Program size, linking matter, and static this()

[Andrei Alexandrescu]

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.

Here's a list of all files in std using static cdtors:

std/__fileinit.d
std/concurrency.d
std/cpuid.d
std/cstream.d
std/datebase.d
std/datetime.d
std/encoding.d
std/internal/math/biguintcore.d
std/internal/math/biguintx86.d
std/internal/processinit.d
std/internal/windows/advapi32.d
std/mmfile.d
std/parallelism.d
std/perf.d
std/socket.d
std/stdiobase.d
std/uri.d

The majority of them don't do a lot of work and are not much used inside 
phobos, so they don't blow up the executable. The main one that could 
receive some attention is std.datetime. It has a few static ctors and a 
lot of classes. Essentially just importing std.datetime or any std 
module that transitively imports std.datetime (and there are many of 
them) ends up linking in most of Phobos and blows the size up from the 
218KB baseline to 700KB.

Jonathan, could I impose on you to replace all static cdtors in 
std.datetime with lazy initialization? I looked through it and it 
strikes me as a reasonably simple job, but I think you'd know better 
what to do than me.

A similar effort could be conducted to reduce or eliminate static cdtors 
from druntime. I made the experiment of commenting them all, and that 
reduced the size of the baseline from 218KB to 200KB. This is a good 
amount, but not as dramatic as what we can get by working on std.datetime.


Thanks,

Andrei