[MachineOutliner] Add "thunk" outlining for AArch64.

When we're outlining a sequence that ends in a call, we can save up to
three instructions in the outlined function by turning the call into
a tail-call. I refer to this as thunk outlining because the resulting
outlined function looks like a thunk; suggestions welcome for a better
name.

In addition to making the outlined function shorter, thunk outlining
allows outlining calls which would otherwise be illegal to outline:
we don't need to save/restore LR, so we don't need to prove anything
about the stack access patterns of the callee.

To make this work effectively, I also added
MachineOutlinerInstrType::LegalTerminator to the generic MachineOutliner
code; this allows treating an arbitrary instruction as a terminator in
the suffix tree.

Differential Revision: https://reviews.llvm.org/D47173

llvm-svn: 333015

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/MachineOutliner.cpp b/llvm/lib/CodeGen/MachineOutliner.cpp
index be12b207b89..5aecc84481a 100644
--- a/llvm/lib/CodeGen/MachineOutliner.cpp
+++ b/llvm/lib/CodeGen/MachineOutliner.cpp
@@ -777,6 +777,13 @@ struct InstructionMapper {
         mapToLegalUnsigned(It);
         break;
 
+      case TargetInstrInfo::MachineOutlinerInstrType::LegalTerminator:
+        mapToLegalUnsigned(It);
+        InstrList.push_back(It);
+        UnsignedVec.push_back(IllegalInstrNumber);
+        IllegalInstrNumber--;
+        break;
+
       case TargetInstrInfo::MachineOutlinerInstrType::Invisible:
         break;
       }