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//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===//
//
// 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, and returns an
// iterator that designates the first element remaining after the block that
// was deleted.
//
// WARNING: The entry node of a function may not be simplified.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Constant.h"
#include "llvm/iPHINode.h"
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <functional>
// PropogatePredecessors - 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. This function returns true (failure) if the Succ BB already
// has a predecessor that is a predecessor of BB.
//
// Assumption: Succ is the single successor for BB.
//
static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!");
// 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!
//
for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ);
PI != PE; ++PI) {
if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end())
return true;
}
// 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);
assert(OldVal && "No entry in PHI for Pred BB!");
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;
}
// 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, and returns an
// iterator that designates the first element remaining after the block that
// was deleted.
//
// WARNING: The entry node of a function may not be simplified.
//
bool SimplifyCFG(BasicBlock *BB) {
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) &&
!BB->hasConstantReferences()) {
//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 this block has no instructions and only a single
// successor. If so, replace block references with successor.
succ_iterator SI(succ_begin(BB));
if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ?
if (BB->front().isTerminator()) { // Terminator is the only 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 (!isa<PHINode>(Succ->front()) ||
!PropogatePredecessorsForPHIs(BB, Succ)) {
//cerr << "Killing Trivial BB: \n" << BB;
BB->replaceAllUsesWith(Succ);
std::string OldName = BB->getName();
// 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;
}
}
}
}
// 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.
//
if (!isa<PHINode>(BB->front()) && !BB->hasConstantReferences()) {
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
// 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();
// Delete the unconditional branch from the predecessor...
OnlyPred->getInstList().pop_back();
// Move all definitions in the succecessor to the predecessor...
OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());
// Make all PHI nodes that refered 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;
}
}
return false;
}
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