On Sat, 17 Dec 2011 07:09:50 +0100, Martin Nowak <d...@dawgfoto.de> wrote:
On Fri, 16 Dec 2011 19:29:18 +0100, 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.
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
We'd need the linker to do anything of this. Unreferenced symbols should
be outputted using
kind of vague linkage (multiobj partly does this).
I-reference-everything stuff link ModuleInfos
should only create weak references. This includes that localClasses
More concrete if we'd output weak defined symbols (null) for what is
referenced
by a ModuleInfo then the linker should not open further object files to
find a definition. But if another definition is linked in it will replace
the weak definition. The program would then need to skip the dummy symbols
(null)
at runtime.
might contain only
part of the actual module. People can use the designated export
attribute to forcefully
output unused symbols.
