Teach LoopSimplify how to merge multiple loop exits into a single exit,

when one of them can be converted to a trivial icmp and conditional
branch.

This addresses what is essentially a phase ordering problem.
SimplifyCFG knows how to do this transformation, but it doesn't do so
if the primary block has any instructions in it other than an icmp and
a branch. In the given testcase, the block contains other instructions,
however they are loop-invariant and can be hoisted. SimplifyCFG doesn't
have LoopInfo though, so it can't hoist them. And, it's important that
the blocks be merged before LoopRotation, as it doesn't support
multiple-exit loops.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74396 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -1492,7 +1492,7 @@ static bool SimplifyCondBranchToTwoReturns(BranchInst *BI) {
 /// and if a predecessor branches to us and one of our successors, fold the
 /// setcc into the predecessor and use logical operations to pick the right
 /// destination.
-static bool FoldBranchToCommonDest(BranchInst *BI) {
+bool llvm::FoldBranchToCommonDest(BranchInst *BI) {
   BasicBlock *BB = BI->getParent();
   Instruction *Cond = dyn_cast<Instruction>(BI->getCondition());
   if (Cond == 0) return false;