So for better or worse (mostly better, but this bug is about the worse), I chose to use GVariant for implementing a userspace filesystem.  For example:

http://git.gnome.org/browse/ostree/tree/src/libostree/ostree-core.h?id=95921bcbf0a6b710dedd107fbacb41090af74ed9#n47

It turns out that the slowest part of my program (currently, after I've fixed various other things), is g_variant_type_string_scan().  Which in turn is called mostly from g_variant_get().  Which in turn is mostly hit when I do this:

http://git.gnome.org/browse/ostree/tree/src/libostree/ostree-core.c?id=95921bcbf0a6b710dedd107fbacb41090af74ed9#n1011

That code gets run *a lot*, because, well, I have a lot of files.  Now, I could probably avoid turning my file objects into GFileInfo, but if I want to use GVariant, I'm not aware of a faster way to parse them.

I think what I'd like is something like:

GVariantReader *g_variant_reader_new ("(uuuu&s@a(ayay))");
void g_variant_reader_get (reader, variant, &uid, &gid, &mode, &rdev,
                 &symlink_target, &ret_xattrs);

Possibly even nicer would be a way to build a relationship between C structures and certain GVariant formats.  For example:

typedef struct {
  guint32 uid;
  guint32 gid;
  ...
  GVariant *xattrs;
} MyFileObject;

MyFileObject mystruct;

g_variant_reader_new ("(uuuu&s@a(ayay))", G_VARIANT_READER_STRUCTURE_BASIC);
g_variant_reader_get (reader, variant, &mystruct);