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https://github.com/c64scene-ar/llvm-6502.git
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009505452b
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2 91177308-0d34-0410-b5e6-96231b3b80d8
240 lines
8.4 KiB
C++
240 lines
8.4 KiB
C++
//===- ConstantProp.cpp - Code to perform Constant Propogation ------------===//
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//
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// This file implements constant propogation and merging:
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//
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// Specifically, this:
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// * Folds multiple identical constants in the constant pool together
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// Note that if one is named and the other is not, that the result gets the
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// original name.
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// * Converts instructions like "add int %1, %2" into a direct def of %3 in
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// the constant pool
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// * Converts conditional branches on a constant boolean value into direct
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// branches.
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// * Converts phi nodes with one incoming def to the incoming def directly
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// . Converts switch statements with one entry into a test & conditional
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// branch
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// . Converts switches on constant values into an unconditional branch.
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//
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// Notice that:
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// * This pass has a habit of making definitions be dead. It is a good idea
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// to to run a DCE pass sometime after running this pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Module.h"
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#include "llvm/Method.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iOther.h"
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#include "llvm/ConstPoolVals.h"
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#include "llvm/ConstantPool.h"
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#include "llvm/Opt/AllOpts.h"
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#include "llvm/Opt/ConstantHandling.h"
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// Merge identical constant values in the constant pool.
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//
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// TODO: We can do better than this simplistic N^2 algorithm...
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//
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static bool MergeConstantPoolReferences(ConstantPool &CP) {
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bool Modified = false;
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for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI) {
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for (ConstantPool::PlaneType::iterator I = (*PI)->begin();
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I != (*PI)->end(); I++) {
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ConstPoolVal *C = *I;
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ConstantPool::PlaneType::iterator J = I;
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for (++J; J != (*PI)->end(); J++) {
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if (C->equals(*J)) {
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Modified = true;
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// Okay we know that *I == *J. So now we need to make all uses of *I
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// point to *J.
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//
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C->replaceAllUsesWith(*J);
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(*PI)->remove(I); // Remove C from constant pool...
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if (C->hasName() && !(*J)->hasName()) // The merged constant inherits
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(*J)->setName(C->getName()); // the old name...
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delete C; // Delete the constant itself.
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break; // Break out of inner for loop
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}
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}
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}
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}
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return Modified;
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}
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inline static bool
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ConstantFoldUnaryInst(Method *M, Method::inst_iterator &DI,
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UnaryOperator *Op, ConstPoolVal *D) {
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ConstPoolVal *ReplaceWith = 0;
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switch (Op->getInstType()) {
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case Instruction::Not: ReplaceWith = !*D; break;
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case Instruction::Neg: ReplaceWith = -*D; break;
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}
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if (!ReplaceWith) return false; // Nothing new to change...
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// Add the new value to the constant pool...
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M->getConstantPool().insert(ReplaceWith);
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// Replaces all of the uses of a variable with uses of the constant.
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Op->replaceAllUsesWith(ReplaceWith);
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// Remove the operator from the list of definitions...
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Op->getParent()->getInstList().remove(DI.getInstructionIterator());
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// The new constant inherits the old name of the operator...
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if (Op->hasName()) ReplaceWith->setName(Op->getName());
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// Delete the operator now...
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delete Op;
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return true;
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}
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inline static bool
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ConstantFoldBinaryInst(Method *M, Method::inst_iterator &DI,
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BinaryOperator *Op,
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ConstPoolVal *D1, ConstPoolVal *D2) {
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ConstPoolVal *ReplaceWith = 0;
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switch (Op->getInstType()) {
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case Instruction::Add: ReplaceWith = *D1 + *D2; break;
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case Instruction::Sub: ReplaceWith = *D1 - *D2; break;
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case Instruction::SetEQ: ReplaceWith = *D1 == *D2; break;
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case Instruction::SetNE: ReplaceWith = *D1 != *D2; break;
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case Instruction::SetLE: ReplaceWith = *D1 <= *D2; break;
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case Instruction::SetGE: ReplaceWith = *D1 >= *D2; break;
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case Instruction::SetLT: ReplaceWith = *D1 < *D2; break;
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case Instruction::SetGT: ReplaceWith = *D1 > *D2; break;
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}
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if (!ReplaceWith) return false; // Nothing new to change...
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// Add the new value to the constant pool...
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M->getConstantPool().insert(ReplaceWith);
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// Replaces all of the uses of a variable with uses of the constant.
