[WebAssembly] Fix liveness tracking info after drop insertion

[aheejin]

Summary

This fixes liveness tracking information after drop instruction
insertion in ExplicitLocals pass.

When a drop instruction is inserted to drop a dead register operand, the
original operand should be marked not dead anymore because it is now
used by the new drop instruction. And the operand to the new drop
instruction should be marked killed instead. This bug caused some
programs to fail when llc is run with -verify-machineinstrs option.

[aheejin]

test/CodeGen/WebAssembly/explicit-locals.mir

15

The reason we need an empty BB here is, to reveal the bug, the block to which a drop will be inserted should have a predecessor.

lib/Target/WebAssembly/WebAssemblyExplicitLocals.cpp

for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end(); I != E;) {
           auto InsertPt = std::next(MachineBasicBlock::iterator(&MI));
           if (MI.getOpcode() == WebAssembly::IMPLICIT_DEF) {
             MI.eraseFromParent();
             Changed = true;
             continue;
           }
           if (UseEmpty[TargetRegisterInfo::virtReg2Index(OldReg)]) {
             unsigned Opc = getDropOpcode(RC);
+            MachineInstr *Drop =
                 BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc))
                     .addReg(NewReg);
+            // After the drop instruction, this reg operand will not be used
+            Drop->getOperand(0).setIsKill();
           } else {
             unsigned LocalId = getLocalId(Reg2Local, CurLocal, OldReg);
             unsigned Opc = getSetLocalOpcode(RC);
             BuildMI(MBB, InsertPt, MI.getDebugLoc(), TII->get(Opc))
                 .addImm(LocalId)
                 .addReg(NewReg);
           }
           MI.getOperand(0).setReg(NewReg);
+          // This register operand is now being used by the inserted drop
+          // instruction, so make it undead.
+          MI.getOperand(0).setIsDead(false);
           MFI.stackifyVReg(NewReg);
           Changed = true;
         }
       }
 
       // Insert get_locals for any uses that aren't stackified yet.
       MachineInstr *InsertPt = &MI;
       for (MachineOperand &MO : reverse(MI.explicit_uses())) {

test/CodeGen/WebAssembly/explicit-locals.mir

+# RUN: llc -mtriple=wasm32-unknown-unknown -run-pass wasm-explicit-locals %s -o - | FileCheck %s
+
+# When a drop instruction is inserted to drop a dead register operand, the
+# original operand should be marked not dead anymore because it is now used by
+# the new drop instruction. And the operand to the new drop instruction should
+# be marked killed instead.
+---
+name: drop_test
+liveins:
+  - { reg: '$arguments' }
+tracksRegLiveness: true
+body: |
+  bb.0:
+    successors: %bb.1
+    liveins: $arguments
+
+  bb.1:
+  ; predecessors: %bb.0
+    ; CHECK-NOT: dead %{{[0-9]+}}
+    ; CHECK: DROP_I32 killed %{{[0-9]+}}
+    dead %0:i32 = CONST_I32 0, implicit-def dead $arguments, implicit $sp32, implicit $sp64
+    RETURN_VOID implicit-def dead $arguments
+...