Skeletal LCSSA pass. This is currently non-functional. Expect functionality

and documentation updates soo.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@28495 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 159 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/LCSSA.cpp b/lib/Transforms/Utils/LCSSA.cpp
new file mode 100644
index 0000000000..5382ec6d6a
--- /dev/null
+++ b/lib/Transforms/Utils/LCSSA.cpp
@@ -0,0 +1,159 @@
+//===-- LCSSA.cpp - Convert loops into loop-closed SSA form         ------===//
+//
+//                     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 transforms loops by placing phi nodes at the end of the loops for
+// all values that are live across the loop boundary.  For example, it turns
+// the left into the right code:
+// 
+// for (...)                for (...)
+//   if (c)                   if(c)
+//     X1 = ...                 X1 = ...
+//   else                     else
+//     X2 = ...                 X2 = ...
+//   X3 = phi(X1, X2)         X3 = phi(X1, X2)
+// ... = X3 + 4              X4 = phi(X3)
+//                           ... = X4 + 4
+//
+// This is still valid LLVM; the extra phi nodes are purely redundant, and will
+// be trivially eliminated by InstCombine.  The major benefit of this 
+// transformation is that it makes many other loop optimizations, such as 
+// LoopUnswitching, simpler.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Pass.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include <iostream>
+#include <set>
+#include <vector>
+
+using namespace llvm;
+
+namespace {
+  class LCSSA : public FunctionPass {
+  public:
+    LoopInfo *LI;  // Loop information
+    
+    // LoopProcessWorklist - List of loops we need to process.
+    std::vector<Loop*> LoopProcessWorklist;
+    
+    virtual bool runOnFunction(Function &F);
+    
+    bool visitLoop(Loop *L, Value *V);
+    
+    /// This transformation requires natural loop information & requires that
+    /// loop preheaders be inserted into the CFG...
+    ///
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequiredID(LoopSimplifyID);
+      AU.addPreservedID(LoopSimplifyID);
+      AU.addRequired<LoopInfo>();
+      AU.addPreserved<LoopInfo>();
+    }
+  private:
+    void addSubloopsToWorklist(Loop* L);
+    std::set<Value*> loopValuesUsedOutsideLoop(Loop *L);
+  };
+  
+  RegisterOpt<LCSSA> X("lcssa", "Loop-Closed SSA Form Pass");
+}
+
+FunctionPass *llvm::createLCSSAPass() { return new LCSSA(); }
+
+bool LCSSA::runOnFunction(Function &F) {
+  bool changed = false;
+  LI = &getAnalysis<LoopInfo>();
+    
+  for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I) {
+    addSubloopsToWorklist(*I);
+    LoopProcessWorklist.push_back(*I);
+  }
+      
+  for (std::vector<Loop*>::iterator I = LoopProcessWorklist.begin(),
+       E = LoopProcessWorklist.end(); I != E; ++I) {
+    std::set<Value*> AffectedValues = loopValuesUsedOutsideLoop(*I);
+    if (!AffectedValues.empty()) {
+      for (std::set<Value*>::iterator VI = AffectedValues.begin(),
+           VE = AffectedValues.end(); VI != VE; ++VI)
+        changed |= visitLoop(*I, *VI);
+    }
+  }
+      
+  return changed;
+}
+
+bool LCSSA::visitLoop(Loop *L, Value* V) {
+  // We will be doing lots of "loop contains block" queries.  Loop::contains is
+  // linear time, use a set to speed this up.
+  std::set<BasicBlock*> LoopBlocks;
+
+  for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
+       BB != E; ++BB)
+    LoopBlocks.insert(*BB);
+  
+  std::vector<BasicBlock*> exitBlocks;
+  L->getExitBlocks(exitBlocks);
+  
+  for (std::vector<BasicBlock*>::iterator BBI = exitBlocks.begin(),
+       BBE = exitBlocks.end(); BBI != BBE; ++BBI) {
+    PHINode *phi = new PHINode(V->getType(), "lcssa");
+    (*BBI)->getInstList().insert((*BBI)->front(), phi);
+    
+    for (pred_iterator PI = pred_begin(*BBI), PE = pred_end(*BBI); PI != PE;
+         ++PI)
+      phi->addIncoming(V, *PI);
+  }
+  
+  for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE;
+       ++UI) {
+    BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
+    if (!LoopBlocks.count(UserBB))
+      ; // FIXME: This should update the SSA form through the rest of the graph.
+  }
+  
+  return false;
+}
+
+void LCSSA::addSubloopsToWorklist(Loop* L) {
+  for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I) {
+    addSubloopsToWorklist(*I);
+    LoopProcessWorklist.push_back(*I);
+  }
+}
+
+/// loopValuesUsedOutsideLoop - Return true if there are any values defined in
+/// the loop that are used by instructions outside of it.
+std::set<Value*> LCSSA::loopValuesUsedOutsideLoop(Loop *L) {
+  std::set<Value*> AffectedValues;
+
+  // We will be doing lots of "loop contains block" queries.  Loop::contains is
+  // linear time, use a set to speed this up.
+  std::set<BasicBlock*> LoopBlocks;
+
+  for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
+       BB != E; ++BB)
+    LoopBlocks.insert(*BB);
+  
+  for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
+       BB != E; ++BB) {
+    for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ++I)
+      for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+           ++UI) {
+        BasicBlock *UserBB = cast<Instruction>(*UI)->getParent();
+        if (!LoopBlocks.count(UserBB))
+          AffectedValues.insert(I);
+      }
+  }
+  return AffectedValues;
+}
\ No newline at end of file