CodeGen: Fix livein calculation in MachineBasicBlock splitAt

Fix and simplify computation of liveins for new block.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D88535

Author: perlfu

llvm/lib/CodeGen/MachineBasicBlock.cpp

@@ -961,9 +961,10 @@ MachineBasicBlock *MachineBasicBlock::splitAt(MachineInstr &MI,
   if (UpdateLiveIns) {
     // Make sure we add any physregs we define in the block as liveins to the
     // new block.
+    MachineBasicBlock::iterator Prev(&MI);
     LiveRegs.init(*MF->getSubtarget().getRegisterInfo());
     LiveRegs.addLiveOuts(*this);
-    for (auto I = rbegin(), E = SplitPoint.getReverse(); I != E; ++I)
+    for (auto I = rbegin(), E = Prev.getReverse(); I != E; ++I)
       LiveRegs.stepBackward(*I);
   }
 

llvm/test/CodeGen/AMDGPU/si-lower-control-flow.mir

@@ -233,7 +233,6 @@ body:             |
   ; GCN:   $exec = S_OR_B64_term $exec, [[COPY3]], implicit-def $scc
   ; GCN: bb.3:
   ; GCN:   successors: %bb.2(0x80000000)
-  ; GCN:   liveins: $vgpr3
   ; GCN:   $vgpr3 = V_MOV_B32_e32 0, implicit $exec
   ; GCN:   $sgpr4_sgpr5 = S_MOV_B64 32
   ; GCN: bb.2: