[gjs/wip/gcampax/70-arg-cache: 5/10] arg-cache: Improve struct packing and add sizeof guard

[Philip Chimento <pchimento src gnome org>]

commit ac2487489c13bd75de3ac6755bcdadcee644249b
Author: Philip Chimento <philip chimento gmail com>
Date:   Sat Apr 18 14:08:48 2020 -0700

    arg-cache: Improve struct packing and add sizeof guard
    
    Since one of these structs will be allocated for every argument of every
    introspected function, squeezing any bytes out of them is good.

 gi/arg-cache.h | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)
---
diff --git a/gi/arg-cache.h b/gi/arg-cache.h
index b4807302..ff5d4d3b 100644
--- a/gi/arg-cache.h
+++ b/gi/arg-cache.h
@@ -39,22 +39,22 @@
 
 struct GjsFunctionCallState;
 
-typedef struct _GjsArgumentCache {
-    bool (*marshal_in)(JSContext* cx, struct _GjsArgumentCache* cache,
+struct GjsArgumentCache {
+    bool (*marshal_in)(JSContext* cx, GjsArgumentCache* cache,
                        GjsFunctionCallState* state, GIArgument* in_argument,
                        JS::HandleValue value);
-    bool (*marshal_out)(JSContext* cx, struct _GjsArgumentCache* cache,
+    bool (*marshal_out)(JSContext* cx, GjsArgumentCache* cache,
                         GjsFunctionCallState* state, GIArgument* out_argument,
                         JS::MutableHandleValue value);
-    bool (*release)(JSContext* cx, struct _GjsArgumentCache* cache,
+    bool (*release)(JSContext* cx, GjsArgumentCache* cache,
                     GjsFunctionCallState* state, GIArgument* in_argument,
                     GIArgument* out_argument);
-    void (*free)(struct _GjsArgumentCache* cache);
+    void (*free)(GjsArgumentCache* cache);
 
     const char* arg_name;
-    int arg_pos;
     GITypeInfo type_info;
 
+    int arg_pos;
     bool skip_in : 1;
     bool skip_out : 1;
     GITransfer transfer : 2;
@@ -65,17 +65,17 @@ typedef struct _GjsArgumentCache {
         // for explicit array only
         struct {
             int length_pos;
-            GITypeTag length_tag;
+            GITypeTag length_tag : 5;
         } array;
 
         struct {
-            GIScopeType scope;
             int closure_pos;
             int destroy_pos;
+            GIScopeType scope : 2;
         } callback;
 
         struct {
-            GITypeTag number_tag;
+            GITypeTag number_tag : 5;
             bool is_unsigned : 1;
         } number;
 
@@ -101,7 +101,21 @@ typedef struct _GjsArgumentCache {
         // out caller allocates (FIXME: should be in object)
         size_t caller_allocates_size;
     } contents;
-} GjsArgumentCache;
+};
+
+// This is a trick to print out the sizes of the structs at compile time, in
+// an error message:
+// template <int s> struct Measure;
+// Measure<sizeof(GjsArgumentCache)> arg_cache_size;
+
+#if defined(__x86_64__) && defined(__clang__)
+// This isn't meant to be comprehensive, but should trip on at least one CI job
+// if sizeof(GjsArgumentCache) is increased. */
+static_assert(sizeof(GjsArgumentCache) <= 136,
+              "Think very hard before increasing the size of GjsArgumentCache. "
+              "One is allocated for every argument to every introspected "
+              "function.");
+#endif  // x86-64 clang
 
 GJS_JSAPI_RETURN_CONVENTION
 bool gjs_arg_cache_build_arg(JSContext* cx, GjsArgumentCache* self,