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/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"
35 ConstantFoldUnaryInst(BasicBlock *BB, BasicBlock::iterator &II,
36 UnaryOperator *Op, Constant *D) {
37 Constant *ReplaceWith =
38 opt::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 =
61 opt::ConstantFoldCastInstruction(D, CI->getType());
63 if (!ReplaceWith) return false; // Nothing new to change...
65 // Replaces all of the uses of a variable with uses of the constant.
66 CI->replaceAllUsesWith(ReplaceWith);
68 // Remove the cast from the list of definitions...
69 CI->getParent()->getInstList().remove(II);
71 // The new constant inherits the old name of the cast...
73 ReplaceWith->setName(CI->getName(), BB->getParent()->getSymbolTableSure());
75 // Delete the cast now...
81 ConstantFoldBinaryInst(BasicBlock *BB, BasicBlock::iterator &II,
83 Constant *D1, Constant *D2) {
84 Constant *ReplaceWith =
85 opt::ConstantFoldBinaryInstruction(Op->getOpcode(), D1, D2);
86 if (!ReplaceWith) return false; // Nothing new to change...
88 // Replaces all of the uses of a variable with uses of the constant.
89 Op->replaceAllUsesWith(ReplaceWith);
91 // Remove the operator from the list of definitions...
92 Op->getParent()->getInstList().remove(II);
94 // The new constant inherits the old name of the operator...
96 ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
98 // Delete the operator now...
103 // ConstantFoldTerminator - If a terminator instruction is predicated on a
104 // constant value, convert it into an unconditional branch to the constant
107 bool 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));
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;
120 //cerr << "Method: " << T->getParent()->getParent()
121 // << "\nRemoving branch from " << T->getParent()
122 // << "\n\nTo: " << OldDest << endl;
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());
129 // Set the unconditional destination, and change the insn to be an
130 // unconditional branch.
131 BI->setUnconditionalDest(Destination);
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 opt::ConstantPropogation::doConstantPropogation(BasicBlock *BB,
160 BasicBlock::iterator &II) {
161 Instruction *Inst = *II;
162 if (BinaryOperator *BInst = dyn_cast<BinaryOperator>(Inst)) {
163 Constant *D1 = dyn_cast<Constant>(Inst->getOperand(0));
164 Constant *D2 = dyn_cast<Constant>(Inst->getOperand(1));
167 return ConstantFoldBinaryInst(BB, II, cast<BinaryOperator>(Inst), D1, D2);
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);
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);
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...
197 // DoConstPropPass - Propogate constants and do constant folding on instructions
198 // this returns true if something was changed, false if nothing was changed.
200 static bool DoConstPropPass(Method *M) {
201 bool SomethingChanged = false;
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;
211 return SomethingChanged;
215 // returns whether or not the underlying method was modified
217 bool opt::ConstantPropogation::doConstantPropogation(Method *M) {
218 bool Modified = false;
220 // Fold constants until we make no progress...
221 while (DoConstPropPass(M)) Modified = true;