[MCP] Remove dead copies from basic blocks with successors. (#86973)

Previously we wouldn't remove dead copies from basic blocks with
successors. The comment said we didn't want to trust the live-in lists.
The comment is very old so I'm not sure if that's still a concern today.

This patch checks the live-in lists and removes copies from
MaybeDeadCopies if they are referenced by any live-ins in any
successors. We only do this if the tracksLiveness property is set. If
that property is not set, we retain the old behavior.

1 files changed, 28 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 65c067e..8dc6781 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -411,6 +411,7 @@ private:
   typedef enum { DebugUse = false, RegularUse = true } DebugType;
 
   void ReadRegister(MCRegister Reg, MachineInstr &Reader, DebugType DT);
+  void readSuccessorLiveIns(const MachineBasicBlock &MBB);
   void ForwardCopyPropagateBlock(MachineBasicBlock &MBB);
   void BackwardCopyPropagateBlock(MachineBasicBlock &MBB);
   void EliminateSpillageCopies(MachineBasicBlock &MBB);
@@ -463,6 +464,22 @@ void MachineCopyPropagation::ReadRegister(MCRegister Reg, MachineInstr &Reader,
   }
 }
 
+void MachineCopyPropagation::readSuccessorLiveIns(
+    const MachineBasicBlock &MBB) {
+  if (MaybeDeadCopies.empty())
+    return;
+
+  // If a copy result is livein to a successor, it is not dead.
+  for (const MachineBasicBlock *Succ : MBB.successors()) {
+    for (const auto &LI : Succ->liveins()) {
+      for (MCRegUnit Unit : TRI->regunits(LI.PhysReg)) {
+        if (MachineInstr *Copy = Tracker.findCopyForUnit(Unit, *TRI))
+          MaybeDeadCopies.remove(Copy);
+      }
+    }
+  }
+}
+
 /// Return true if \p PreviousCopy did copy register \p Src to register \p Def.
 /// This fact may have been obscured by sub register usage or may not be true at
 /// all even though Src and Def are subregisters of the registers used in
@@ -914,10 +931,17 @@ void MachineCopyPropagation::ForwardCopyPropagateBlock(MachineBasicBlock &MBB) {
       Tracker.clobberRegister(Reg, *TRI, *TII, UseCopyInstr);
   }
 
-  // If MBB doesn't have successors, delete the copies whose defs are not used.
-  // If MBB does have successors, then conservative assume the defs are live-out
-  // since we don't want to trust live-in lists.
-  if (MBB.succ_empty()) {
+  bool TracksLiveness = MRI->tracksLiveness();
+
+  // If liveness is tracked, we can use the live-in lists to know which
+  // copies aren't dead.
+  if (TracksLiveness)
+    readSuccessorLiveIns(MBB);
+
+  // If MBB doesn't have succesor, delete copies whose defs are not used.
+  // If MBB does have successors, we can only delete copies if we are able to
+  // use liveness information from successors to confirm they are really dead.
+  if (MBB.succ_empty() || TracksLiveness) {
     for (MachineInstr *MaybeDead : MaybeDeadCopies) {
       LLVM_DEBUG(dbgs() << "MCP: Removing copy due to no live-out succ: ";
                  MaybeDead->dump());