Codegen: Don't tail-duplicate blocks with un-analyzable fallthrough.

If AnalyzeBranch can't analyze a block and it is possible to
fallthrough, then duplicating the block doesn't make sense, as only one
block can be the layout predecessor for the un-analyzable fallthrough.

Submitted wit a test case, but NOTE: the test case doesn't currently
fail. However, the test case fails with D20505 and would have saved me
some time debugging.

llvm-svn: 278288

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/TailDuplicator.cpp b/llvm/lib/CodeGen/TailDuplicator.cpp
index 059c1dc5483..029bd138f53 100644
--- a/llvm/lib/CodeGen/TailDuplicator.cpp
+++ b/llvm/lib/CodeGen/TailDuplicator.cpp
@@ -525,6 +525,16 @@ bool TailDuplicator::shouldTailDuplicate(const MachineFunction &MF,
   else
     MaxDuplicateCount = TailDuplicateSize;
 
+  // If the block to be duplicated ends in an unanalyzable fallthrough, don't
+  // duplicate it.
+  // A similar check is necessary in MachineBlockPlacement to make sure pairs of
+  // blocks with unanalyzable fallthrough get layed out contiguously.
+  MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
+  SmallVector<MachineOperand, 4> PredCond;
+  if (TII->analyzeBranch(TailBB, PredTBB, PredFBB, PredCond, true)
+      && TailBB.canFallThrough())
+    return false;
+
   // If the target has hardware branch prediction that can handle indirect
   // branches, duplicating them can often make them predictable when there
   // are common paths through the code.  The limit needs to be high enough