Teach SimplifyCFG how to eliminate duplicate PHI nodes within a block.

This reduces codesize on a variety of codes by 1-2% on x86-64. It also
helps clean up after SSAUpdater.

llvm-svn: 85626

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -24,6 +24,7 @@
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
@@ -1748,6 +1749,63 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) {
   return true;
 }
 
+/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
+/// nodes in this block. This doesn't try to be clever about PHI nodes
+/// which differ only in the order of the incoming values, but instcombine
+/// orders them so it usually won't matter.
+static bool EliminateDuplicatePHINodes(BasicBlock *BB) {
+  bool Changed = false;
+
+  // Map from PHI hash values to PHI nodes. If multiple PHIs have
+  // the same hash value, the element is the first PHI in the
+  // linked list in CollisionMap.
+  DenseMap<uintptr_t, PHINode *> HashMap;
+
+  // Maintain linked lists of PHI nodes with common hash values.
+  DenseMap<PHINode *, PHINode *> CollisionMap;
+
+  // Examine each PHI.
+  for (BasicBlock::iterator I = BB->begin();
+       PHINode *PN = dyn_cast<PHINode>(I++); ) {
+    // Compute a hash value on the operands. Instcombine will likely have sorted
+    // them, which helps expose duplicates, but we have to check all the
+    // operands to be safe in case instcombine hasn't run.
+    uintptr_t Hash = 0;
+    for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) {
+      // This hash algorithm is quite weak as hash functions go, but it seems
+      // to do a good enough job for this particular purpose, and is very quick.
+      Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I));
+      Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
+    }
+    // If we've never seen this hash value before, it's a unique PHI.
+    std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair =
+      HashMap.insert(std::make_pair(Hash, PN));
+    if (Pair.second) continue;
+    // Otherwise it's either a duplicate or a hash collision.
+    for (PHINode *OtherPN = Pair.first->second; ; ) {
+      if (OtherPN->isIdenticalTo(PN)) {
+        // A duplicate. Replace this PHI with its duplicate.
+        PN->replaceAllUsesWith(OtherPN);
+        PN->eraseFromParent();
+        Changed = true;
+        break;
+      }
+      // A non-duplicate hash collision.
+      DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN);
+      if (I == CollisionMap.end()) {
+        // Set this PHI to be the head of the linked list of colliding PHIs.
+        PHINode *Old = Pair.first->second;
+        Pair.first->second = PN;
+        CollisionMap[PN] = Old;
+        break;
+      }
+      // Procede to the next PHI in the list.
+      OtherPN = I->second;
+    }
+  }
+
+  return Changed;
+}
 
 /// SimplifyCFG - This function is used to do simplification of a CFG.  For
 /// example, it adjusts branches to branches to eliminate the extra hop, it
@@ -1777,6 +1835,9 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
   // away...
   Changed |= ConstantFoldTerminator(BB);
 
+  // Check for and eliminate duplicate PHI nodes in this block.
+  Changed |= EliminateDuplicatePHINodes(BB);
+
   // If there is a trivial two-entry PHI node in this basic block, and we can
   // eliminate it, do so now.
   if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))