This file implements the function DemoteRegToStack(), which takes a

virtual register computed by an Instruction& X and replaces it with
a slot in the stack frame, allocated via alloca.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@4964 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/DemoteRegToStack.cpp

@@ -0,0 +1,193 @@
+//===- DemoteRegToStack.cpp - Move a virtual reg. to stack ------*- C++ -*-===//
+// 
+// This file provide the function DemoteRegToStack().
+// This function takes a virtual register computed by an
+// Instruction& X and replaces it with a slot in the stack frame,
+// allocated via alloca. It returns the pointer to the AllocaInst inserted.
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/DemoteRegToStack.h"
+#include "llvm/Function.h"
+#include "llvm/BasicBlock.h"
+#include "llvm/iMemory.h"
+#include "llvm/iPHINode.h"
+#include "llvm/iTerminators.h"
+#include "llvm/Type.h"
+#include "Support/hash_set"
+#include <stack>
+
+
+//---------------------------------------------------------------------------- 
+// function DemoteRegToStack()
+//
+//---------------------------------------------------------------------------- 
+
+typedef hash_set<PHINode*>           PhiSet;
+typedef hash_set<PHINode*>::iterator PhiSetIterator;
+
+// Helper function to push a phi *and* all its operands to the worklist!
+// Do not push an instruction if it is already in the result set of Phis to go.
+inline void PushOperandsOnWorkList(std::stack<Instruction*>& workList,
+                                   PhiSet& phisToGo, PHINode* phiN)
+{
+  for (User::op_iterator OI=phiN->op_begin(), OE=phiN->op_end();
+       OI != OE; ++OI)
+    if (Instruction* opI = dyn_cast<Instruction>(OI->get()))
+      if (!isa<PHINode>(opI) ||
+          phisToGo.find(cast<PHINode>(opI)) == phisToGo.end())
+        workList.push(opI);
+}
+
+void FindPhis(Instruction& X, PhiSet& phisToGo)
+{
+  std::stack<Instruction*> workList;
+  workList.push(&X);
+
+  // Handle the case that X itself is a Phi!
+  if (PHINode* phiX = dyn_cast<PHINode>(&X))
+    {
+      phisToGo.insert(phiX);
+      PushOperandsOnWorkList(workList, phisToGo, phiX);
+    }
+
+  // Now use a worklist to find all phis reachable from X, and
+  // (recursively) all phis reachable from operands of such phis.
+  for (Instruction* workI; !workList.empty(); workList.pop())
+    {
+      workI = workList.top();
+      for (Value::use_iterator UI=workI->use_begin(), UE=workI->use_end();
+           UI != UE; ++UI)
+        if (PHINode* phiN = dyn_cast<PHINode>(*UI))
+          if (phisToGo.find(phiN) == phisToGo.end())
+            { // Seeing this phi for the first time: it must go!
+              phisToGo.insert(phiN);
+              workList.push(phiN);
+              PushOperandsOnWorkList(workList, phisToGo, phiN);
+            }
+    }
+}
+
+
+// Create the Alloca for X
+AllocaInst* CreateAllocaForX(Instruction& X)
+{
+  Function* parentFunc = X.getParent()->getParent();
+  Instruction* entryInst = parentFunc->getEntryNode().begin();
+  return new AllocaInst(X.getType(), /*arraySize*/ NULL,
+                        X.hasName()? X.getName()+std::string("OnStack")
+                                   : "DemotedTmp",
+                        entryInst);
+}
+
+// Insert loads before all uses of I, except uses in Phis
+// since all such Phis *must* be deleted.
+void LoadBeforeUses(Instruction* def, AllocaInst* XSlot)
+{
+  for (unsigned nPhis = 0; def->use_size() - nPhis > 0; )
+    {
+      Instruction* useI = cast<Instruction>(def->use_back());
+      if (!isa<PHINode>(useI))
+        {
+          LoadInst* loadI =
+            new LoadInst(XSlot, std::string("Load")+XSlot->getName(), useI);
+          useI->replaceUsesOfWith(def, loadI);
+        }
+      else
+        ++nPhis;
+    }
+}
+
+void AddLoadsAndStores(AllocaInst* XSlot, Instruction& X, PhiSet& phisToGo)
+{
+  for (PhiSetIterator PI=phisToGo.begin(), PE=phisToGo.end(); PI != PE; ++PI)
+    {
+      PHINode* pn = *PI;
+
+      // First, insert loads before all uses except uses in Phis.
