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

1 files changed, 7 insertions, 0 deletions

diff --git a/include/llvm/Transforms/Utils/Local.h b/include/llvm/Transforms/Utils/Local.h
index 98a68f6461..dd423fa3b1 100644
--- a/include/llvm/Transforms/Utils/Local.h
+++ b/include/llvm/Transforms/Utils/Local.h
@@ -19,6 +19,7 @@ namespace llvm {
 
 class User;
 class BasicBlock;
+class BranchInst;
 class Instruction;
 class Value;
 class Pass;
@@ -94,6 +95,12 @@ void MergeBasicBlockIntoOnlyPred(BasicBlock *BB);
 ///
 bool SimplifyCFG(BasicBlock *BB);
 
+/// FoldBranchToCommonDest - If this basic block is ONLY a setcc and a branch,
+/// 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.
+bool FoldBranchToCommonDest(BranchInst *BI);
+
 /// DemoteRegToStack - This function takes a virtual register computed by an
 /// Instruction and replaces it with a slot in the stack frame, allocated via
 /// alloca.  This allows the CFG to be changed around without fear of