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llvm-6502/lib/Transforms/Utils/SimplifyCFG.cpp
Chris Lattner 570751c2a7 Fold code like: 
if (C)
    V1 |= V2;

into:
  Vx = V1 | V2;
  V1 = select C, V1, Vx

when the expression can be evaluated unconditionally and is *cheap* to
execute.  This limited form of if conversion is quite handy in lots of cases.
For example, it turns this testcase into straight-line code:

int in0 ; int in1 ; int in2 ; int in3 ;
int in4 ; int in5 ; int in6 ; int in7 ;
int in8 ; int in9 ; int in10; int in11;
int in12; int in13; int in14; int in15;
long output;

void mux(void) {
  output =
      (in0   ?  0x00000001 : 0) | (in1   ?  0x00000002 : 0) |
      (in2   ?  0x00000004 : 0) | (in3   ?  0x00000008 : 0) |
      (in4   ?  0x00000010 : 0) | (in5   ?  0x00000020 : 0) |
      (in6   ?  0x00000040 : 0) | (in7   ?  0x00000080 : 0) |
      (in8   ?  0x00000100 : 0) | (in9   ?  0x00000200 : 0) |
      (in10  ?  0x00000400 : 0) | (in11  ?  0x00000800 : 0) |
      (in12  ?  0x00001000 : 0) | (in13  ?  0x00002000 : 0) |
      (in14  ?  0x00004000 : 0) | (in15  ?  0x00008000 : 0) ;
}


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12798 91177308-0d34-0410-b5e6-96231b3b80d8
2004-04-09 22:50:22 +00:00

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//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Peephole optimize the CFG.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constants.h"
#include "llvm/Instructions.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <functional>
#include <set>
using namespace llvm;
// PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
// predecessors from "BB". This is a little tricky because "Succ" has PHI
// nodes, which need to have extra slots added to them to hold the merge edges
// from BB's predecessors, and BB itself might have had PHI nodes in it. This
// function returns true (failure) if the Succ BB already has a predecessor that
// is a predecessor of BB and incoming PHI arguments would not be discernible.
//
// Assumption: Succ is the single successor for BB.
//
static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
if (!isa<PHINode>(Succ->front()))
return false; // We can make the transformation, no problem.
// If there is more than one predecessor, and there are PHI nodes in
// the successor, then we need to add incoming edges for the PHI nodes
//
const std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB));
// Check to see if one of the predecessors of BB is already a predecessor of
// Succ. If so, we cannot do the transformation if there are any PHI nodes
// with incompatible values coming in from the two edges!
//
for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI)
if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) {
// Loop over all of the PHI nodes checking to see if there are
// incompatible values coming in.
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
// Loop up the entries in the PHI node for BB and for *PI if the values
// coming in are non-equal, we cannot merge these two blocks (instead we
// should insert a conditional move or something, then merge the
// blocks).
int Idx1 = PN->getBasicBlockIndex(BB);
int Idx2 = PN->getBasicBlockIndex(*PI);
assert(Idx1 != -1 && Idx2 != -1 &&
"Didn't have entries for my predecessors??");
if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2))
return true; // Values are not equal...
}
}
// Loop over all of the PHI nodes in the successor BB
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
Value *OldVal = PN->removeIncomingValue(BB, false);
assert(OldVal && "No entry in PHI for Pred BB!");
// If this incoming value is one of the PHI nodes in BB...
if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
PHINode *OldValPN = cast<PHINode>(OldVal);
for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
End = BBPreds.end(); PredI != End; ++PredI) {
PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI);
}
} else {
for (std::vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(),
End = BBPreds.end(); PredI != End; ++PredI) {
// Add an incoming value for each of the new incoming values...
PN->addIncoming(OldVal, *PredI);
}
}
}
return false;
}
/// GetIfCondition - Given a basic block (BB) with two predecessors (and
/// presumably PHI nodes in it), check to see if the merge at this block is due
/// to an "if condition". If so, return the boolean condition that determines
/// which entry into BB will be taken. Also, return by references the block
/// that will be entered from if the condition is true, and the block that will
/// be entered if the condition is false.