+      // Do this first because new stores will appear as uses also!
+      LoadBeforeUses(pn, XSlot);
+
+      // For every incoming operand of the Phi, insert a store either
+      // just after the instruction defining the value or just before the
+      // predecessor of the Phi if the value is a formal, not an instruction.
+      // 
+      for (unsigned i=0, N=pn->getNumIncomingValues(); i < N; ++i)
+        {
+          Value* phiOp = pn->getIncomingValue(i);
+          if (phiOp != &X &&
+              (!isa<PHINode>(phiOp) ||
+               phisToGo.find(cast<PHINode>(phiOp)) == phisToGo.end()))
+            { // This operand is not a phi that will be deleted: need to store.
+              assert(!isa<TerminatorInst>(phiOp));
+
+              Instruction* storeBefore;
+              if (Instruction* I = dyn_cast<Instruction>(phiOp))
+                { // phiOp is an instruction, store its result right after it.
+                  assert(I->getNext() && "Non-terminator without successor?");
+                  storeBefore = I->getNext();
+                }
+              else
+                { // If not, it must be a formal: store it at the end of the
+                  // predecessor block of the Phi (*not* at function entry!).
+                  storeBefore = pn->getIncomingBlock(i)->getTerminator();
+                }
+              
+              // Create instr. to store the value of phiOp before `insertBefore'
+              StoreInst* storeI = new StoreInst(phiOp, XSlot, storeBefore);
+            }
+        }
+    }
+}
+
+void DeletePhis(PhiSet& phisToGo)
+{
+  for (PhiSetIterator PI=phisToGo.begin(), PE=phisToGo.end(); PI != PE; ++PI)
+    {
+      assert((*PI)->use_size() == 0 && "This PHI should be DEAD!");
+      (*PI)->getParent()->getInstList().remove(*PI);
+      delete *PI;
+    }
+  phisToGo.clear();
+}
+
+//---------------------------------------------------------------------------- 
+// function DemoteRegToStack()
+// 
+// This function takes a virtual register computed by an
+// Instruction& X and replaces it with a slot in the stack frame,
+// allocated via alloca.  It has to:
+// (1) Identify all Phi operations that have X as an operand and
+//     transitively other Phis that use such Phis; 
+// (2) Store all values merged with X via Phi operations to the stack slot;
+// (3) Load the value from the stack slot just before any use of X or any
+//     of the Phis that were eliminated; and
+// (4) Delete all the Phis, which should all now be dead.
+//
+// Returns the pointer to the alloca inserted to create a stack slot for X.
+//---------------------------------------------------------------------------- 
+
+AllocaInst* DemoteRegToStack(Instruction& X)
+{
+  if (X.getType() == Type::VoidTy)
+    return NULL;                             // nothing to do!
+
+  // Find all Phis involving X or recursively using such Phis or Phis
+  // involving operands of such Phis (essentially all Phis in the "web" of X)
+  PhiSet phisToGo;
+  FindPhis(X, phisToGo);
+
+  // Create a stack slot to hold X
+  AllocaInst* XSlot = CreateAllocaForX(X);
+
+  // Insert loads before all uses of X and (*only then*) insert store after X
+  assert(X.getNext() && "Non-terminator (since non-void) with no successor?");
+  LoadBeforeUses(&X, XSlot);
+  StoreInst* storeI = new StoreInst(&X, XSlot, X.getNext());
+
+  // Do the same for all the phis that will be deleted
+  AddLoadsAndStores(XSlot, X, phisToGo);
+
+  // Delete the phis and return the alloca instruction
+  DeletePhis(phisToGo);
+  return XSlot;
+}