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llvm-project/llvm/lib/Transforms/Utils/Local.cpp

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//===-- Local.cpp - Functions to perform local transformations ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This family of functions perform various local transformations to the
// program.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/GlobalAlias.h"
#include "llvm/GlobalVariable.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Intrinsics.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/DebugInfo.h"
#include "llvm/Analysis/ProfileInfo.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/MathExtras.h"
using namespace llvm;
//===----------------------------------------------------------------------===//
// Local analysis.
//
/// isSafeToLoadUnconditionally - Return true if we know that executing a load
/// from this value cannot trap. If it is not obviously safe to load from the
/// specified pointer, we do a quick local scan of the basic block containing
/// ScanFrom, to determine if the address is already accessed.
bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) {
// If it is an alloca it is always safe to load from.
if (isa<AllocaInst>(V)) return true;
// If it is a global variable it is mostly safe to load from.
if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V))
// Don't try to evaluate aliases. External weak GV can be null.
return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage();
// Otherwise, be a little bit agressive by scanning the local block where we
// want to check to see if the pointer is already being loaded or stored
// from/to. If so, the previous load or store would have already trapped,
// so there is no harm doing an extra load (also, CSE will later eliminate
// the load entirely).
BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
while (BBI != E) {
--BBI;
// If we see a free or a call which may write to memory (i.e. which might do
// a free) the pointer could be marked invalid.
if (isa<FreeInst>(BBI) ||
(isa<CallInst>(BBI) && BBI->mayWriteToMemory() &&
!isa<DbgInfoIntrinsic>(BBI)))
return false;
if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
if (LI->getOperand(0) == V) return true;
} else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
if (SI->getOperand(1) == V) return true;
}
}
return false;
}
//===----------------------------------------------------------------------===//
// Local constant propagation.
//
// ConstantFoldTerminator - If a terminator instruction is predicated on a
// constant value, convert it into an unconditional branch to the constant
// destination.
//
bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
TerminatorInst *T = BB->getTerminator();
// Branch - See if we are conditional jumping on constant
if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
if (BI->isUnconditional()) return false; // Can't optimize uncond branch
BasicBlock *Dest1 = BI->getSuccessor(0);
BasicBlock *Dest2 = BI->getSuccessor(1);
if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
// Are we branching on constant?
// YES. Change to unconditional branch...
BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1;
//cerr << "Function: " << T->getParent()->getParent()
// << "\nRemoving branch from " << T->getParent()
// << "\n\nTo: " << OldDest << endl;
// Let the basic block know that we are letting go of it. Based on this,
// it will adjust it's PHI nodes.
assert(BI->getParent() && "Terminator not inserted in block!");
OldDest->removePredecessor(BI->getParent());
// Set the unconditional destination, and change the insn to be an
// unconditional branch.
BI->setUnconditionalDest(Destination);
return true;
} else if (Dest2 == Dest1) { // Conditional branch to same location?
// This branch matches something like this:
// br bool %cond, label %Dest, label %Dest
// and changes it into: br label %Dest
// Let the basic block know that we are letting go of one copy of it.
assert(BI->getParent() && "Terminator not inserted in block!");
Dest1->removePredecessor(BI->getParent());
// Change a conditional branch to unconditional.
BI->setUnconditionalDest(Dest1);
return true;
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
// If we are switching on a constant, we can convert the switch into a
// single branch instruction!
ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest
BasicBlock *DefaultDest = TheOnlyDest;
assert(TheOnlyDest == SI->getDefaultDest() &&
"Default destination is not successor #0?");
// Figure out which case it goes to...
for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
// Found case matching a constant operand?
if (SI->getSuccessorValue(i) == CI) {
TheOnlyDest = SI->getSuccessor(i);
break;
}
// Check to see if this branch is going to the same place as the default
// dest. If so, eliminate it as an explicit compare.
if (SI->getSuccessor(i) == DefaultDest) {
// Remove this entry...
DefaultDest->removePredecessor(SI->getParent());
SI->removeCase(i);
--i; --e; // Don't skip an entry...
continue;
}
// Otherwise, check to see if the switch only branches to one destination.
// We do this by reseting "TheOnlyDest" to null when we find two non-equal
// destinations.
if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
}
if (CI && !TheOnlyDest) {
// Branching on a constant, but not any of the cases, go to the default
// successor.
TheOnlyDest = SI->getDefaultDest();
}
// If we found a single destination that we can fold the switch into, do so
// now.
if (TheOnlyDest) {
// Insert the new branch..
BranchInst::Create(TheOnlyDest, SI);
BasicBlock *BB = SI->getParent();
// Remove entries from PHI nodes which we no longer branch to...
for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
// Found case matching a constant operand?
