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/iOther.h"
31 #include "llvm/ConstPoolVals.h"
32 #include "llvm/ConstantPool.h"
34 // Merge identical constant values in the constant pool.
36 // TODO: We can do better than this simplistic N^2 algorithm...
38 bool opt::DoConstantPoolMerging(Method *M) {
39 return DoConstantPoolMerging(M->getConstantPool());
42 bool opt::DoConstantPoolMerging(ConstantPool &CP) {
43 bool Modified = false;
44 for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI) {
45 for (ConstantPool::PlaneType::iterator I = (*PI)->begin();
46 I != (*PI)->end(); ++I) {
49 ConstantPool::PlaneType::iterator J = I;
50 for (++J; J != (*PI)->end(); ++J) {
53 // Okay we know that *I == *J. So now we need to make all uses of *I
56 C->replaceAllUsesWith(*J);
58 (*PI)->remove(I); // Remove C from constant pool...
60 if (C->hasName() && !(*J)->hasName()) // The merged constant inherits
61 (*J)->setName(C->getName()); // the old name...
63 delete C; // Delete the constant itself.
64 break; // Break out of inner for loop
73 ConstantFoldUnaryInst(Method *M, Method::inst_iterator &DI,
74 UnaryOperator *Op, ConstPoolVal *D) {
75 ConstPoolVal *ReplaceWith =
76 opt::ConstantFoldUnaryInstruction(Op->getOpcode(), D);
78 if (!ReplaceWith) return false; // Nothing new to change...
81 // Add the new value to the constant pool...
82 M->getConstantPool().insert(ReplaceWith);
84 // Replaces all of the uses of a variable with uses of the constant.
85 Op->replaceAllUsesWith(ReplaceWith);
87 // Remove the operator from the list of definitions...
88 Op->getParent()->getInstList().remove(DI.getInstructionIterator());
90 // The new constant inherits the old name of the operator...
91 if (Op->hasName()) ReplaceWith->setName(Op->getName());
93 // Delete the operator now...
99 ConstantFoldBinaryInst(Method *M, Method::inst_iterator &DI,
101 ConstPoolVal *D1, ConstPoolVal *D2) {
102 ConstPoolVal *ReplaceWith =
103 opt::ConstantFoldBinaryInstruction(Op->getOpcode(), D1, D2);
104 if (!ReplaceWith) return false; // Nothing new to change...
106 // Add the new value to the constant pool...
107 M->getConstantPool().insert(ReplaceWith);
109 // Replaces all of the uses of a variable with uses of the constant.
110 Op->replaceAllUsesWith(ReplaceWith);
112 // Remove the operator from the list of definitions...
113 Op->getParent()->getInstList().remove(DI.getInstructionIterator());
115 // The new constant inherits the old name of the operator...
116 if (Op->hasName()) ReplaceWith->setName(Op->getName());
118 // Delete the operator now...
123 // ConstantFoldTerminator - If a terminator instruction is predicated on a
124 // constant value, convert it into an unconditional branch to the constant
127 bool opt::ConstantFoldTerminator(TerminatorInst *T) {
128 // Branch - See if we are conditional jumping on constant
129 if (T->getOpcode() == Instruction::Br) {
130 BranchInst *BI = (BranchInst*)T;
131 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
132 BasicBlock *Dest1 = BI->getOperand(0)->castBasicBlockAsserting();
133 BasicBlock *Dest2 = BI->getOperand(1)->castBasicBlockAsserting();
135 if (BI->getCondition()->isConstant()) { // Are we branching on constant?
136 // YES. Change to unconditional branch...
137 ConstPoolBool *Cond = (ConstPoolBool*)BI->getCondition();
138 BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
139 BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
141 //cerr << "Method: " << T->getParent()->getParent()
142 // << "\nRemoving branch from " << T->getParent()
143 // << "\n\nTo: " << OldDest << endl;
145 // Let the basic block know that we are letting go of it. Based on this,
146 // it will adjust it's PHI nodes.
147 assert(BI->getParent() && "Terminator not inserted in block!");
148 OldDest->removePredecessor(BI->getParent());
150 // Set the unconditional destination, and change the insn to be an
151 // unconditional branch.
152 BI->setUnconditionalDest(Destination);
154 } else if (Dest2 == Dest1) { // Conditional branch to same location?
155 // This branch matches something like this:
156 // br bool %cond, label %Dest, label %Dest
157 // and changes it into: br label %Dest
159 // Let the basic block know that we are letting go of one copy of it.
160 assert(BI->getParent() && "Terminator not inserted in block!");
161 Dest1->removePredecessor(BI->getParent());
163 // Change a conditional branch to unconditional.
164 BI->setUnconditionalDest(Dest1);
171 // ConstantFoldInstruction - If an instruction references constants, try to fold
175 ConstantFoldInstruction(Method *M, Method::inst_iterator &II) {
176 Instruction *Inst = *II;
177 if (Inst->isBinaryOp()) {
178 ConstPoolVal *D1 = Inst->getOperand(0)->castConstant();
179 ConstPoolVal *D2 = Inst->getOperand(1)->castConstant();
182 return ConstantFoldBinaryInst(M, II, (BinaryOperator*)Inst, D1, D2);
184 } else if (Inst->isUnaryOp()) {
185 ConstPoolVal *D = Inst->getOperand(0)->castConstant();
186 if (D) return ConstantFoldUnaryInst(M, II, (UnaryOperator*)Inst, D);
187 } else if (Inst->isTerminator()) {
188 return opt::ConstantFoldTerminator((TerminatorInst*)Inst);
190 } else if (Inst->isPHINode()) {
191 PHINode *PN = (PHINode*)Inst; // If it's a PHI node and only has one operand
192 // Then replace it directly with that operand.
193 assert(PN->getOperand(0) && "PHI Node must have at least one operand!");
194 if (PN->getNumOperands() == 1) { // If the PHI Node has exactly 1 operand
195 Value *V = PN->getOperand(0);
196 PN->replaceAllUsesWith(V); // Replace all uses of this PHI
197 // Unlink from basic block
198 PN->getParent()->getInstList().remove(II.getInstructionIterator());
199 if (PN->hasName()) V->setName(PN->getName()); // Inherit PHINode name
200 delete PN; // Finally, delete the node...
207 // DoConstPropPass - Propogate constants and do constant folding on instructions
208 // this returns true if something was changed, false if nothing was changed.
210 static bool DoConstPropPass(Method *M) {
211 bool SomethingChanged = false;
214 Method::inst_iterator It = M->inst_begin();
215 while (It != M->inst_end())
216 if (ConstantFoldInstruction(M, It)) {
217 SomethingChanged = true; // If returned true, iter is already incremented
219 // Incrementing the iterator in an unchecked manner could mess up the
220 // internals of 'It'. To make sure everything is happy, tell it we might
222 It.resyncInstructionIterator();
227 for (Method::iterator BBIt = M->begin(); BBIt != M->end(); ++BBIt) {
228 BasicBlock *BB = *BBIt;
230 reduce_apply_bool(BB->begin(), BB->end(),
231 bind1st(ConstantFoldInstruction, M));
234 return SomethingChanged;
238 // returns true on failure, false on success...
240 bool opt::DoConstantPropogation(Method *M) {
241 bool Modified = false;
243 // Fold constants until we make no progress...
244 while (DoConstPropPass(M)) Modified = true;
246 // Merge identical constants last: this is important because we may have just
247 // introduced constants that already exist!
249 Modified |= DoConstantPoolMerging(M->getConstantPool());