When inserting PHI nodes, don't insert any phi nodes that are obviously

unneccesary.  This allows us to delete several hundred phi nodes of the
form PHI(x,x,x,undef) from 253.perlbmk and probably other programs as well.

This implements Mem2Reg/UndefValuesMerge.ll

llvm-svn: 17098

1 files changed, 31 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
index 6423b762da9..49499c65e0d 100644
--- a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
+++ b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -24,6 +24,7 @@
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/AliasSetTracker.h"
 #include "llvm/ADT/StringExtras.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/StableBasicBlockNumbering.h"
 #include <algorithm>
@@ -94,6 +95,12 @@ namespace {
 
     void run();
 
+    /// dominates - Return true if BB1 dominates BB2 using the DT.
+    ///
+    bool dominates(BasicBlock *BB1, BasicBlock *BB2) const {
+      return DT[BB1]->dominates(DT[BB2]);
+    }
+
   private:
     void MarkDominatingPHILive(BasicBlock *BB, unsigned AllocaNum,
                                std::set<PHINode*> &DeadPHINodes);
@@ -316,7 +323,7 @@ void PromoteMem2Reg::run() {
   // code.  Unfortunately, there may be blocks which are not reachable, which
   // the renamer hasn't traversed.  If this is the case, the PHI nodes may not
   // have incoming values for all predecessors.  Loop over all PHI nodes we have
-  // created, inserting null constants if they are missing any incoming values.
+  // created, inserting undef values if they are missing any incoming values.
   //
   for (std::map<BasicBlock*, std::vector<PHINode *> >::iterator I = 
          NewPhiNodes.begin(), E = NewPhiNodes.end(); I != E; ++I) {
@@ -325,27 +332,41 @@ void PromoteMem2Reg::run() {
     std::vector<PHINode*> &PNs = I->second;
     assert(!PNs.empty() && "Empty PHI node list??");
 
+    // Loop over all of the PHI nodes and see if there are any that we can get
+    // rid of because they merge all of the same incoming values.  This can
+    // happen due to undef values coming into the PHI nodes.
+    PHINode *SomePHI = 0;
+    for (unsigned i = 0, e = PNs.size(); i != e; ++i)
+      if (PNs[i]) {
+        if (Value *V = hasConstantValue(PNs[i])) {
+          if (!isa<Instruction>(V) ||
+              dominates(cast<Instruction>(V)->getParent(), I->first)) {
+            PNs[i]->replaceAllUsesWith(V);
+            PNs[i]->eraseFromParent();
+            PNs[i] = 0;
+          }
+        }
+        if (PNs[i])
+          SomePHI = PNs[i];
+      }
+
     // Only do work here if there the PHI nodes are missing incoming values.  We
     // know that all PHI nodes that were inserted in a block will have the same
     // number of incoming values, so we can just check any PHI node.
-    PHINode *FirstPHI;
-    for (unsigned i = 0; (FirstPHI = PNs[i]) == 0; ++i)
-      /*empty*/;
-
-    if (Preds.size() != FirstPHI->getNumIncomingValues()) {
+    if (SomePHI && Preds.size() != SomePHI->getNumIncomingValues()) {
       // Ok, now we know that all of the PHI nodes are missing entries for some
       // basic blocks.  Start by sorting the incoming predecessors for efficient
       // access.
       std::sort(Preds.begin(), Preds.end());
 
-      // Now we loop through all BB's which have entries in FirstPHI and remove
+      // Now we loop through all BB's which have entries in SomePHI and remove
       // them from the Preds list.
-      for (unsigned i = 0, e = FirstPHI->getNumIncomingValues(); i != e; ++i) {
+      for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
         // Do a log(n) search of the Preds list for the entry we want.
         std::vector<BasicBlock*>::iterator EntIt =
           std::lower_bound(Preds.begin(), Preds.end(),
-                           FirstPHI->getIncomingBlock(i));
-        assert(EntIt != Preds.end() && *EntIt == FirstPHI->getIncomingBlock(i)&&
+                           SomePHI->getIncomingBlock(i));
+        assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i)&&
                "PHI node has entry for a block which is not a predecessor!");
 
         // Remove the entry