BasicBlock *Succ = SI->getSuccessor(i);
if (Succ == TheOnlyDest)
TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
else
Succ->removePredecessor(BB);
}
// Delete the old switch...
BB->getInstList().erase(SI);
return true;
} else if (SI->getNumSuccessors() == 2) {
// Otherwise, we can fold this switch into a conditional branch
// instruction if it has only one non-default destination.
Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(),
SI->getSuccessorValue(1), "cond");
// Insert the new branch...
BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI);
// Delete the old switch...
SI->eraseFromParent();
return true;
}
}
return false;
}
//===----------------------------------------------------------------------===//
// Local dead code elimination...
//
/// isInstructionTriviallyDead - Return true if the result produced by the
/// instruction is not used, and the instruction has no side effects.
///
bool llvm::isInstructionTriviallyDead(Instruction *I) {
if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
// We don't want debug info removed by anything this general.
if (isa<DbgInfoIntrinsic>(I)) return false;
if (!I->mayHaveSideEffects()) return true;
// Special case intrinsics that "may have side effects" but can be deleted
// when dead.
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
// Safe to delete llvm.stacksave if dead.
if (II->getIntrinsicID() == Intrinsic::stacksave)
return true;
return false;
}
/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
/// trivially dead instruction, delete it. If that makes any of its operands
/// trivially dead, delete them too, recursively.
void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I || !I->use_empty() || !isInstructionTriviallyDead(I))
return;
SmallVector<Instruction*, 16> DeadInsts;
DeadInsts.push_back(I);
while (!DeadInsts.empty()) {
I = DeadInsts.pop_back_val();
// Null out all of the instruction's operands to see if any operand becomes
// dead as we go.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
Value *OpV = I->getOperand(i);
I->setOperand(i, 0);
if (!OpV->use_empty()) continue;
// If the operand is an instruction that became dead as we nulled out the
// operand, and if it is 'trivially' dead, delete it in a future loop
// iteration.
if (Instruction *OpI = dyn_cast<Instruction>(OpV))
if (isInstructionTriviallyDead(OpI))
DeadInsts.push_back(OpI);
}
I->eraseFromParent();
}
}
/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
/// dead PHI node, due to being a def-use chain of single-use nodes that
/// either forms a cycle or is terminated by a trivially dead instruction,
/// delete it. If that makes any of its operands trivially dead, delete them
/// too, recursively.
void
llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) {
// We can remove a PHI if it is on a cycle in the def-use graph
// where each node in the cycle has degree one, i.e. only one use,
// and is an instruction with no side effects.
if (!PN->hasOneUse())
return;
SmallPtrSet<PHINode *, 4> PHIs;
PHIs.insert(PN);
for (Instruction *J = cast<Instruction>(*PN->use_begin());
J->hasOneUse() && !J->mayHaveSideEffects();
J = cast<Instruction>(*J->use_begin()))
// If we find a PHI more than once, we're on a cycle that
// won't prove fruitful.
if (PHINode *JP = dyn_cast<PHINode>(J))
if (!PHIs.insert(cast<PHINode>(JP))) {
// Break the cycle and delete the PHI and its operands.
JP->replaceAllUsesWith(UndefValue::get(JP->getType()));
RecursivelyDeleteTriviallyDeadInstructions(JP);
break;
}
}
//===----------------------------------------------------------------------===//
// Control Flow Graph Restructuring...
//
/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
/// predecessor is known to have one successor (DestBB!). Eliminate the edge
/// between them, moving the instructions in the predecessor into DestBB and
/// deleting the predecessor block.
///
void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
// If BB has single-entry PHI nodes, fold them.
while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
Value *NewVal = PN->getIncomingValue(0);
// Replace self referencing PHI with undef, it must be dead.
if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
PN->replaceAllUsesWith(NewVal);
PN->eraseFromParent();
}
BasicBlock *PredBB = DestBB->getSinglePredecessor();
assert(PredBB && "Block doesn't have a single predecessor!");
// Splice all the instructions from PredBB to DestBB.
PredBB->getTerminator()->eraseFromParent();
DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
// Anything that branched to PredBB now branches to DestBB.
PredBB->replaceAllUsesWith(DestBB);
if (P) {
ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
if (PI) {
PI->replaceAllUses(PredBB, DestBB);
PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB));
}
}
// Nuke BB.
PredBB->eraseFromParent();
}
/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used
/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled
/// with the DbgInfoIntrinsic that use the instruction I.
bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I,
SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) {
if (DbgInUses)
DbgInUses->clear();
for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE;
++UI) {
if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) {
if (DbgInUses)
DbgInUses->push_back(DI);
} else {
if (DbgInUses)
DbgInUses->clear();
return false;
}
}
return true;
}
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