///
///
static Value *GetIfCondition(BasicBlock *BB,
BasicBlock *&IfTrue, BasicBlock *&IfFalse) {
assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 &&
"Function can only handle blocks with 2 predecessors!");
BasicBlock *Pred1 = *pred_begin(BB);
BasicBlock *Pred2 = *++pred_begin(BB);
// We can only handle branches. Other control flow will be lowered to
// branches if possible anyway.
if (!isa<BranchInst>(Pred1->getTerminator()) ||
!isa<BranchInst>(Pred2->getTerminator()))
return 0;
BranchInst *Pred1Br = cast<BranchInst>(Pred1->getTerminator());
BranchInst *Pred2Br = cast<BranchInst>(Pred2->getTerminator());
// Eliminate code duplication by ensuring that Pred1Br is conditional if
// either are.
if (Pred2Br->isConditional()) {
// If both branches are conditional, we don't have an "if statement". In
// reality, we could transform this case, but since the condition will be
// required anyway, we stand no chance of eliminating it, so the xform is
// probably not profitable.
if (Pred1Br->isConditional())
return 0;
std::swap(Pred1, Pred2);
std::swap(Pred1Br, Pred2Br);
}
if (Pred1Br->isConditional()) {
// If we found a conditional branch predecessor, make sure that it branches
// to BB and Pred2Br. If it doesn't, this isn't an "if statement".
if (Pred1Br->getSuccessor(0) == BB &&
Pred1Br->getSuccessor(1) == Pred2) {
IfTrue = Pred1;
IfFalse = Pred2;
} else if (Pred1Br->getSuccessor(0) == Pred2 &&
Pred1Br->getSuccessor(1) == BB) {
IfTrue = Pred2;
IfFalse = Pred1;
} else {
// We know that one arm of the conditional goes to BB, so the other must
// go somewhere unrelated, and this must not be an "if statement".
return 0;
}
// The only thing we have to watch out for here is to make sure that Pred2
// doesn't have incoming edges from other blocks. If it does, the condition
// doesn't dominate BB.
if (++pred_begin(Pred2) != pred_end(Pred2))
return 0;
return Pred1Br->getCondition();
}
// Ok, if we got here, both predecessors end with an unconditional branch to
// BB. Don't panic! If both blocks only have a single (identical)
// predecessor, and THAT is a conditional branch, then we're all ok!
if (pred_begin(Pred1) == pred_end(Pred1) ||
++pred_begin(Pred1) != pred_end(Pred1) ||
pred_begin(Pred2) == pred_end(Pred2) ||
++pred_begin(Pred2) != pred_end(Pred2) ||
*pred_begin(Pred1) != *pred_begin(Pred2))
return 0;
// Otherwise, if this is a conditional branch, then we can use it!
BasicBlock *CommonPred = *pred_begin(Pred1);
if (BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator())) {
assert(BI->isConditional() && "Two successors but not conditional?");
if (BI->getSuccessor(0) == Pred1) {
IfTrue = Pred1;
IfFalse = Pred2;
} else {
IfTrue = Pred2;
IfFalse = Pred1;
}
return BI->getCondition();
}
return 0;
}
// If we have a merge point of an "if condition" as accepted above, return true
// if the specified value dominates the block. We don't handle the true
// generality of domination here, just a special case which works well enough
// for us.
static bool DominatesMergePoint(Value *V, BasicBlock *BB, bool AllowAggressive){
Instruction *I = dyn_cast<Instruction>(V);
if (!I) return true; // Non-instructions all dominate instructions.
BasicBlock *PBB = I->getParent();
// We don't want to allow wierd loops that might have the "if condition" in
// the bottom of this block.
if (PBB == BB) return false;
// If this instruction is defined in a block that contains an unconditional
// branch to BB, then it must be in the 'conditional' part of the "if
// statement".
if (BranchInst *BI = dyn_cast<BranchInst>(PBB->getTerminator()))
if (BI->isUnconditional() && BI->getSuccessor(0) == BB) {
if (!AllowAggressive) return false;
// Okay, it looks like the instruction IS in the "condition". Check to
// see if its a cheap instruction to unconditionally compute, and if it
// only uses stuff defined outside of the condition. If so, hoist it out.
switch (I->getOpcode()) {
default: return false; // Cannot hoist this out safely.
case Instruction::Load:
// We can hoist loads that are non-volatile and obviously cannot trap.
if (cast<LoadInst>(I)->isVolatile())
return false;
if (!isa<AllocaInst>(I->getOperand(0)) &&
!isa<Constant>(I->getOperand(0)) &&
!isa<GlobalValue>(I->getOperand(0)))
return false;
// Finally, we have to check to make sure there are no instructions
// before the load in its basic block, as we are going to hoist the loop
// out to its predecessor.
if (PBB->begin() != BasicBlock::iterator(I))
return false;
break;
case Instruction::Add:
case Instruction::Sub:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Shl:
case Instruction::Shr:
break; // These are all cheap and non-trapping instructions.
