On Fri, 16 Dec 2011 13:29:18 -0500, Andrei Alexandrescu
<seewebsiteforem...@erdani.org> wrote:
Hello,
Late last night Walter and I figured a few interesting tidbits of
information. Allow me to give some context, discuss them, and sketch a
few approaches for improving things.
A while ago Walter wanted to enable function-level linking, i.e. only
get the needed functions from a given (and presumably large) module. So
he arranged things that a library contains many small object "files"
(that actually are generated from a single .d file and never exist on
disk, only inside the library file, which can be considered an archive
like tar). Then the linker would only pick the used object "files" from
the library and link those in. Unfortunately that didn't have nearly the
expected impact - essentially the size of most binaries stayed the same.
The mystery was unsolved, and Walter needed to move on to other things.
One particularly annoying issue is that even programs that don't
ostensibly use anything from an imported module may balloon inexplicably
in size. Consider:
import std.path;
void main(){}
This program, after stripping and all, has some 750KB in size. Removing
the import line reduces the size to 218KB. That includes the runtime
support, garbage collector, and such, and I'll consider it a baseline.
(A similar but separate discussion could be focused on reducing the
baseline size, but herein I'll consider it constant.)
What we'd simply want is to be able to import stuff without blatantly
paying for what we don't use. If a program imports std.path and uses no
function from it, it should be as large as a program without the import.
Furthermore, the increase should be incremental - using 2-3 functions
from std.path should only increase the executable size by a little, not
suddenly link in all code in that module.
But in experiments it seemed like program size would increase in sudden
amounts when certain modules were included. After much investigation we
figured that the following fateful causal sequence happened:
1. Some modules define static constructors with "static this()" or
"static shared this()", and/or static destructors.
2. These constructors/destructors are linked in automatically whenever a
module is included.
3. Importing a module with a static constructor (or destructor) will
generate its ModuleInfo structure, which contains static information
about all module members. In particular, it keeps virtual table pointers
for all classes defined inside the module.
4. That means generating ModuleInfo refers all virtual functions defined
in that module, whether they're used or not.
5. The phenomenon is transitive, e.g. even if std.path has no static
constructors but imports std.datetime which does, a ModuleInfo is
generated for std.path too, in addition to the one for std.datetime. So
now classes inside std.path (if any) will be all linked in.
6. It follows that a module that defines classes which in turn use other
functions in other modules, and has static constructors (or includes
other modules that do) will baloon the size of the executable suddenly.
There are a few approaches that we can use to improve the state of
affairs.
A. On the library side, use static constructors and destructors
sparingly inside druntime and std. We can use lazy initialization
instead of compulsively initializing library internals. I think this is
often a worthy thing to do in any case (dynamic libraries etc) because
it only does work if and when work needs to be done at the small cost of
a check upon each use.
B. On the compiler side, we could use a similar lazy initialization
trick to only refer class methods in the module if they're actually
needed. I'm being vague here because I'm not sure what and how that can
be done.
I disagree with this assessment. It's good to know the cause of the
problem, but let's look at the root issue -- reflection. The only reason
to include class information for classes not being referenced is to be
able to construct/use classes at runtime instead of at compile time. But
if you look at D's runtime reflection capabilities, they are quite poor.
You can only construct a class at runtime if it has a zero-arg constructor.
So essentially, we are paying the penalty of having runtime reflection in
terms of bloat, but get very very little benefit.
I think there are two things that need to be considered:
1. We eventually should have some reasonably complete runtime reflection
capability
2. Runtime reflection and shared libraries go hand-in-hand. With shared
library support, the bloat penalty isn't nearly as significant.
I don't think the right answer is to avoid using features of the language
because the compiler/runtime has some design deficiencies. At some point
these deficiencies will be fixed, and then we are left with a library that
has seemingly odd design choices that we can't change.
-Steve
