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"
35 ConstantFoldUnaryInst(BasicBlock *BB, BasicBlock::iterator &II,
36 UnaryOperator *Op, Constant *D) {
37 Constant *ReplaceWith = ConstantFoldUnaryInstruction(Op->getOpcode(), D);
39 if (!ReplaceWith) return false; // Nothing new to change...
41 // Replaces all of the uses of a variable with uses of the constant.
42 Op->replaceAllUsesWith(ReplaceWith);
44 // Remove the operator from the list of definitions...
45 Op->getParent()->getInstList().remove(II);
47 // The new constant inherits the old name of the operator...
49 ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
51 // Delete the operator now...
57 ConstantFoldCast(BasicBlock *BB, BasicBlock::iterator &II,
58 CastInst *CI, Constant *D) {
59 Constant *ReplaceWith = ConstantFoldCastInstruction(D, CI->getType());
61 if (!ReplaceWith) return false; // Nothing new to change...
63 // Replaces all of the uses of a variable with uses of the constant.
64 CI->replaceAllUsesWith(ReplaceWith);
66 // Remove the cast from the list of definitions...
67 CI->getParent()->getInstList().remove(II);
69 // The new constant inherits the old name of the cast...
71 ReplaceWith->setName(CI->getName(), BB->getParent()->getSymbolTableSure());
73 // Delete the cast now...
79 ConstantFoldBinaryInst(BasicBlock *BB, BasicBlock::iterator &II,
81 Constant *D1, Constant *D2) {
82 Constant *ReplaceWith = ConstantFoldBinaryInstruction(Op->getOpcode(), D1,D2);
83 if (!ReplaceWith) return false; // Nothing new to change...
85 // Replaces all of the uses of a variable with uses of the constant.
86 Op->replaceAllUsesWith(ReplaceWith);
88 // Remove the operator from the list of definitions...
89 Op->getParent()->getInstList().remove(II);
91 // The new constant inherits the old name of the operator...
93 ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
95 // Delete the operator now...
100 // ConstantFoldTerminator - If a terminator instruction is predicated on a
101 // constant value, convert it into an unconditional branch to the constant
104 bool ConstantFoldTerminator(BasicBlock *BB, BasicBlock::iterator &II,
106 // Branch - See if we are conditional jumping on constant
107 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
108 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
109 BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
110 BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
112 if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
113 // Are we branching on constant?
114 // YES. Change to unconditional branch...
115 BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
116 BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
118 //cerr << "Function: " << T->getParent()->getParent()
119 // << "\nRemoving branch from " << T->getParent()
120 // << "\n\nTo: " << OldDest << endl;
122 // Let the basic block know that we are letting go of it. Based on this,
123 // it will adjust it's PHI nodes.
124 assert(BI->getParent() && "Terminator not inserted in block!");
125 OldDest->removePredecessor(BI->getParent());
127 // Set the unconditional destination, and change the insn to be an
128 // unconditional branch.
129 BI->setUnconditionalDest(Destination);
130 II = BB->end()-1; // Update instruction iterator!
134 // FIXME: TODO: This doesn't work if the destination has PHI nodes with
135 // different incoming values on each branch!
137 else if (Dest2 == Dest1) { // Conditional branch to same location?
138 // This branch matches something like this:
139 // br bool %cond, label %Dest, label %Dest
140 // and changes it into: br label %Dest
142 // Let the basic block know that we are letting go of one copy of it.
143 assert(BI->getParent() && "Terminator not inserted in block!");
144 Dest1->removePredecessor(BI->getParent());
146 // Change a conditional branch to unconditional.
147 BI->setUnconditionalDest(Dest1);
155 // ConstantFoldInstruction - If an instruction references constants, try to fold
158 bool doConstantPropogation(BasicBlock *BB, BasicBlock::iterator &II) {
159 Instruction *Inst = *II;
160 if (isa<BinaryOperator>(Inst)) {
161 Constant *D1 = dyn_cast<Constant>(Inst->getOperand(0));
162 Constant *D2 = dyn_cast<Constant>(Inst->getOperand(1));
165 return ConstantFoldBinaryInst(BB, II, cast<BinaryOperator>(Inst), D1, D2);
167 } else if (CastInst *CI = dyn_cast<CastInst>(Inst)) {
168 Constant *D = dyn_cast<Constant>(CI->getOperand(0));
169 if (D) return ConstantFoldCast(BB, II, CI, D);
171 } else if (UnaryOperator *UInst = dyn_cast<UnaryOperator>(Inst)) {
172 Constant *D = dyn_cast<Constant>(UInst->getOperand(0));
173 if (D) return ConstantFoldUnaryInst(BB, II, UInst, D);
174 } else if (TerminatorInst *TInst = dyn_cast<TerminatorInst>(Inst)) {
175 return ConstantFoldTerminator(BB, II, TInst);
177 } else if (PHINode *PN = dyn_cast<PHINode>(Inst)) {
178 // If it's a PHI node and only has one operand
179 // Then replace it directly with that operand.
180 assert(PN->getNumOperands() && "PHI Node must have at least one operand!");
181 if (PN->getNumOperands() == 1) { // If the PHI Node has exactly 1 operand
182 Value *V = PN->getOperand(0);
183 PN->replaceAllUsesWith(V); // Replace all uses of this PHI
184 // Unlink from basic block
185 PN->getParent()->getInstList().remove(II);
186 if (PN->hasName()) // Inherit PHINode name
187 V->setName(PN->getName(), BB->getParent()->getSymbolTableSure());
188 delete PN; // Finally, delete the node...
195 // DoConstPropPass - Propogate constants and do constant folding on instructions
196 // this returns true if something was changed, false if nothing was changed.
198 static bool DoConstPropPass(Function *F) {
199 bool SomethingChanged = false;
201 for (Function::iterator BBI = F->begin(); BBI != F->end(); ++BBI) {
202 BasicBlock *BB = *BBI;
203 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); )
204 if (doConstantPropogation(BB, I))
205 SomethingChanged = true;
209 return SomethingChanged;
213 struct ConstantPropogation : public FunctionPass {
214 const char *getPassName() const { return "Simple Constant Propogation"; }
216 inline bool runOnFunction(Function *F) {
217 bool Modified = false;
219 // Fold constants until we make no progress...
220 while (DoConstPropPass(F)) Modified = true;
225 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
226 // FIXME: This pass does not preserve the CFG because it folds terminator
233 Pass *createConstantPropogationPass() {
234 return new ConstantPropogation();