}
// Okay, we can only really hoist these out if their operands are not
// defined in the conditional region.
for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
if (!DominatesMergePoint(I->getOperand(i), BB, false))
return false;
// Okay, it's safe to do this!
}
return true;
}
// GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq
// instructions that compare a value against a constant, return the value being
// compared, and stick the constant into the Values vector.
static Value *GatherConstantSetEQs(Value *V, std::vector<Constant*> &Values) {
if (Instruction *Inst = dyn_cast<Instruction>(V))
if (Inst->getOpcode() == Instruction::SetEQ) {
if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
Values.push_back(C);
return Inst->getOperand(0);
} else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
Values.push_back(C);
return Inst->getOperand(1);
}
} else if (Inst->getOpcode() == Instruction::Or) {
if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values))
if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values))
if (LHS == RHS)
return LHS;
}
return 0;
}
// GatherConstantSetNEs - Given a potentially 'and'd together collection of
// setne instructions that compare a value against a constant, return the value
// being compared, and stick the constant into the Values vector.
static Value *GatherConstantSetNEs(Value *V, std::vector<Constant*> &Values) {
if (Instruction *Inst = dyn_cast<Instruction>(V))
if (Inst->getOpcode() == Instruction::SetNE) {
if (Constant *C = dyn_cast<Constant>(Inst->getOperand(1))) {
Values.push_back(C);
return Inst->getOperand(0);
} else if (Constant *C = dyn_cast<Constant>(Inst->getOperand(0))) {
Values.push_back(C);
return Inst->getOperand(1);
}
} else if (Inst->getOpcode() == Instruction::Cast) {
// Cast of X to bool is really a comparison against zero.
assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!");
Values.push_back(Constant::getNullValue(Inst->getOperand(0)->getType()));
return Inst->getOperand(0);
} else if (Inst->getOpcode() == Instruction::And) {
if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values))
if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values))
if (LHS == RHS)
return LHS;
}
return 0;
}
/// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a
/// bunch of comparisons of one value against constants, return the value and
/// the constants being compared.
static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal,
std::vector<Constant*> &Values) {
if (Cond->getOpcode() == Instruction::Or) {
CompVal = GatherConstantSetEQs(Cond, Values);
// Return true to indicate that the condition is true if the CompVal is
// equal to one of the constants.
return true;
} else if (Cond->getOpcode() == Instruction::And) {
CompVal = GatherConstantSetNEs(Cond, Values);
// Return false to indicate that the condition is false if the CompVal is
// equal to one of the constants.
return false;
}
return false;
}
/// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and
/// has no side effects, nuke it. If it uses any instructions that become dead
/// because the instruction is now gone, nuke them too.
static void ErasePossiblyDeadInstructionTree(Instruction *I) {
if (isInstructionTriviallyDead(I)) {
std::vector<Value*> Operands(I->op_begin(), I->op_end());
I->getParent()->getInstList().erase(I);
for (unsigned i = 0, e = Operands.size(); i != e; ++i)
if (Instruction *OpI = dyn_cast<Instruction>(Operands[i]))
ErasePossiblyDeadInstructionTree(OpI);
}
}
/// SafeToMergeTerminators - Return true if it is safe to merge these two
/// terminator instructions together.
///
static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
if (SI1 == SI2) return false; // Can't merge with self!
// It is not safe to merge these two switch instructions if they have a common
// successor, and if that successor has a PHI node, and if that PHI node has
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
if (SI1Succs.count(*I))
for (BasicBlock::iterator BBI = (*I)->begin();
PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
if (PN->getIncomingValueForBlock(SI1BB) !=
PN->getIncomingValueForBlock(SI2BB))
return false;
return true;
}
/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
/// now be entries in it from the 'NewPred' block. The values that will be
/// flowing into the PHI nodes will be the same as those coming in from
/// ExistPred, and existing predecessor of Succ.
