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237 lines
8.5 KiB
C++
237 lines
8.5 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/Transforms/Scalar/ConstantProp.h"
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#include "llvm/ConstantHandling.h"
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#include "llvm/Module.h"
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#include "llvm/Function.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iPHINode.h"
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#include "llvm/iOther.h"
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#include "llvm/Pass.h"
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inline static bool
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ConstantFoldUnaryInst(BasicBlock *BB, BasicBlock::iterator &II,
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UnaryOperator *Op, Constant *D) {
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Constant *ReplaceWith = ConstantFoldUnaryInstruction(Op->getOpcode(), D);
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if (!ReplaceWith) return false; // Nothing new to change...
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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(II);
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// The new constant inherits the old name of the operator...
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if (Op->hasName())
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ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
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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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ConstantFoldCast(BasicBlock *BB, BasicBlock::iterator &II,
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CastInst *CI, Constant *D) {
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Constant *ReplaceWith = ConstantFoldCastInstruction(D, CI->getType());
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if (!ReplaceWith) return false; // Nothing new to change...
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// Replaces all of the uses of a variable with uses of the constant.
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CI->replaceAllUsesWith(ReplaceWith);
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// Remove the cast from the list of definitions...
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CI->getParent()->getInstList().remove(II);
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// The new constant inherits the old name of the cast...
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if (CI->hasName())
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ReplaceWith->setName(CI->getName(), BB->getParent()->getSymbolTableSure());
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// Delete the cast now...
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delete CI;
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return true;
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}
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inline static bool
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ConstantFoldBinaryInst(BasicBlock *BB, BasicBlock::iterator &II,
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BinaryOperator *Op,
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Constant *D1, Constant *D2) {
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Constant *ReplaceWith = ConstantFoldBinaryInstruction(Op->getOpcode(), D1,D2);
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if (!ReplaceWith) return false; // Nothing new to change...
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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(II);
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// The new constant inherits the old name of the operator...
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if (Op->hasName())
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ReplaceWith->setName(Op->getName(), BB->getParent()->getSymbolTableSure());
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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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// ConstantFoldTerminator - If a terminator instruction is predicated on a
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// constant value, convert it into an unconditional branch to the constant
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// destination.
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//
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bool ConstantFoldTerminator(BasicBlock *BB, BasicBlock::iterator &II,
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TerminatorInst *T) {
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// Branch - See if we are conditional jumping on constant
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if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
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if (BI->isUnconditional()) return false; // Can't optimize uncond branch
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BasicBlock *Dest1 = cast<BasicBlock>(BI->getOperand(0));
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BasicBlock *Dest2 = cast<BasicBlock>(BI->getOperand(1));
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if (ConstantBool *Cond = dyn_cast<ConstantBool>(BI->getCondition())) {
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// Are we branching on constant?
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// YES. Change to unconditional branch...
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BasicBlock *Destination = Cond->getValue() ? Dest1 : Dest2;
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BasicBlock *OldDest = Cond->getValue() ? Dest2 : Dest1;
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//cerr << "Function: " << T->getParent()->getParent()
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// << "\nRemoving branch from " << T->getParent()
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// << "\n\nTo: " << OldDest << endl;
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// Let the basic block know that we are letting go of it. Based on this,
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// it will adjust it's PHI nodes.
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assert(BI->getParent() && "Terminator not inserted in block!");
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OldDest->removePredecessor(BI->getParent());
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// Set the unconditional destination, and change the insn to be an
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// unconditional branch.
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BI->setUnconditionalDest(Destination);
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II = BB->end()-1; // Update instruction iterator!
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return true;
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}
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#if 0
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// FIXME: TODO: This doesn't work if the destination has PHI nodes with
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// different incoming values on each branch!
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//
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else if (Dest2 == Dest1) { // Conditional branch to same location?
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// This branch matches something like this:
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// br bool %cond, label %Dest, label %Dest
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// and changes it into: br label %Dest
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// Let the basic block know that we are letting go of one copy of it.
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assert(BI->getParent() && "Terminator not inserted in block!");
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Dest1->removePredecessor(BI->getParent());
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// Change a conditional branch to unconditional.
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BI->setUnconditionalDest(Dest1);
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return true;
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}
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#endif
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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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bool doConstantPropogation(BasicBlock *BB, BasicBlock::iterator &II) {
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Instruction *Inst = *II;
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if (isa<BinaryOperator>(Inst)) {
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Constant *D1 = dyn_cast<Constant>(Inst->getOperand(0));
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Constant *D2 = dyn_cast<Constant>(Inst->getOperand(1));
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if (D1 && D2)
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return ConstantFoldBinaryInst(BB, II, cast<BinaryOperator>(Inst), D1, D2);
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} else if (CastInst *CI = dyn_cast<CastInst>(Inst)) {
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Constant *D = dyn_cast<Constant>(CI->getOperand(0));
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if (D) return ConstantFoldCast(BB, II, CI, D);
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} else if (UnaryOperator *UInst = dyn_cast<UnaryOperator>(Inst)) {
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Constant *D = dyn_cast<Constant>(UInst->getOperand(0));
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if (D) return ConstantFoldUnaryInst(BB, II, UInst, D);
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} else if (TerminatorInst *TInst = dyn_cast<TerminatorInst>(Inst)) {
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return ConstantFoldTerminator(BB, II, TInst);
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} else if (PHINode *PN = dyn_cast<PHINode>(Inst)) {
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// 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->getNumOperands() && "PHI Node must have at least one operand!");
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if (PN->getNumOperands() == 1) { // 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);
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if (PN->hasName()) // Inherit PHINode name
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V->setName(PN->getName(), BB->getParent()->getSymbolTableSure());
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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(Function *F) {
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bool SomethingChanged = false;
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for (Function::iterator BBI = F->begin(); BBI != F->end(); ++BBI) {
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BasicBlock *BB = *BBI;
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for (BasicBlock::iterator I = BB->begin(); I != BB->end(); )
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if (doConstantPropogation(BB, I))
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SomethingChanged = true;
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else
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++I;
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}
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return SomethingChanged;
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}
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namespace {
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struct ConstantPropogation : public FunctionPass {
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const char *getPassName() const { return "Simple Constant Propogation"; }
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inline bool runOnFunction(Function *F) {
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bool Modified = false;
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// Fold constants until we make no progress...
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while (DoConstPropPass(F)) Modified = true;
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return Modified;
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}
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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// FIXME: This pass does not preserve the CFG because it folds terminator
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// instructions!
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//AU.preservesCFG();
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}
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};
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}
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Pass *createConstantPropogationPass() {
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return new ConstantPropogation();
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}
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