[Relay][VM] Fix compilation of If-Elses (#5040)

src/relay/backend/vm/compiler.cc

@@ -366,7 +366,9 @@ class VMFunctionCompiler : ExprFunctor<void(const Expr& expr)> {
     this->Emit(Instruction::If(test_register, target_register, 0, 0));
     this->VisitExpr(if_node->true_branch);
 
-    size_t true_register = last_register_;
+    // It saves the result of If-Else expression.
+    auto merge_register = NewRegister();
+    Emit(Instruction::Move(last_register_, merge_register));
     Emit(Instruction::Goto(0));
 
     // Finally store how many instructions there are in the
@@ -378,7 +380,7 @@ class VMFunctionCompiler : ExprFunctor<void(const Expr& expr)> {
     size_t false_register = last_register_;
 
     // In else-branch, override the then-branch register
-    Emit(Instruction::Move(false_register, true_register));
+    Emit(Instruction::Move(false_register, merge_register));
     // Compute the total number of instructions
     // after generating false.
     auto after_false = this->instructions_.size();
@@ -397,7 +399,7 @@ class VMFunctionCompiler : ExprFunctor<void(const Expr& expr)> {
     // Patch the Goto.
     this->instructions_[after_true - 1].pc_offset = (after_false - after_true) + 1;
 
-    this->last_register_ = true_register;
+    this->last_register_ = merge_register;
   }
 
   void EmitShapeFunc(Function func, Array<Expr> inputs, Array<Expr> outputs) {

tests/python/relay/test_vm.py

@@ -142,6 +142,25 @@ def test_simple_if():
     # diff
     check_result([x_data, y_data], y_data, mod=mod)
 
+def test_multiple_ifs():
+    mod = tvm.IRModule({})
+    b = relay.var('b')
+    v0 = relay.var('v0')
+    v1 = relay.var('v1')
+    v2 = relay.var('v2')
+    v3 = relay.var('v3')
+    out = relay.Tuple([v2, v3])
+    out = relay.Let(v3, relay.If(b, v1, v0), out)
+    out = relay.Let(v2, relay.If(b, v0, v1), out)
+    out = relay.Let(v1, relay.Tuple([relay.const(1)]), out)
+    out = relay.Let(v0, relay.Tuple([relay.const(0)]), out)
+    fn = relay.Function([b], out)
+    mod['main'] = fn
+    ctx = tvm.runtime.ndarray.context('llvm', 0)
+    vm = relay.create_executor(ctx=ctx, mod=mod, kind='vm')
+    res = vmobj_to_list(vm.evaluate()(False))
+    assert(res == [1, 0])
+
 def test_simple_call():
     mod = tvm.IRModule({})
     sum_up = relay.GlobalVar('sum_up')