static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
BasicBlock *ExistPred) {
assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
Value *V = PN->getIncomingValueForBlock(ExistPred);
PN->addIncoming(V, NewPred);
}
}
// isValueEqualityComparison - Return true if the specified terminator checks to
// see if a value is equal to constant integer value.
static Value *isValueEqualityComparison(TerminatorInst *TI) {
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
// Do not permit merging of large switch instructions into their
// predecessors unless there is only one predecessor.
if (SI->getNumSuccessors() * std::distance(pred_begin(SI->getParent()),
pred_end(SI->getParent())) > 128)
return 0;
return SI->getCondition();
}
if (BranchInst *BI = dyn_cast<BranchInst>(TI))
if (BI->isConditional() && BI->getCondition()->hasOneUse())
if (SetCondInst *SCI = dyn_cast<SetCondInst>(BI->getCondition()))
if ((SCI->getOpcode() == Instruction::SetEQ ||
SCI->getOpcode() == Instruction::SetNE) &&
isa<ConstantInt>(SCI->getOperand(1)))
return SCI->getOperand(0);
return 0;
}
// Given a value comparison instruction, decode all of the 'cases' that it
// represents and return the 'default' block.
static BasicBlock *
GetValueEqualityComparisonCases(TerminatorInst *TI,
std::vector<std::pair<ConstantInt*,
BasicBlock*> > &Cases) {
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
Cases.reserve(SI->getNumCases());
for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i)
Cases.push_back(std::make_pair(cast<ConstantInt>(SI->getCaseValue(i)),
SI->getSuccessor(i)));
return SI->getDefaultDest();
}
BranchInst *BI = cast<BranchInst>(TI);
SetCondInst *SCI = cast<SetCondInst>(BI->getCondition());
Cases.push_back(std::make_pair(cast<ConstantInt>(SCI->getOperand(1)),
BI->getSuccessor(SCI->getOpcode() ==
Instruction::SetNE)));
return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ);
}
// FoldValueComparisonIntoPredecessors - The specified terminator is a value
// equality comparison instruction (either a switch or a branch on "X == c").
// See if any of the predecessors of the terminator block are value comparisons
// on the same value. If so, and if safe to do so, fold them together.
static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) {
BasicBlock *BB = TI->getParent();
Value *CV = isValueEqualityComparison(TI); // CondVal
assert(CV && "Not a comparison?");
bool Changed = false;
std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.back();
Preds.pop_back();
// See if the predecessor is a comparison with the same value.
TerminatorInst *PTI = Pred->getTerminator();
Value *PCV = isValueEqualityComparison(PTI); // PredCondVal
if (PCV == CV && SafeToMergeTerminators(TI, PTI)) {
// Figure out which 'cases' to copy from SI to PSI.
std::vector<std::pair<ConstantInt*, BasicBlock*> > BBCases;
BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases);
std::vector<std::pair<ConstantInt*, BasicBlock*> > PredCases;
BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases);
// Based on whether the default edge from PTI goes to BB or not, fill in
// PredCases and PredDefault with the new switch cases we would like to
// build.
std::vector<BasicBlock*> NewSuccessors;
if (PredDefault == BB) {
// If this is the default destination from PTI, only the edges in TI
// that don't occur in PTI, or that branch to BB will be activated.
std::set<ConstantInt*> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].second != BB)
PTIHandled.insert(PredCases[i].first);
else {
// The default destination is BB, we don't need explicit targets.
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i; --e;
}
// Reconstruct the new switch statement we will be building.
if (PredDefault != BBDefault) {
PredDefault->removePredecessor(Pred);
PredDefault = BBDefault;
NewSuccessors.push_back(BBDefault);
}
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (!PTIHandled.count(BBCases[i].first) &&
BBCases[i].second != BBDefault) {
PredCases.push_back(BBCases[i]);
NewSuccessors.push_back(BBCases[i].second);
}
} else {
// If this is not the default destination from PSI, only the edges
// in SI that occur in PSI with a destination of BB will be
// activated.
std::set<ConstantInt*> PTIHandled;
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
if (PredCases[i].second == BB) {
PTIHandled.insert(PredCases[i].first);
std::swap(PredCases[i], PredCases.back());
PredCases.pop_back();
--i; --e;
}
// Okay, now we know which constants were sent to BB from the
// predecessor. Figure out where they will all go now.
for (unsigned i = 0, e = BBCases.size(); i != e; ++i)
if (PTIHandled.count(BBCases[i].first)) {
// If this is one we are capable of getting...
