Add the skeleton of a better PHI elimination pass.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43542 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/Passes.h

@@ -44,6 +44,14 @@ namespace llvm {
   /// these register allocator like this: AU.addRequiredID(PHIEliminationID);
   ///
   extern const PassInfo *PHIEliminationID;
+  
+  /// StrongPHIElimination pass - This pass eliminates machine instruction PHI
+  /// nodes by inserting copy instructions.  This destroys SSA information, but
+  /// is the desired input for some register allocators.  This pass is
+  /// "required" by these register allocator like this:
+  ///    AU.addRequiredID(PHIEliminationID);
+  ///  This pass is still in development
+  extern const PassInfo *StrongPHIEliminationID;
 
   /// SimpleRegisterCoalescing pass.  Aggressively coalesces every register
   /// copy it can.

lib/CodeGen/StrongPHIElimination.cpp

@@ -0,0 +1,112 @@
+//===- StrongPhiElimination.cpp - Eliminate PHI nodes by inserting copies -===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by Owen Anderson and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass eliminates machine instruction PHI nodes by inserting copy
+// instructions, using an intelligent copy-folding technique based on
+// dominator information.  This is technique is derived from:
+// 
+//    Budimlic, et al. Fast copy coalescing and live-range identification.
+//    In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
+//    Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
+//    PLDI '02. ACM, New York, NY, 25-32.
+//    DOI= http://doi.acm.org/10.1145/512529.512534
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "strongphielim"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstr.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetMachine.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Support/Compiler.h"
+using namespace llvm;
+
+
+namespace {
+  struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
+    static char ID; // Pass identification, replacement for typeid
+    StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}
+
+    bool runOnMachineFunction(MachineFunction &Fn) {
+      computeDFS(Fn);
+      
+      
+      return false;
+    }
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequired<MachineDominatorTree>();
+      MachineFunctionPass::getAnalysisUsage(AU);
+    }
+    
+    virtual void releaseMemory() {
+      preorder.clear();
+      maxpreorder.clear();
+    }
+
+  private:
+    DenseMap<MachineBasicBlock*, unsigned> preorder;
+    DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
+    
+    void computeDFS(MachineFunction& MF);
+  };
+
+  char StrongPHIElimination::ID = 0;
+  RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
+                  "Eliminate PHI nodes for register allocation, intelligently");
+}
+
+const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();
+
+/// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
+/// of the given MachineFunction.  These numbers are then used in other parts
+/// of the PHI elimination process.
+void StrongPHIElimination::computeDFS(MachineFunction& MF) {
+  SmallPtrSet<MachineDomTreeNode*, 8> frontier;
+  SmallPtrSet<MachineDomTreeNode*, 8> visited;
+  
+  unsigned time = 0;
+  
+  MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
+  
+  MachineDomTreeNode* node = DT.getRootNode();
+  
+  std::vector<MachineDomTreeNode*> worklist;
+  worklist.push_back(node);
+  
+  while (!worklist.empty()) {
+    MachineDomTreeNode* currNode = worklist.back();
+    
+    if (!frontier.count(currNode)) {
+      frontier.insert(currNode);
+      ++time;
+      preorder.insert(std::make_pair(currNode->getBlock(), time));
+    }
+    
+    bool inserted = false;
+    for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
+         I != E; ++I)
+      if (!frontier.count(*I) && !visited.count(*I)) {
+        worklist.push_back(*I);
+        inserted = true;
+        break;
+      }
+    
+    if (!inserted) {
+      frontier.erase(currNode);
+      visited.insert(currNode);
+      maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
+      
+      worklist.pop_back();
+    }
+  }
+}