ConstantProp.cpp revision 697954c15da58bd8b186dbafdedd8b06db770201 
1//===- ConstantProp.cpp - Code to perform Constant Propogation ------------===//
2//
3// This file implements constant propogation and merging:
4//
5// Specifically, this:
6//   * Folds multiple identical constants in the constant pool together
7//     Note that if one is named and the other is not, that the result gets the
8//     original name.
9//   * Converts instructions like "add int %1, %2" into a direct def of %3 in
10//     the constant pool
11//   * Converts conditional branches on a constant boolean value into direct
12//     branches.
13//   * Converts phi nodes with one incoming def to the incoming def directly
14//   . Converts switch statements with one entry into a test & conditional
15//     branch
16//   . Converts switches on constant values into an unconditional branch.
17//
18// Notice that:
19//   * This pass has a habit of making definitions be dead.  It is a good idea
20//     to to run a DCE pass sometime after running this pass.
21//
22//===----------------------------------------------------------------------===//
23
24#include "llvm/Optimizations/ConstantProp.h"
25#include "llvm/Optimizations/ConstantHandling.h"
26#include "llvm/Module.h"
27#include "llvm/Method.h"
28#include "llvm/BasicBlock.h"
29#include "llvm/iTerminators.h"
30#include "llvm/iPHINode.h"
31#include "llvm/iOther.h"
32#include "llvm/ConstantVals.h"
33
34inline static bool
35ConstantFoldUnaryInst(BasicBlock *BB, BasicBlock::iterator &II,
36                      UnaryOperator *Op, Constant *D) {
37  Constant *ReplaceWith =
38    opt::ConstantFoldUnaryInstruction(Op->getOpcode(), D);
39
40  if (!ReplaceWith) return false;   // Nothing new to change...
41
42  // Replaces all of the uses of a variable with uses of the constant.
43  Op->replaceAllUsesWith(ReplaceWith);
44
45  // Remove the operator from the list of definitions...
46  Op->getParent()->getInstList().remove(II);
47
48  // The new constant inherits the old name of the operator...
49  if (Op->hasName())
50    ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
51
52  // Delete the operator now...
53  delete Op;
54  return true;
55}
56
57inline static bool
58ConstantFoldCast(BasicBlock *BB, BasicBlock::iterator &II,
59                 CastInst *CI, Constant *D) {
60  Constant *ReplaceWith =
61    opt::ConstantFoldCastInstruction(D, CI->getType());
62
63  if (!ReplaceWith) return false;   // Nothing new to change...
64
65  // Replaces all of the uses of a variable with uses of the constant.
66  CI->replaceAllUsesWith(ReplaceWith);
67
68  // Remove the cast from the list of definitions...
69  CI->getParent()->getInstList().remove(II);
70
71  // The new constant inherits the old name of the cast...
72  if (CI->hasName())
73    ReplaceWith->setName(CI->getName(), BB->getParent()->getSymbolTableSure());
74
75  // Delete the cast now...
76  delete CI;
77  return true;
78}
79
80inline static bool
81ConstantFoldBinaryInst(BasicBlock *BB, BasicBlock::iterator &II,
82		       BinaryOperator *Op,
83		       Constant *D1, Constant *D2) {
84  Constant *ReplaceWith =
85    opt::ConstantFoldBinaryInstruction(Op->getOpcode(), D1, D2);
86  if (!ReplaceWith) return false;   // Nothing new to change...
87
88  // Replaces all of the uses of a variable with uses of the constant.
89  Op->replaceAllUsesWith(ReplaceWith);
90
91  // Remove the operator from the list of definitions...
92  Op->getParent()->getInstList().remove(II);
93
94  // The new constant inherits the old name of the operator...
95  if (Op->hasName())
96    ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
97
98  // Delete the operator now...
99  delete Op;
100  return true;
101}
102
103// ConstantFoldTerminator - If a terminator instruction is predicated on a
104// constant value, convert it into an unconditional branch to the constant
105// destination.
106//
107bool opt::ConstantFoldTerminator(TerminatorInst *T) {
108  // Branch - See if we are conditional jumping on constant
109  if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
110    if (BI->isUnconditional()) return false;  // Can't optimize uncond branch
111    BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
112    BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
113
114    if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
115      // Are we branching on constant?
116      // YES.  Change to unconditional branch...