PredCases.push_back(BBCases[i]);
NewSuccessors.push_back(BBCases[i].second);
PTIHandled.erase(BBCases[i].first);// This constant is taken care of
}
// If there are any constants vectored to BB that TI doesn't handle,
// they must go to the default destination of TI.
for (std::set<ConstantInt*>::iterator I = PTIHandled.begin(),
E = PTIHandled.end(); I != E; ++I) {
PredCases.push_back(std::make_pair(*I, BBDefault));
NewSuccessors.push_back(BBDefault);
}
}
// Okay, at this point, we know which new successor Pred will get. Make
// sure we update the number of entries in the PHI nodes for these
// successors.
for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i)
AddPredecessorToBlock(NewSuccessors[i], Pred, BB);
// Now that the successors are updated, create the new Switch instruction.
SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PTI);
for (unsigned i = 0, e = PredCases.size(); i != e; ++i)
NewSI->addCase(PredCases[i].first, PredCases[i].second);
Pred->getInstList().erase(PTI);
// Okay, last check. If BB is still a successor of PSI, then we must
// have an infinite loop case. If so, add an infinitely looping block
// to handle the case to preserve the behavior of the code.
BasicBlock *InfLoopBlock = 0;
for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i)
if (NewSI->getSuccessor(i) == BB) {
if (InfLoopBlock == 0) {
// Insert it at the end of the loop, because it's either code,
// or it won't matter if it's hot. :)
InfLoopBlock = new BasicBlock("infloop", BB->getParent());
new BranchInst(InfLoopBlock, InfLoopBlock);
}
NewSI->setSuccessor(i, InfLoopBlock);
}
Changed = true;
}
}
return Changed;
}
// SimplifyCFG - This function is used to do simplification of a CFG. For
// example, it adjusts branches to branches to eliminate the extra hop, it
// eliminates unreachable basic blocks, and does other "peephole" optimization
// of the CFG. It returns true if a modification was made.
//
// WARNING: The entry node of a function may not be simplified.
//
bool llvm::SimplifyCFG(BasicBlock *BB) {
bool Changed = false;
Function *M = BB->getParent();
assert(BB && BB->getParent() && "Block not embedded in function!");
assert(BB->getTerminator() && "Degenerate basic block encountered!");
assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
// Remove basic blocks that have no predecessors... which are unreachable.
if (pred_begin(BB) == pred_end(BB) ||
*pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
//cerr << "Removing BB: \n" << BB;
// Loop through all of our successors and make sure they know that one
// of their predecessors is going away.
for_each(succ_begin(BB), succ_end(BB),
std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB));
while (!BB->empty()) {
Instruction &I = BB->back();
// If this instruction is used, replace uses with an arbitrary
// constant value. Because control flow can't get here, we don't care
// what we replace the value with. Note that since this block is
// unreachable, and all values contained within it must dominate their
// uses, that all uses will eventually be removed.
if (!I.use_empty())
// Make all users of this instruction reference the constant instead
I.replaceAllUsesWith(Constant::getNullValue(I.getType()));
// Remove the instruction from the basic block
BB->getInstList().pop_back();
}
M->getBasicBlockList().erase(BB);
return true;
}
// Check to see if we can constant propagate this terminator instruction
// away...
Changed |= ConstantFoldTerminator(BB);
// Check to see if this block has no non-phi instructions and only a single
// successor. If so, replace references to this basic block with references
// to the successor.
succ_iterator SI(succ_begin(BB));
if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes...
while (isa<PHINode>(*BBI)) ++BBI;
if (BBI->isTerminator()) { // Terminator is the only non-phi instruction!
BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor
if (Succ != BB) { // Arg, don't hurt infinite loops!
// If our successor has PHI nodes, then we need to update them to
// include entries for BB's predecessors, not for BB itself.
// Be careful though, if this transformation fails (returns true) then
// we cannot do this transformation!
//
if (!PropagatePredecessorsForPHIs(BB, Succ)) {
//cerr << "Killing Trivial BB: \n" << BB;
std::string OldName = BB->getName();
std::vector<BasicBlock*>
OldSuccPreds(pred_begin(Succ), pred_end(Succ));
// Move all PHI nodes in BB to Succ if they are alive, otherwise
// delete them.
while (PHINode *PN = dyn_cast<PHINode>(&BB->front()))
if (PN->use_empty())
BB->getInstList().erase(BB->begin()); // Nuke instruction...
else {
// The instruction is alive, so this means that Succ must have
// *ONLY* had BB as a predecessor, and the PHI node is still valid
// now. Simply move it into Succ, because we know that BB
// strictly dominated Succ.
