condition is a known constant, we only visit the reachable successors.
+///
+static void AddReachableCodeToWorklist(BasicBlock *BB, 
+                                       std::set<BasicBlock*> &Visited,
+                                       std::vector<Instruction*> &WorkList) {
+  // We have now visited this block!  If we've already been here, bail out.
+  if (!Visited.insert(BB).second) return;
+    
+  for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E; ) {
+    Instruction *Inst = BBI++;
+    
+    // DCE instruction if trivially dead.
+    if (isInstructionTriviallyDead(Inst)) {
+      ++NumDeadInst;
+      DEBUG(std::cerr << "IC: DCE: " << *Inst);
+      Inst->eraseFromParent();
+      continue;
+    }
+    
+    // ConstantProp instruction if trivially constant.
+    if (Constant *C = ConstantFoldInstruction(Inst)) {
+      DEBUG(std::cerr << "IC: ConstFold to: " << *C << " from: " << *Inst);
+      Inst->replaceAllUsesWith(C);
+      ++NumConstProp;
+      Inst->eraseFromParent();
+      continue;
+    }
+    
+    WorkList.push_back(Inst);
+  }
+
+  // Recursively visit successors.  If this is a branch or switch on a constant,
+  // only visit the reachable successor.
+  TerminatorInst *TI = BB->getTerminator();
+  if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
+    if (BI->isConditional() && isa<ConstantBool>(BI->getCondition())) {
+      bool CondVal = cast<ConstantBool>(BI->getCondition())->getValue();
+      AddReachableCodeToWorklist(BI->getSuccessor(!CondVal), Visited, WorkList);
+      return;
+    }
+  } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
+    if (ConstantInt *Cond = dyn_cast<ConstantInt>(SI->getCondition())) {
+      // See if this is an explicit destination.
+      for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i)
+        if (SI->getCaseValue(i) == Cond) {
+          AddReachableCodeToWorklist(SI->getSuccessor(i), Visited, WorkList);
+          return;
+        }
+      
+      // Otherwise it is the default destination.
+      AddReachableCodeToWorklist(SI->getSuccessor(0), Visited, WorkList);
+      return;
+    }
+  }
+  
+  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
+    AddReachableCodeToWorklist(TI->getSuccessor(i), Visited, WorkList);
+}
+
 bool InstCombiner::runOnFunction(Function &F) {
   bool Changed = false;
   TD = &getAnalysis<TargetData>();
 
   {
-    // Populate the worklist with the reachable instructions.
+    // Do a depth-first traversal of the function, populate the worklist with
+    // the reachable instructions.  Ignore blocks that are not reachable.  Keep
+    // track of which blocks we visit.
     std::set<BasicBlock*> Visited;
-    for (df_ext_iterator<BasicBlock*> BB = df_ext_begin(&F.front(), Visited),
-           E = df_ext_end(&F.front(), Visited); BB != E; ++BB)
-      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
-        WorkList.push_back(I);
+    AddReachableCodeToWorklist(F.begin(), Visited, WorkList);
 
     // Do a quick scan over the function.  If we find any blocks that are
     // unreachable, remove any instructions inside of them.  This prevents
@@ -7397,7 +7462,7 @@ bool InstCombiner::runOnFunction(Function &F) {
       ReplaceInstUsesWith(*I, C);
 
       ++NumConstProp;
-      I->getParent()->getInstList().erase(I);
+      I->eraseFromParent();
       removeFromWorkList(I);
       continue;
     }