Add a prototype of a new peephole optimizing pass that uses LazyValue info to simplify PHIs and select's.

This pass addresses the missed optimizations from PR2581 and PR4420.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112325 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 113 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/ValuePropagation.cpp b/lib/Transforms/Scalar/ValuePropagation.cpp
new file mode 100644
index 0000000000..c1384bb1d9
--- /dev/null
+++ b/lib/Transforms/Scalar/ValuePropagation.cpp
@@ -0,0 +1,113 @@
+//===- ValuePropagation.cpp - Propagate information derived control flow --===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the Value Propagation pass.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "value-propagation"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/Pass.h"
+#include "llvm/Analysis/LazyValueInfo.h"
+#include "llvm/Transforms/Utils/Local.h"
+using namespace llvm;
+
+namespace {
+  class ValuePropagation : public FunctionPass {
+    LazyValueInfo *LVI;
+    
+    bool processSelect(SelectInst *SI);
+    bool processPHI(PHINode *P);
+    
+  public:
+    static char ID;
+    ValuePropagation(): FunctionPass(ID) { }
+    
+    bool runOnFunction(Function &F);
+    
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      AU.addRequired<LazyValueInfo>();
+    }
+  };
+}
+
+char ValuePropagation::ID = 0;
+INITIALIZE_PASS(ValuePropagation, "value-propagation",
+                "Value Propagation", false, false);
+
+// Public interface to the Value Propagation pass
+Pass *llvm::createValuePropagationPass() {
+  return new ValuePropagation();
+}
+
+bool ValuePropagation::processSelect(SelectInst *S) {
+  Constant *C = LVI->getConstant(S->getOperand(0), S->getParent());
+  if (!C) return false;
+  
+  ConstantInt *CI = dyn_cast<ConstantInt>(C);
+  if (!CI) return false;
+  
+  if (CI->isZero()) {
+    S->replaceAllUsesWith(S->getOperand(2));
+    S->eraseFromParent();
+  } else if (CI->isOne()) {
+    S->replaceAllUsesWith(S->getOperand(1));
+    S->eraseFromParent();
+  } else {
+    assert(0 && "Select on constant is neither 0 nor 1?");
+  }
+  
+  return true;
+}
+
+bool ValuePropagation::processPHI(PHINode *P) {
+  bool changed = false;
+  
+  BasicBlock *BB = P->getParent();
+  for (unsigned i = 0; i < P->getNumIncomingValues(); ++i) {
+    Constant *C = LVI->getConstantOnEdge(P->getIncomingValue(i),
+                                         P->getIncomingBlock(i),
+                                         BB);
+    if (!C || C == P->getIncomingValue(i)) continue;
+    
+    P->setIncomingValue(i, C);
+    changed = true;
+  }
+  
+  if (Value *ConstVal = P->hasConstantValue()) {
+    P->replaceAllUsesWith(ConstVal);
+    P->eraseFromParent();
+    changed = true;
+  }
+  
+  return changed;
+}
+
+bool ValuePropagation::runOnFunction(Function &F) {
+  LVI = &getAnalysis<LazyValueInfo>();
+  
+  bool changed = false;
+  
+  for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
+    for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
+      Instruction *II = BI++;
+      if (SelectInst *SI = dyn_cast<SelectInst>(II))
+        changed |= processSelect(SI);
+      else if (PHINode *P = dyn_cast<PHINode>(II))
+        changed |= processPHI(P);
+    }
+  
+  if (changed)
+    for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
+      SimplifyInstructionsInBlock(FI);
+  
+  return changed;
+}
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