117      BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
118      BasicBlock *OldDest     = Cond->getValue() ? Dest2 : Dest1;
119
120      //cerr << "Method: " << T->getParent()->getParent()
121      //     << "\nRemoving branch from " << T->getParent()
122      //     << "\n\nTo: " << OldDest << endl;
123
124      // Let the basic block know that we are letting go of it.  Based on this,
125      // it will adjust it's PHI nodes.
126      assert(BI->getParent() && "Terminator not inserted in block!");
127      OldDest->removePredecessor(BI->getParent());
128
129      // Set the unconditional destination, and change the insn to be an
130      // unconditional branch.
131      BI->setUnconditionalDest(Destination);
132      return true;
133    }
134#if 0
135    // FIXME: TODO: This doesn't work if the destination has PHI nodes with
136    // different incoming values on each branch!
137    //
138    else if (Dest2 == Dest1) {       // Conditional branch to same location?
139      // This branch matches something like this:
140      //     br bool %cond, label %Dest, label %Dest
141      // and changes it into:  br label %Dest
142
143      // Let the basic block know that we are letting go of one copy of it.
144      assert(BI->getParent() && "Terminator not inserted in block!");
145      Dest1->removePredecessor(BI->getParent());
146
147      // Change a conditional branch to unconditional.
148      BI->setUnconditionalDest(Dest1);
149      return true;
150    }
151#endif
152  }
153  return false;
154}
155
156// ConstantFoldInstruction - If an instruction references constants, try to fold
157// them together...
158//
159bool opt::ConstantPropogation::doConstantPropogation(BasicBlock *BB,
160						     BasicBlock::iterator &II) {
161  Instruction *Inst = *II;
162  if (isa<BinaryOperator>(Inst)) {
163    Constant *D1 = dyn_cast<Constant>(Inst->getOperand(0));
164    Constant *D2 = dyn_cast<Constant>(Inst->getOperand(1));
165
166    if (D1 && D2)
167      return ConstantFoldBinaryInst(BB, II, cast<BinaryOperator>(Inst), D1, D2);
168
169  } else if (CastInst *CI = dyn_cast<CastInst>(Inst)) {
170    Constant *D = dyn_cast<Constant>(CI->getOperand(0));
171    if (D) return ConstantFoldCast(BB, II, CI, D);
172
173  } else if (UnaryOperator *UInst = dyn_cast<UnaryOperator>(Inst)) {
174    Constant *D = dyn_cast<Constant>(UInst->getOperand(0));
175    if (D) return ConstantFoldUnaryInst(BB, II, UInst, D);
176  } else if (TerminatorInst *TInst = dyn_cast<TerminatorInst>(Inst)) {
177    return opt::ConstantFoldTerminator(TInst);
178
179  } else if (PHINode *PN = dyn_cast<PHINode>(Inst)) {
180    // If it's a PHI node and only has one operand
181    // Then replace it directly with that operand.
182    assert(PN->getNumOperands() && "PHI Node must have at least one operand!");
183    if (PN->getNumOperands() == 1) {    // If the PHI Node has exactly 1 operand
184      Value *V = PN->getOperand(0);
185      PN->replaceAllUsesWith(V);                 // Replace all uses of this PHI
186                                                 // Unlink from basic block
187      PN->getParent()->getInstList().remove(II);
188      if (PN->hasName())                         // Inherit PHINode name
189	V->setName(PN->getName(), BB->getParent()->getSymbolTableSure());
190      delete PN;                                 // Finally, delete the node...
191      return true;
192    }
193  }
194  return false;
195}
196
197// DoConstPropPass - Propogate constants and do constant folding on instructions
198// this returns true if something was changed, false if nothing was changed.
199//
200static bool DoConstPropPass(Method *M) {
201  bool SomethingChanged = false;
202
203  for (Method::iterator BBI = M->begin(); BBI != M->end(); ++BBI) {
204    BasicBlock *BB = *BBI;
205    for (BasicBlock::iterator I = BB->begin(); I != BB->end(); )
206      if (opt::ConstantPropogation::doConstantPropogation(BB, I))
207	SomethingChanged = true;
208      else
209	++I;
210  }
211  return SomethingChanged;
212}
213
214
215// returns whether or not the underlying method was modified
216//
217bool opt::ConstantPropogation::doConstantPropogation(Method *M) {
218  bool Modified = false;
219
220  // Fold constants until we make no progress...
221  while (DoConstPropPass(M)) Modified = true;
222
223  return Modified;
224}
225