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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@8093 91177308-0d34-0410-b5e6-96231b3b80d8
176 lines
6.4 KiB
C++
176 lines
6.4 KiB
C++
//===-- Local.cpp - Functions to perform local transformations ------------===//
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//
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// This family of functions perform various local transformations to the
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// program.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Utils/Local.h"
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#include "llvm/iTerminators.h"
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#include "llvm/iOperators.h"
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#include "llvm/ConstantHandling.h"
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//===----------------------------------------------------------------------===//
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// Local constant propagation...
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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 doConstantPropagation(BasicBlock::iterator &II) {
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if (Constant *C = ConstantFoldInstruction(II)) {
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// Replaces all of the uses of a variable with uses of the constant.
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II->replaceAllUsesWith(C);
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// Remove the instruction from the basic block...
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II = II->getParent()->getInstList().erase(II);
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return true;
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}
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return false;
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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) {
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TerminatorInst *T = BB->getTerminator();
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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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return true;
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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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} else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
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// If we are switching on a constant, we can convert the switch into a
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// single branch instruction!
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ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
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BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
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BasicBlock *DefaultDest = TheOnlyDest;
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assert(TheOnlyDest == SI->getDefaultDest() &&
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"Default destination is not successor #0?");
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// Figure out which case it goes to...
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for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
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// Found case matching a constant operand?
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if (SI->getSuccessorValue(i) == CI) {
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TheOnlyDest = SI->getSuccessor(i);
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break;
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}
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// Check to see if this branch is going to the same place as the default
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// dest. If so, eliminate it as an explicit compare.
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if (SI->getSuccessor(i) == DefaultDest) {
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// Remove this entry...
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DefaultDest->removePredecessor(SI->getParent());
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SI->removeCase(i);
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--i; --e; // Don't skip an entry...
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continue;
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}
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// Otherwise, check to see if the switch only branches to one destination.
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// We do this by reseting "TheOnlyDest" to null when we find two non-equal
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// destinations.
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if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
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}
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if (CI && !TheOnlyDest) {
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// Branching on a constant, but not any of the cases, go to the default
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// successor.
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TheOnlyDest = SI->getDefaultDest();
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}
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// If we found a single destination that we can fold the switch into, do so
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// now.
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if (TheOnlyDest) {
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// Insert the new branch..
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new BranchInst(TheOnlyDest, SI);
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BasicBlock *BB = SI->getParent();
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// Remove entries from PHI nodes which we no longer branch to...
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for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
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// Found case matching a constant operand?
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BasicBlock *Succ = SI->getSuccessor(i);
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if (Succ == TheOnlyDest)
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TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
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else
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Succ->removePredecessor(BB);
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}
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// Delete the old switch...
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BB->getInstList().erase(SI);
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return true;
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} else if (SI->getNumSuccessors() == 2) {
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// Otherwise, we can fold this switch into a conditional branch
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// instruction if it has only one non-default destination.
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Value *Cond = new SetCondInst(Instruction::SetEQ, SI->getCondition(),
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SI->getSuccessorValue(1), "cond", SI);
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// Insert the new branch...
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new BranchInst(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
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// Delete the old switch...
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SI->getParent()->getInstList().erase(SI);
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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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//===----------------------------------------------------------------------===//
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// Local dead code elimination...
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//
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bool isInstructionTriviallyDead(Instruction *I) {
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return I->use_empty() && !I->mayWriteToMemory() && !isa<TerminatorInst>(I);
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}
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// dceInstruction - Inspect the instruction at *BBI and figure out if it's
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// [trivially] dead. If so, remove the instruction and update the iterator
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// to point to the instruction that immediately succeeded the original
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// instruction.
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//
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bool dceInstruction(BasicBlock::iterator &BBI) {
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// Look for un"used" definitions...
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if (isInstructionTriviallyDead(BBI)) {
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BBI = BBI->getParent()->getInstList().erase(BBI); // Bye bye
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return true;
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}
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return false;
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}
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