Track liveness of unallocatable, unreserved registers in machine DCE.

Reserved registers are unpredictable, and are treated as always live by machine
DCE.

Allocatable registers are never reserved, and can be used for virtual registers.

Unreserved, unallocatable registers can not be used for virtual registers, but
otherwise behave like a normal allocatable register. Most targets only have
the flag register in this set.

llvm-svn: 112649

1 files changed, 8 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
index 138b83d4b66..318d922adeb 100644
--- a/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
+++ b/llvm/lib/CodeGen/DeadMachineInstructionElim.cpp
@@ -80,9 +80,8 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
   TRI = MF.getTarget().getRegisterInfo();
   TII = MF.getTarget().getInstrInfo();
 
-  // Compute a bitvector to represent all non-allocatable physregs.
-  BitVector NonAllocatableRegs = TRI->getAllocatableSet(MF);
-  NonAllocatableRegs.flip();
+  // Treat reserved registers as always live.
+  BitVector ReservedRegs = TRI->getReservedRegs(MF);
 
   // Loop over all instructions in all blocks, from bottom to top, so that it's
   // more likely that chains of dependent but ultimately dead instructions will
@@ -91,9 +90,8 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
        I != E; ++I) {
     MachineBasicBlock *MBB = &*I;
 
-    // Start out assuming that all non-allocatable registers are live
-    // out of this block.
-    LivePhysRegs = NonAllocatableRegs;
+    // Start out assuming that reserved registers are live out of this block.
+    LivePhysRegs = ReservedRegs;
 
     // Also add any explicit live-out physregs for this block.
     if (!MBB->empty() && MBB->back().getDesc().isReturn())
@@ -104,6 +102,10 @@ bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
           LivePhysRegs.set(Reg);
       }
 
+    // FIXME: Add live-ins from sucessors to LivePhysRegs. Normally, physregs
+    // are not live across blocks, but some targets (x86) can have flags live
+    // out of a block.
+
     // Now scan the instructions and delete dead ones, tracking physreg
     // liveness as we go.
     for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),