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Op->replaceAllUsesWith(ReplaceWith);
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// Remove the operator from the list of definitions...
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Op->getParent()->getInstList().remove(DI.getInstructionIterator());
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// The new constant inherits the old name of the operator...
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if (Op->hasName()) ReplaceWith->setName(Op->getName());
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// Delete the operator now...
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delete Op;
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return true;
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}
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inline static bool ConstantFoldTerminator(TerminatorInst *T) {
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// Branch - See if we are conditional jumping on constant
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if (T->getInstType() == Instruction::Br) {
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BranchInst *BI = (BranchInst*)T;
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if (!BI->isUnconditional() && // Are we branching on constant?
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BI->getOperand(2)->getValueType() == Value::ConstantVal) {
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// YES. Change to unconditional branch...
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ConstPoolBool *Cond = (ConstPoolBool*)BI->getOperand(2);
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Value *Destination = BI->getOperand(Cond->getValue() ? 0 : 1);
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BI->setOperand(0, Destination); // Set the unconditional destination
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BI->setOperand(1, 0); // Clear the conditional destination
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BI->setOperand(2, 0); // Clear the condition...
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return true;
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}
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}
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return false;
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}
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// ConstantFoldInstruction - If an instruction references constants, try to fold
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// them together...
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//
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inline static bool
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ConstantFoldInstruction(Method *M, Method::inst_iterator &II) {
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Instruction *Inst = *II;
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if (Inst->isBinaryOp()) {
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Value *D1, *D2;
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if (((D1 = Inst->getOperand(0))->getValueType() == Value::ConstantVal) &
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((D2 = Inst->getOperand(1))->getValueType() == Value::ConstantVal))
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return ConstantFoldBinaryInst(M, II, (BinaryOperator*)Inst,
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(ConstPoolVal*)D1, (ConstPoolVal*)D2);
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} else if (Inst->isUnaryOp()) {
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Value *D;
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if ((D = Inst->getOperand(0))->getValueType() == Value::ConstantVal)
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return ConstantFoldUnaryInst(M, II, (UnaryOperator*)Inst,
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(ConstPoolVal*)D);
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} else if (Inst->isTerminator()) {
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return ConstantFoldTerminator((TerminatorInst*)Inst);
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} else if (Inst->getInstType() == Instruction::PHINode) {
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PHINode *PN = (PHINode*)Inst; // If it's a PHI node and only has one operand
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// Then replace it directly with that operand.
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assert(PN->getOperand(0) && "PHI Node must have at least one operand!");
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if (PN->getOperand(1) == 0) { // If the PHI Node has exactly 1 operand
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Value *V = PN->getOperand(0);
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PN->replaceAllUsesWith(V); // Replace all uses of this PHI
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// Unlink from basic block
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PN->getParent()->getInstList().remove(II.getInstructionIterator());
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if (PN->hasName()) V->setName(PN->getName()); // Inherit PHINode name
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delete PN; // Finally, delete the node...
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return true;
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}
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}
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return false;
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}
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// DoConstPropPass - Propogate constants and do constant folding on instructions
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// this returns true if something was changed, false if nothing was changed.
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//
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static bool DoConstPropPass(Method *M) {
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bool SomethingChanged = false;
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#if 1
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Method::inst_iterator It = M->inst_begin();
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while (It != M->inst_end())
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if (ConstantFoldInstruction(M, It)) {
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SomethingChanged = true; // If returned true, iter is already incremented
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// Incrementing the iterator in an unchecked manner could mess up the
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// internals of 'It'. To make sure everything is happy, tell it we might
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// have broken it.
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It.resyncInstructionIterator();
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} else {
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++It;
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}
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#else
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Method::BasicBlocksType::iterator BBIt = M->getBasicBlocks().begin();
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for (; BBIt != M->getBasicBlocks().end(); BBIt++) {
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BasicBlock *BB = *BBIt;
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BasicBlock::InstListType::iterator DI = BB->getInstList().begin();
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for (; DI != BB->getInstList().end(); DI++)
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SomethingChanged |= ConstantFoldInstruction(M, DI);
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}
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#endif
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return SomethingChanged;
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}
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// returns true on failure, false on success...
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//
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bool DoConstantPropogation(Method *M) {
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bool Modified = false;
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// Fold constants until we make no progress...
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while (DoConstPropPass(M)) Modified = true;
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// Merge identical constants last: this is important because we may have just
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// introduced constants that already exist!
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//
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Modified |= MergeConstantPoolReferences(M->getConstantPool());
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return Modified;
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}
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