[MachineOutliner] Teach outliner to set live-ins

Preserving liveness can be useful even late in the pipeline, if we're
doing substantial optimization work afterwards. (See, for example,
D76065.) Teach MachineOutliner how to correctly set live-ins on the
basic block in outlined functions.

Differential Revision: https://reviews.llvm.org/D78605

1 files changed, 27 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/MachineOutliner.cpp b/llvm/lib/CodeGen/MachineOutliner.cpp
index bd30bd4..e15dcc3 100644
--- a/llvm/lib/CodeGen/MachineOutliner.cpp
+++ b/llvm/lib/CodeGen/MachineOutliner.cpp
@@ -1178,12 +1178,35 @@ MachineFunction *MachineOutliner::createOutlinedFunction(
     MBB.insert(MBB.end(), NewMI);
   }
 
-  TII.buildOutlinedFrame(MBB, MF, OF);
-
-  // Outlined functions shouldn't preserve liveness.
-  MF.getProperties().reset(MachineFunctionProperties::Property::TracksLiveness);
+  // Set normal properties for a late MachineFunction.
+  MF.getProperties().reset(MachineFunctionProperties::Property::IsSSA);
+  MF.getProperties().set(MachineFunctionProperties::Property::NoPHIs);
+  MF.getProperties().set(MachineFunctionProperties::Property::NoVRegs);
+  MF.getProperties().set(MachineFunctionProperties::Property::TracksLiveness);
   MF.getRegInfo().freezeReservedRegs(MF);
 
+  // Compute live-in set for outlined fn
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
+  LivePhysRegs LiveIns(TRI);
+  for (auto &Cand : OF.Candidates) {
+    // Figure out live-ins at the first instruction.
+    MachineBasicBlock &OutlineBB = *Cand.front()->getParent();
+    LivePhysRegs CandLiveIns(TRI);
+    CandLiveIns.addLiveOuts(OutlineBB);
+    for (const MachineInstr &MI :
+         reverse(make_range(Cand.front(), OutlineBB.end())))
+      CandLiveIns.stepBackward(MI);
+
+    // The live-in set for the outlined function is the union of the live-ins
+    // from all the outlining points.
+    for (MCPhysReg Reg : make_range(CandLiveIns.begin(), CandLiveIns.end()))
+      LiveIns.addReg(Reg);
+  }
+  addLiveIns(MBB, LiveIns);
+
+  TII.buildOutlinedFrame(MBB, MF, OF);
+
   // If there's a DISubprogram associated with this outlined function, then
   // emit debug info for the outlined function.
   if (DISubprogram *SP = getSubprogramOrNull(OF)) {