Insert IMPLICIT_DEFS for undef uses in tail merging

Tail merging can convert an undef use into a normal one when creating a
common tail. Doing so can make the register live out from a block which
previously contained the undef use. To keep the liveness up-to-date,
insert IMPLICIT_DEFs in such blocks when necessary.

To enable this patch the computeLiveIns() function which used to
compute live-ins for a block and set them immediately is split into new
functions:
- computeLiveIns() just computes the live-ins in a LivePhysRegs set.
- addLiveIns() applies the live-ins to a block live-in list.
- computeAndAddLiveIns() is a convenience function combining the other
  two functions and behaving like computeLiveIns() before this patch.

Based on a patch by Krzysztof Parzyszek <kparzysz@codeaurora.org>

Differential Revision: https://reviews.llvm.org/D37034

llvm-svn: 312668

1 files changed, 17 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/LivePhysRegs.cpp b/llvm/lib/CodeGen/LivePhysRegs.cpp
index 870d8cc71c7..ed05acfac4d 100644
--- a/llvm/lib/CodeGen/LivePhysRegs.cpp
+++ b/llvm/lib/CodeGen/LivePhysRegs.cpp
@@ -218,16 +218,22 @@ void LivePhysRegs::addLiveIns(const MachineBasicBlock &MBB) {
 }
 
 void llvm::computeLiveIns(LivePhysRegs &LiveRegs,
-                          const MachineRegisterInfo &MRI,
-                          MachineBasicBlock &MBB) {
+                          const MachineBasicBlock &MBB) {
+  const MachineFunction &MF = *MBB.getParent();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
   const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
-  assert(MBB.livein_empty());
   LiveRegs.init(TRI);
   LiveRegs.addLiveOutsNoPristines(MBB);
-  for (MachineInstr &MI : make_range(MBB.rbegin(), MBB.rend()))
+  for (const MachineInstr &MI : make_range(MBB.rbegin(), MBB.rend()))
     LiveRegs.stepBackward(MI);
+}
 
-  for (unsigned Reg : LiveRegs) {
+void llvm::addLiveIns(MachineBasicBlock &MBB, const LivePhysRegs &LiveRegs) {
+  assert(MBB.livein_empty() && "Expected empty live-in list");
+  const MachineFunction &MF = *MBB.getParent();
+  const MachineRegisterInfo &MRI = MF.getRegInfo();
+  const TargetRegisterInfo &TRI = *MRI.getTargetRegisterInfo();
+  for (MCPhysReg Reg : LiveRegs) {
     if (MRI.isReserved(Reg))
       continue;
     // Skip the register if we are about to add one of its super registers.
@@ -243,3 +249,9 @@ void llvm::computeLiveIns(LivePhysRegs &LiveRegs,
     MBB.addLiveIn(Reg);
   }
 }
+
+void llvm::computeAndAddLiveIns(LivePhysRegs &LiveRegs,
+                                MachineBasicBlock &MBB) {
+  computeLiveIns(LiveRegs, MBB);
+  addLiveIns(MBB, LiveRegs);
+}