[MBP] do not reorder and move up loop latch block

Do not reorder and move up a loop latch block before a loop header
when optimising for size because this will generate an extra 
unconditional branch.

Differential Revision: https://reviews.llvm.org/D22521

llvm-svn: 278840

llvm/lib/CodeGen/MachineBlockPlacement.cpp

@@ -966,6 +966,16 @@ void MachineBlockPlacement::buildChain(
 MachineBasicBlock *
 MachineBlockPlacement::findBestLoopTop(MachineLoop &L,
                                        const BlockFilterSet &LoopBlockSet) {
+  // Placing the latch block before the header may introduce an extra branch
+  // that skips this block the first time the loop is executed, which we want
+  // to avoid when optimising for size.
+  // FIXME: in theory there is a case that does not introduce a new branch,
+  // i.e. when the layout predecessor does not fallthrough to the loop header.
+  // In practice this never happens though: there always seems to be a preheader
+  // that can fallthrough and that is also placed before the header.
+  if (F->getFunction()->optForSize())
+    return L.getHeader();
+
   // Check that the header hasn't been fused with a preheader block due to
   // crazy branches. If it has, we need to start with the header at the top to
   // prevent pulling the preheader into the loop body.