[BranchFolding] Tail common all identical unreachable blocks

Summary:
Blocks ending in unreachable are typically cold because they end the
program or throw an exception, so merging them with other identical
blocks is usually profitable because it reduces the size of cold code.
MachineBlockPlacement generally does not arrange to fall through to such
blocks, so commoning these blocks will not introduce additional
unconditional branches.

Reviewers: hans, iteratee, haicheng

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29153

llvm-svn: 295105

1 files changed, 20 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index b196bf98d27..d148a9dbac5 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -523,6 +523,17 @@ static unsigned CountTerminators(MachineBasicBlock *MBB,
   return NumTerms;
 }
 
+/// A no successor, non-return block probably ends in unreachable and is cold.
+/// Also consider a block that ends in an indirect branch to be a return block,
+/// since many targets use plain indirect branches to return.
+static bool blockEndsInUnreachable(const MachineBasicBlock *MBB) {
+  if (!MBB->succ_empty())
+    return false;
+  if (MBB->empty())
+    return true;
+  return !(MBB->back().isReturn() || MBB->back().isIndirectBranch());
+}
+
 /// ProfitableToMerge - Check if two machine basic blocks have a common tail
 /// and decide if it would be profitable to merge those tails.  Return the
 /// length of the common tail and iterators to the first common instruction
@@ -577,6 +588,15 @@ ProfitableToMerge(MachineBasicBlock *MBB1, MachineBasicBlock *MBB2,
       return true;
   }
 
+  // If these are identical non-return blocks with no successors, merge them.
+  // Such blocks are typically cold calls to noreturn functions like abort, and
+  // are unlikely to become a fallthrough target after machine block placement.
+  // Tail merging these blocks is unlikely to create additional unconditional
+  // branches, and will reduce the size of this cold code.
+  if (I1 == MBB1->begin() && I2 == MBB2->begin() &&
+      blockEndsInUnreachable(MBB1) && blockEndsInUnreachable(MBB2))
+    return true;
+
   // If one of the blocks can be completely merged and happens to be in
   // a position where the other could fall through into it, merge any number
   // of instructions, because it can be done without a branch.