BB->getInstList().remove(BB->begin());
Succ->getInstList().push_front(PN);
// We need to add new entries for the PHI node to account for
// predecessors of Succ that the PHI node does not take into
// account. At this point, since we know that BB dominated succ,
// this means that we should any newly added incoming edges should
// use the PHI node as the value for these edges, because they are
// loop back edges.
for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i)
if (OldSuccPreds[i] != BB)
PN->addIncoming(PN, OldSuccPreds[i]);
}
// Everything that jumped to BB now goes to Succ...
BB->replaceAllUsesWith(Succ);
// Delete the old basic block...
M->getBasicBlockList().erase(BB);
if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can
Succ->setName(OldName);
//cerr << "Function after removal: \n" << M;
return true;
}
}
}
}
// If this is a returning block with only PHI nodes in it, fold the return
// instruction into any unconditional branch predecessors.
//
// If any predecessor is a conditional branch that just selects among
// different return values, fold the replace the branch/return with a select
// and return.
if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) {
BasicBlock::iterator BBI = BB->getTerminator();
if (BBI == BB->begin() || isa<PHINode>(--BBI)) {
// Find predecessors that end with branches.
std::vector<BasicBlock*> UncondBranchPreds;
std::vector<BranchInst*> CondBranchPreds;
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
TerminatorInst *PTI = (*PI)->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
if (BI->isUnconditional())
UncondBranchPreds.push_back(*PI);
else
CondBranchPreds.push_back(BI);
}
// If we found some, do the transformation!
if (!UncondBranchPreds.empty()) {
while (!UncondBranchPreds.empty()) {
BasicBlock *Pred = UncondBranchPreds.back();
UncondBranchPreds.pop_back();
Instruction *UncondBranch = Pred->getTerminator();
// Clone the return and add it to the end of the predecessor.
Instruction *NewRet = RI->clone();
Pred->getInstList().push_back(NewRet);
// If the return instruction returns a value, and if the value was a
// PHI node in "BB", propagate the right value into the return.
if (NewRet->getNumOperands() == 1)
if (PHINode *PN = dyn_cast<PHINode>(NewRet->getOperand(0)))
if (PN->getParent() == BB)
NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred));
// Update any PHI nodes in the returning block to realize that we no
// longer branch to them.
BB->removePredecessor(Pred);
Pred->getInstList().erase(UncondBranch);
}
// If we eliminated all predecessors of the block, delete the block now.
if (pred_begin(BB) == pred_end(BB))
// We know there are no successors, so just nuke the block.
M->getBasicBlockList().erase(BB);
return true;
}
// Check out all of the conditional branches going to this return
// instruction. If any of them just select between returns, change the
// branch itself into a select/return pair.
while (!CondBranchPreds.empty()) {
BranchInst *BI = CondBranchPreds.back();
CondBranchPreds.pop_back();
BasicBlock *TrueSucc = BI->getSuccessor(0);
BasicBlock *FalseSucc = BI->getSuccessor(1);
BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc;
// Check to see if the non-BB successor is also a return block.
if (isa<ReturnInst>(OtherSucc->getTerminator())) {
// Check to see if there are only PHI instructions in this block.
BasicBlock::iterator OSI = OtherSucc->getTerminator();
if (OSI == OtherSucc->begin() || isa<PHINode>(--OSI)) {
// Okay, we found a branch that is going to two return nodes. If
// there is no return value for this function, just change the
// branch into a return.
if (RI->getNumOperands() == 0) {
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
new ReturnInst(0, BI);
BI->getParent()->getInstList().erase(BI);
return true;
}
// Otherwise, figure out what the true and false return values are
// so we can insert a new select instruction.
Value *TrueValue = TrueSucc->getTerminator()->getOperand(0);
Value *FalseValue = FalseSucc->getTerminator()->getOperand(0);
// Unwrap any PHI nodes in the return blocks.
if (PHINode *TVPN = dyn_cast<PHINode>(TrueValue))
if (TVPN->getParent() == TrueSucc)
TrueValue = TVPN->getIncomingValueForBlock(BI->getParent());
if (PHINode *FVPN = dyn_cast<PHINode>(FalseValue))
if (FVPN->getParent() == FalseSucc)
FalseValue = FVPN->getIncomingValueForBlock(BI->getParent());
TrueSucc->removePredecessor(BI->getParent());
FalseSucc->removePredecessor(BI->getParent());
// Insert a new select instruction.
