[Codegen] Skip wb emission when they are no children objects

Author: vchuravy

src/cgutils.cpp

@@ -3239,6 +3239,9 @@ static void emit_write_barrier(jl_codectx_t &ctx, Value *parent, Value *ptr)
 
 static void emit_write_barrier(jl_codectx_t &ctx, Value *parent, ArrayRef<Value*> ptrs)
 {
+    // if there are no child objects we can skip emission
+    if (ptrs.empty())
+        return;
     SmallVector<Value*, 8> decay_ptrs;
     decay_ptrs.push_back(maybe_decay_untracked(ctx, emit_bitcast(ctx, parent, T_prjlvalue)));
     for (auto ptr : ptrs) {

src/codegen.cpp

@@ -4972,7 +4972,7 @@ static Value *get_current_task(jl_codectx_t &ctx)
     const int ptls_offset = offsetof(jl_task_t, gcstack);
     return ctx.builder.CreateInBoundsGEP(
         T_pjlvalue, emit_bitcast(ctx, ctx.pgcstack, T_ppjlvalue),
-        ConstantInt::get(T_size, -ptls_offset / sizeof(void *)),
+        ConstantInt::get(T_size, -(ptls_offset / sizeof(void *))),
         "current_task");
 }