[mono] Reduce number of reallocs inside interp_create_var_explicit (#100801)

During startup we realloc the vars table a lot, so pre-reserve some spare space in it to minimize the number of reallocs

Author: kg

src/mono/mono/mini/interp/transform.c

@@ -419,11 +419,12 @@ interp_create_var_explicit (TransformData *td, MonoType *type, int size)
 	if (td->vars_size == td->vars_capacity) {
 		td->vars_capacity *= 2;
 		if (td->vars_capacity == 0)
-			td->vars_capacity = 2;
+			td->vars_capacity = 16;
 		td->vars = (InterpVar*) g_realloc (td->vars, td->vars_capacity * sizeof (InterpVar));
 	}
 	int mt = mono_mint_type (type);
 	InterpVar *local = &td->vars [td->vars_size];
+	// FIXME: We don't need to do this memset unless we realloc'd, since we malloc0 vars initially
 	memset (local, 0, sizeof (InterpVar));
 
 	local->type = type;
@@ -4342,15 +4343,18 @@ interp_method_compute_offsets (TransformData *td, InterpMethod *imethod, MonoMet
 	int num_args = sig->hasthis + sig->param_count;
 	int num_il_locals = header->num_locals;
 	int num_locals = num_args + num_il_locals;
+	// HACK: Pre-reserve extra space to reduce the number of times we realloc during codegen, since it's expensive
+	// 64 vars * 72 bytes = 4608 bytes. Many methods need less than this
+	int target_vars_capacity = num_locals + 64;
 
 	imethod->local_offsets = (guint32*)g_malloc (num_il_locals * sizeof(guint32));
-	td->vars = (InterpVar*)g_malloc0 (num_locals * sizeof (InterpVar));
+	td->vars = (InterpVar*)g_malloc0 (target_vars_capacity * sizeof (InterpVar));
 	td->vars_size = num_locals;
-	td->vars_capacity = td->vars_size;
+	td->vars_capacity = target_vars_capacity;
 
-	td->renamable_vars = (InterpRenamableVar*)g_malloc (num_locals * sizeof (InterpRenamableVar));
+	td->renamable_vars = (InterpRenamableVar*)g_malloc (target_vars_capacity * sizeof (InterpRenamableVar));
 	td->renamable_vars_size = 0;
-	td->renamable_vars_capacity = num_locals;
+	td->renamable_vars_capacity = target_vars_capacity;
 	offset = 0;
 
 	/*