Value *NewRetVal = new SelectInst(BI->getCondition(), TrueValue,
FalseValue, "retval", BI);
new ReturnInst(NewRetVal, BI);
BI->getParent()->getInstList().erase(BI);
return true;
}
}
}
}
} else if (UnwindInst *UI = dyn_cast<UnwindInst>(BB->begin())) {
// Check to see if the first instruction in this block is just an unwind.
// If so, replace any invoke instructions which use this as an exception
// destination with call instructions.
//
std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.back();
if (InvokeInst *II = dyn_cast<InvokeInst>(Pred->getTerminator()))
if (II->getUnwindDest() == BB) {
// Insert a new branch instruction before the invoke, because this
// is now a fall through...
BranchInst *BI = new BranchInst(II->getNormalDest(), II);
Pred->getInstList().remove(II); // Take out of symbol table
// Insert the call now...
std::vector<Value*> Args(II->op_begin()+3, II->op_end());
CallInst *CI = new CallInst(II->getCalledValue(), Args,
II->getName(), BI);
// If the invoke produced a value, the Call now does instead
II->replaceAllUsesWith(CI);
delete II;
Changed = true;
}
Preds.pop_back();
}
// If this block is now dead, remove it.
if (pred_begin(BB) == pred_end(BB)) {
// We know there are no successors, so just nuke the block.
M->getBasicBlockList().erase(BB);
return true;
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
if (isValueEqualityComparison(SI))
if (FoldValueComparisonIntoPredecessors(SI))
return SimplifyCFG(BB) || 1;
} else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
if (Value *CompVal = isValueEqualityComparison(BB->getTerminator())) {
// This block must be empty, except for the setcond inst, if it exists.
BasicBlock::iterator I = BB->begin();
if (&*I == BI ||
(&*I == cast<Instruction>(BI->getCondition()) &&
&*++I == BI))
if (FoldValueComparisonIntoPredecessors(BI))
return SimplifyCFG(BB) || 1;
}
}
// Merge basic blocks into their predecessor if there is only one distinct
// pred, and if there is only one distinct successor of the predecessor, and
// if there are no PHI nodes.
//
pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
BasicBlock *OnlyPred = *PI++;
for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
if (*PI != OnlyPred) {
OnlyPred = 0; // There are multiple different predecessors...
break;
}
BasicBlock *OnlySucc = 0;
if (OnlyPred && OnlyPred != BB && // Don't break self loops
OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) {
// Check to see if there is only one distinct successor...
succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred));
OnlySucc = BB;
for (; SI != SE; ++SI)
if (*SI != OnlySucc) {
OnlySucc = 0; // There are multiple distinct successors!
break;
}
}
if (OnlySucc) {
//cerr << "Merging: " << BB << "into: " << OnlyPred;
TerminatorInst *Term = OnlyPred->getTerminator();
// Resolve any PHI nodes at the start of the block. They are all
// guaranteed to have exactly one entry if they exist, unless there are
// multiple duplicate (but guaranteed to be equal) entries for the
// incoming edges. This occurs when there are multiple edges from
// OnlyPred to OnlySucc.
//
while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
PN->replaceAllUsesWith(PN->getIncomingValue(0));
BB->getInstList().pop_front(); // Delete the phi node...
}
// Delete the unconditional branch from the predecessor...
OnlyPred->getInstList().pop_back();
// Move all definitions in the successor to the predecessor...
OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
// Make all PHI nodes that referred to BB now refer to Pred as their
// source...
BB->replaceAllUsesWith(OnlyPred);
std::string OldName = BB->getName();
// Erase basic block from the function...
M->getBasicBlockList().erase(BB);
// Inherit predecessors name if it exists...
if (!OldName.empty() && !OnlyPred->hasName())
OnlyPred->setName(OldName);
return true;
}
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *BI = dyn_cast<BranchInst>((*PI)->getTerminator()))
// Change br (X == 0 | X == 1), T, F into a switch instruction.
if (BI->isConditional() && isa<Instruction>(BI->getCondition())) {
Instruction *Cond = cast<Instruction>(BI->getCondition());
// If this is a bunch of seteq's or'd together, or if it's a bunch of
// 'setne's and'ed together, collect them.
