1 //===- ConstantProp.cpp - Code to perform Constant Propogation ------------===//
3 // This file implements constant propogation and merging:
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
9 // * Converts instructions like "add int %1, %2" into a direct def of %3 in
11 // * Converts conditional branches on a constant boolean value into direct
13 // * Converts phi nodes with one incoming def to the incoming def directly
14 // . Converts switch statements with one entry into a test & conditional
16 // . Converts switches on constant values into an unconditional branch.
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.
22 //===----------------------------------------------------------------------===//
24 #include "llvm/Transforms/Scalar/ConstantProp.h"
25 #include "llvm/ConstantHandling.h"
26 #include "llvm/Module.h"
27 #include "llvm/Function.h"
28 #include "llvm/BasicBlock.h"
29 #include "llvm/iTerminators.h"
30 #include "llvm/iPHINode.h"
31 #include "llvm/iOther.h"
32 #include "llvm/Pass.h"
33 #include "llvm/ConstantVals.h"
36 ConstantFoldUnaryInst(BasicBlock *BB, BasicBlock::iterator &II,
37 UnaryOperator *Op, Constant *D) {
38 Constant *ReplaceWith = ConstantFoldUnaryInstruction(Op->getOpcode(), D);
40 if (!ReplaceWith) return false; // Nothing new to change...
42 // Replaces all of the uses of a variable with uses of the constant.
43 Op->replaceAllUsesWith(ReplaceWith);
45 // Remove the operator from the list of definitions...
46 Op->getParent()->getInstList().remove(II);
48 // The new constant inherits the old name of the operator...
50 ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
52 // Delete the operator now...
58 ConstantFoldCast(BasicBlock *BB, BasicBlock::iterator &II,
59 CastInst *CI, Constant *D) {
60 Constant *ReplaceWith = ConstantFoldCastInstruction(D, CI->getType());
62 if (!ReplaceWith) return false; // Nothing new to change...
64 // Replaces all of the uses of a variable with uses of the constant.
65 CI->replaceAllUsesWith(ReplaceWith);
67 // Remove the cast from the list of definitions...
68 CI->getParent()->getInstList().remove(II);
70 // The new constant inherits the old name of the cast...
72 ReplaceWith->setName(CI->getName(), BB->getParent()->getSymbolTableSure());
74 // Delete the cast now...
80 ConstantFoldBinaryInst(BasicBlock *BB, BasicBlock::iterator &II,
82 Constant *D1, Constant *D2) {
83 Constant *ReplaceWith = ConstantFoldBinaryInstruction(Op->getOpcode(), D1,D2);
84 if (!ReplaceWith) return false; // Nothing new to change...
86 // Replaces all of the uses of a variable with uses of the constant.
87 Op->replaceAllUsesWith(ReplaceWith);
89 // Remove the operator from the list of definitions...
90 Op->getParent()->getInstList().remove(II);
92 // The new constant inherits the old name of the operator...
94 ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
96 // Delete the operator now...
101 // ConstantFoldTerminator - If a terminator instruction is predicated on a
102 // constant value, convert it into an unconditional branch to the constant
105 bool ConstantFoldTerminator(BasicBlock *BB, BasicBlock::iterator &II,
107 // Branch - See if we are conditional jumping on constant
108 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
109 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
110 BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
111 BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
113 if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
114 // Are we branching on constant?
115 // YES. Change to unconditional branch...
116 BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
117 BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
119 //cerr << "Function: " << T->getParent()->getParent()
120 // << "\nRemoving branch from " << T->getParent()
121 // << "\n\nTo: " << OldDest << endl;
123 // Let the basic block know that we are letting go of it. Based on this,
124 // it will adjust it's PHI nodes.
125 assert(BI->getParent() && "Terminator not inserted in block!");
126 OldDest->removePredecessor(BI->getParent());
128 // Set the unconditional destination, and change the insn to be an
129 // unconditional branch.
130 BI->setUnconditionalDest(Destination);
131 II = BB->end()-1; // Update instruction iterator!
135 // FIXME: TODO: This doesn't work if the destination has PHI nodes with
136 // different incoming values on each branch!
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
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());
147 // Change a conditional branch to unconditional.
148 BI->setUnconditionalDest(Dest1);
156 // ConstantFoldInstruction - If an instruction references constants, try to fold
159 bool doConstantPropogation(BasicBlock *BB, BasicBlock::iterator &II) {
160 Instruction *Inst = *II;
161 if (isa<BinaryOperator>(Inst)) {
162 Constant *D1 = dyn_cast<Constant>(Inst->getOperand(0));
163 Constant *D2 = dyn_cast<Constant>(Inst->getOperand(1));
166 return ConstantFoldBinaryInst(BB, II, cast<BinaryOperator>(Inst), D1, D2);
168 } else if (CastInst *CI = dyn_cast<CastInst>(Inst)) {
169 Constant *D = dyn_cast<Constant>(CI->getOperand(0));
170 if (D) return ConstantFoldCast(BB, II, CI, D);
172 } else if (UnaryOperator *UInst = dyn_cast<UnaryOperator>(Inst)) {
173 Constant *D = dyn_cast<Constant>(UInst->getOperand(0));
174 if (D) return ConstantFoldUnaryInst(BB, II, UInst, D);
175 } else if (TerminatorInst *TInst = dyn_cast<TerminatorInst>(Inst)) {
176 return ConstantFoldTerminator(BB, II, TInst);
178 } else if (PHINode *PN = dyn_cast<PHINode>(Inst)) {
179 // If it's a PHI node and only has one operand
180 // Then replace it directly with that operand.
181 assert(PN->getNumOperands() && "PHI Node must have at least one operand!");
182 if (PN->getNumOperands() == 1) { // If the PHI Node has exactly 1 operand
183 Value *V = PN->getOperand(0);
184 PN->replaceAllUsesWith(V); // Replace all uses of this PHI
185 // Unlink from basic block
186 PN->getParent()->getInstList().remove(II);
187 if (PN->hasName()) // Inherit PHINode name
188 V->setName(PN->getName(), BB->getParent()->getSymbolTableSure());
189 delete PN; // Finally, delete the node...
196 // DoConstPropPass - Propogate constants and do constant folding on instructions
197 // this returns true if something was changed, false if nothing was changed.
199 static bool DoConstPropPass(Function *F) {
200 bool SomethingChanged = false;
202 for (Function::iterator BBI = F->begin(); BBI != F->end(); ++BBI) {
203 BasicBlock *BB = *BBI;
204 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); )
205 if (doConstantPropogation(BB, I))
206 SomethingChanged = true;
210 return SomethingChanged;
214 struct ConstantPropogation : public MethodPass {
215 inline bool runOnMethod(Function *F) {
216 bool Modified = false;
218 // Fold constants until we make no progress...
219 while (DoConstPropPass(F)) Modified = true;
226 Pass *createConstantPropogationPass() {
227 return new ConstantPropogation();