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343 lines
13 KiB
C++
343 lines
13 KiB
C++
//===-- Local.cpp - Functions to perform local transformations ------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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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/Constants.h"
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#include "llvm/GlobalAlias.h"
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#include "llvm/GlobalVariable.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Instructions.h"
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#include "llvm/Intrinsics.h"
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#include "llvm/IntrinsicInst.h"
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#include "llvm/LLVMContext.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/Analysis/ConstantFolding.h"
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#include "llvm/Analysis/DebugInfo.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Support/GetElementPtrTypeIterator.h"
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#include "llvm/Support/MathExtras.h"
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using namespace llvm;
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//===----------------------------------------------------------------------===//
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// Local analysis.
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//
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/// isSafeToLoadUnconditionally - Return true if we know that executing a load
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/// from this value cannot trap. If it is not obviously safe to load from the
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/// specified pointer, we do a quick local scan of the basic block containing
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/// ScanFrom, to determine if the address is already accessed.
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bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) {
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// If it is an alloca it is always safe to load from.
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if (isa<AllocaInst>(V)) return true;
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// If it is a global variable it is mostly safe to load from.
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if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V))
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// Don't try to evaluate aliases. External weak GV can be null.
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return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage();
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// Otherwise, be a little bit agressive by scanning the local block where we
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// want to check to see if the pointer is already being loaded or stored
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// from/to. If so, the previous load or store would have already trapped,
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// so there is no harm doing an extra load (also, CSE will later eliminate
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// the load entirely).
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BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
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while (BBI != E) {
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--BBI;
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// If we see a free or a call which may write to memory (i.e. which might do
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// a free) the pointer could be marked invalid.
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if (isa<FreeInst>(BBI) ||
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(isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
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!isa<DbgInfoIntrinsic>(BBI)))
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return false;
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if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
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if (LI->getOperand(0) == V) return true;
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} else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
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if (SI->getOperand(1) == V) 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 constant propagation.
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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 llvm::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 = BI->getSuccessor(0);
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BasicBlock *Dest2 = BI->getSuccessor(1);
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if (ConstantInt *Cond = dyn_cast<ConstantInt>(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->getZExtValue() ? Dest1 : Dest2;
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BasicBlock *OldDest = Cond->getZExtValue() ? 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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BranchInst::Create(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 ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(),
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SI->getSuccessorValue(1), "cond");
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// Insert the new branch...
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BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
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// Delete the old switch...
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SI->eraseFromParent();
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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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/// isInstructionTriviallyDead - Return true if the result produced by the
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/// instruction is not used, and the instruction has no side effects.
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///
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bool llvm::isInstructionTriviallyDead(Instruction *I) {
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if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
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// We don't want debug info removed by anything this general.
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if (isa<DbgInfoIntrinsic>(I)) return false;
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if (!I->mayHaveSideEffects()) return true;
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// Special case intrinsics that "may have side effects" but can be deleted
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// when dead.
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if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
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// Safe to delete llvm.stacksave if dead.
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if (II->getIntrinsicID() == Intrinsic::stacksave)
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return true;
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return false;
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}
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/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
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/// trivially dead instruction, delete it. If that makes any of its operands
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/// trivially dead, delete them too, recursively.
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void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) {
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Instruction *I = dyn_cast<Instruction>(V);
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if (!I || !I->use_empty() || !isInstructionTriviallyDead(I))
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return;
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SmallVector<Instruction*, 16> DeadInsts;
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DeadInsts.push_back(I);
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while (!DeadInsts.empty()) {
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I = DeadInsts.pop_back_val();
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// Null out all of the instruction's operands to see if any operand becomes
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// dead as we go.
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for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
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Value *OpV = I->getOperand(i);
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I->setOperand(i, 0);
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if (!OpV->use_empty()) continue;
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// If the operand is an instruction that became dead as we nulled out the
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// operand, and if it is 'trivially' dead, delete it in a future loop
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// iteration.
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if (Instruction *OpI = dyn_cast<Instruction>(OpV))
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if (isInstructionTriviallyDead(OpI))
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DeadInsts.push_back(OpI);
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}
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I->eraseFromParent();
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}
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}
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/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
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/// dead PHI node, due to being a def-use chain of single-use nodes that
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/// either forms a cycle or is terminated by a trivially dead instruction,
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/// delete it. If that makes any of its operands trivially dead, delete them
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/// too, recursively.
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void
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llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) {
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// We can remove a PHI if it is on a cycle in the def-use graph
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// where each node in the cycle has degree one, i.e. only one use,
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// and is an instruction with no side effects.
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if (!PN->hasOneUse())
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return;
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SmallPtrSet<PHINode *, 4> PHIs;
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PHIs.insert(PN);
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for (Instruction *J = cast<Instruction>(*PN->use_begin());
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J->hasOneUse() && !J->mayHaveSideEffects();
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J = cast<Instruction>(*J->use_begin()))
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// If we find a PHI more than once, we're on a cycle that
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// won't prove fruitful.
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if (PHINode *JP = dyn_cast<PHINode>(J))
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if (!PHIs.insert(cast<PHINode>(JP))) {
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// Break the cycle and delete the PHI and its operands.
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JP->replaceAllUsesWith(UndefValue::get(JP->getType()));
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RecursivelyDeleteTriviallyDeadInstructions(JP);
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break;
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}
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}
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//===----------------------------------------------------------------------===//
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// Control Flow Graph Restructuring...
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//
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/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
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/// predecessor is known to have one successor (DestBB!). Eliminate the edge
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/// between them, moving the instructions in the predecessor into DestBB and
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/// deleting the predecessor block.
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///
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void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) {
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// If BB has single-entry PHI nodes, fold them.
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while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
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Value *NewVal = PN->getIncomingValue(0);
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// Replace self referencing PHI with undef, it must be dead.
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if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
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PN->replaceAllUsesWith(NewVal);
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PN->eraseFromParent();
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}
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BasicBlock *PredBB = DestBB->getSinglePredecessor();
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assert(PredBB && "Block doesn't have a single predecessor!");
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// Splice all the instructions from PredBB to DestBB.
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PredBB->getTerminator()->eraseFromParent();
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DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
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// Anything that branched to PredBB now branches to DestBB.
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PredBB->replaceAllUsesWith(DestBB);
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// Nuke BB.
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PredBB->eraseFromParent();
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}
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/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used
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/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled
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/// with the DbgInfoIntrinsic that use the instruction I.
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bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I,
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SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) {
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if (DbgInUses)
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DbgInUses->clear();
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for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE;
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++UI) {
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if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) {
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if (DbgInUses)
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DbgInUses->push_back(DI);
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} else {
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if (DbgInUses)
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DbgInUses->clear();
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return false;
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}
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}
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return true;
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}
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