Value *CompVal = 0;
std::vector<Constant*> Values;
bool TrueWhenEqual = GatherValueComparisons(Cond, CompVal, Values);
if (CompVal && CompVal->getType()->isInteger()) {
// There might be duplicate constants in the list, which the switch
// instruction can't handle, remove them now.
std::sort(Values.begin(), Values.end());
Values.erase(std::unique(Values.begin(), Values.end()), Values.end());
// Figure out which block is which destination.
BasicBlock *DefaultBB = BI->getSuccessor(1);
BasicBlock *EdgeBB = BI->getSuccessor(0);
if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB);
// Create the new switch instruction now.
SwitchInst *New = new SwitchInst(CompVal, DefaultBB, BI);
// Add all of the 'cases' to the switch instruction.
for (unsigned i = 0, e = Values.size(); i != e; ++i)
New->addCase(Values[i], EdgeBB);
// We added edges from PI to the EdgeBB. As such, if there were any
// PHI nodes in EdgeBB, they need entries to be added corresponding to
// the number of edges added.
for (BasicBlock::iterator BBI = EdgeBB->begin();
PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI) {
Value *InVal = PN->getIncomingValueForBlock(*PI);
for (unsigned i = 0, e = Values.size()-1; i != e; ++i)
PN->addIncoming(InVal, *PI);
}
// Erase the old branch instruction.
(*PI)->getInstList().erase(BI);
// Erase the potentially condition tree that was used to computed the
// branch condition.
ErasePossiblyDeadInstructionTree(Cond);
return true;
}
}
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
if (PHINode *PN = dyn_cast<PHINode>(BB->begin()))
if (PN->getNumIncomingValues() == 2) {
// Ok, this is a two entry PHI node. Check to see if this is a simple "if
// statement", which has a very simple dominance structure. Basically, we
// are trying to find the condition that is being branched on, which
// subsequently causes this merge to happen. We really want control
// dependence information for this check, but simplifycfg can't keep it up
// to date, and this catches most of the cases we care about anyway.
//
BasicBlock *IfTrue, *IfFalse;
if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) {
//std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: "
// << IfTrue->getName() << " F: " << IfFalse->getName() << "\n";
// Figure out where to insert instructions as necessary.
BasicBlock::iterator AfterPHIIt = BB->begin();
while (isa<PHINode>(AfterPHIIt)) ++AfterPHIIt;
BasicBlock::iterator I = BB->begin();
while (PHINode *PN = dyn_cast<PHINode>(I)) {
++I;
// If we can eliminate this PHI by directly computing it based on the
// condition, do so now. We can't eliminate PHI nodes where the
// incoming values are defined in the conditional parts of the branch,
// so check for this.
//
if (DominatesMergePoint(PN->getIncomingValue(0), BB, true) &&
DominatesMergePoint(PN->getIncomingValue(1), BB, true)) {
Value *TrueVal =
PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse);
Value *FalseVal =
PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue);
// If one of the incoming values is defined in the conditional
// region, move it into it's predecessor block, which we know is
// safe.
if (!DominatesMergePoint(TrueVal, BB, false)) {
Instruction *TrueI = cast<Instruction>(TrueVal);
BasicBlock *OldBB = TrueI->getParent();
OldBB->getInstList().remove(TrueI);
BasicBlock *NewBB = *pred_begin(OldBB);
NewBB->getInstList().insert(NewBB->getTerminator(), TrueI);
}
if (!DominatesMergePoint(FalseVal, BB, false)) {
Instruction *FalseI = cast<Instruction>(FalseVal);
BasicBlock *OldBB = FalseI->getParent();
OldBB->getInstList().remove(FalseI);
BasicBlock *NewBB = *pred_begin(OldBB);
NewBB->getInstList().insert(NewBB->getTerminator(), FalseI);
}
// Change the PHI node into a select instruction.
BasicBlock::iterator InsertPos = PN;
while (isa<PHINode>(InsertPos)) ++InsertPos;
std::string Name = PN->getName(); PN->setName("");
PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal,
Name, InsertPos));
BB->getInstList().erase(PN);
Changed = true;
}
}
}
}
return Changed;
}
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