/// shouldEliminateUnconditionalBranch - Return true if this branch looks
/// attractive to eliminate. We eliminate the branch if the destination basic
/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
/// since one of these is a terminator instruction, this means that we will add
/// up to 4 instructions to the new block.
///
/// We don't count PHI nodes in the count since they will be removed when the
/// contents of the block are copied over.
///
bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI,
unsigned Threshold) {
BranchInst *BI = dyn_cast<BranchInst>(TI);
if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch!
BasicBlock *Dest = BI->getSuccessor(0);
if (Dest == BI->getParent()) return false; // Do not loop infinitely!
// Do not inline a block if we will just get another branch to the same block!
TerminatorInst *DTI = Dest->getTerminator();
if (BranchInst *DBI = dyn_cast<BranchInst>(DTI))
if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
return false; // Do not loop infinitely!
// FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack,
// because doing so would require breaking critical edges. This should be
// fixed eventually.
if (!DTI->use_empty())
return false;
// Do not bother with blocks with only a single predecessor: simplify
// CFG will fold these two blocks together!
pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
++PI;
if (PI == PE) return false; // Exactly one predecessor!
BasicBlock::iterator I = Dest->getFirstNonPHI();
for (unsigned Size = 0; I != Dest->end(); ++I) {
if (Size == Threshold) return false; // The block is too large.
// Don't tail duplicate call instructions. They are very large compared to
// other instructions.
if (isa<CallInst>(I) || isa<InvokeInst>(I)) return false;
// Also alloca and malloc.
if (isa<AllocaInst>(I)) return false;
// Some vector instructions can expand into a number of instructions.
if (isa<ShuffleVectorInst>(I) || isa<ExtractElementInst>(I) ||
isa<InsertElementInst>(I)) return false;
// Only count instructions that are not debugger intrinsics.
if (!isa<DbgInfoIntrinsic>(I)) ++Size;
}
// Do not tail duplicate a block that has thousands of successors into a block
// with a single successor if the block has many other predecessors. This can
// cause an N^2 explosion in CFG edges (and PHI node entries), as seen in
// cases that have a large number of indirect gotos.
unsigned NumSuccs = DTI->getNumSuccessors();
if (NumSuccs > 8) {
unsigned TooMany = 128;
if (NumSuccs >= TooMany) return false;
TooMany = TooMany/NumSuccs;
for (; PI != PE; ++PI)
if (TooMany-- == 0) return false;
}
// If this unconditional branch is a fall-through, be careful about
// tail duplicating it. In particular, we don't want to taildup it if the
// original block will still be there after taildup is completed: doing so
// would eliminate the fall-through, requiring unconditional branches.
Function::iterator DestI = Dest;
if (&*--DestI == BI->getParent()) {
// The uncond branch is a fall-through. Tail duplication of the block is
// will eliminate the fall-through-ness and end up cloning the terminator
// at the end of the Dest block. Since the original Dest block will
// continue to exist, this means that one or the other will not be able to
// fall through. One typical example that this helps with is code like:
// if (a)
// foo();
// if (b)
// foo();
// Cloning the 'if b' block into the end of the first foo block is messy.
// The messy case is when the fall-through block falls through to other
// blocks. This is what we would be preventing if we cloned the block.
DestI = Dest;
if (++DestI != Dest->getParent()->end()) {
BasicBlock *DestSucc = DestI;
// If any of Dest's successors are fall-throughs, don't do this xform.
for (succ_iterator SI = succ_begin(Dest), SE = succ_end(Dest);
SI != SE; ++SI)
if (*SI == DestSucc)
return false;
}
}
// Finally, check that we haven't redirected to this target block earlier;
// there are cases where we loop forever if we don't check this (PR 2323).
if (!CycleDetector.insert(Dest))
return false;
return true;
}
