Nice article! In fact, linker time is really annoying , specially with LTCG, when code gen. is required even after a small change.

Probably the whole "c++ -> obj->linker" architecture is old and is the cause of most c++ and linker times, isn't it?

It's great that you're looking at the linker as it's a big bottleneck on larger projects. I'm in games development and the link time is easily the biggest time waster.

Parallelising the pdb generation seems like a good start, but I think it would also be good if you could review the algorithmic complexity of the debug information compilation and symbol management as well to see if there are any easy wins there. Especially when you start using templates you end up with lots and lots of redundant debug data and empirically it seems to affect link quite a bit.

Now, if you compile a global c++ object in a cpp, it will be linked into the exe if added to the linker as an obj file.

When this obj file is taken into a lib and we use that lib to link with the exe then the linker wont choose it (there is no reference to it from outside, because the static object registers itself as a service into a singleton type manager).

Now there is not even an option to the linker to take all obj from a given lib file.

There is a big hack in the studio against this:

Linker/General/Use Library Dependency Inputs

But this is a joke when using big libraries...

Even ATL and MFC uses "workarounds", because of this problem (define in the header, use the first link attributes in the header files).

Only per object file (cpp) __pragma init_seg(...) available to set the initialization order.

No way to set this per c++ object basis like:

CMyObject __declspec(linkinit(56)) MyObject;

#pragma comment(lib, "dir/file.lib") does not work as a relative path:

only simple "file.lib" would be searched in LIBPATH-es,

"dir/file.lib" automatically considered as global path...

A second thread for writing the PDB is nice, but still won't make much use of today's multi-core CPUs. Is there no way the bulk of the work can be split across many threads? It's annoying to see Task Manager reporting 12.5% CPU usage for link.exe on a Core i7... still, I'm glad it's getting some attention.

I saw this a few days ago: http://stackoverflow.com/questions/1401342/why-is-linker-optimization-so-poor/1401374#1401374 and I think it's actually a good question. How come LTCG takes significantly longer than compiling if you simply include all the same code into a single translation unit?

And what can you do (are you doing?) to improve this?

Do any of the test case include machine generated source code files?  Please provide some test performance numbers when generating a lexer/parser using yacc or another compiler generator for a decently large language (c++, c, pascal, etc).

Most comiler/linker pairs do miserably with generated code given the tens of thousands of symbol names and large amounts of goto code.

Unable to turn of warnings for 3rd party libraries:

xmaencoder.lib(wmachmtx.obj) : warning LNK4099: PDB 'wmachmtx.pdb' was not found with 'C:\Program Files (x86)\Microsoft Xbox 360 SDK\lib\win32\vs2008\xmaencoder.lib' or at 'C:\projects\Tachyon\_Lib\DebugWin32\wmachmtx.pdb'; linking object as if no debug info

xmaencoder.lib(resampthreads.obj) : warning LNK4099: PDB 'audiosrc.pdb' was not found with 'C:\Program Files (x86)\Microsoft Xbox 360 SDK\lib\win32\vs2008\xmaencoder.lib' or at 'C:\projects\Tachyon\_Lib\DebugWin32\audiosrc.pdb'; linking object as if no debug info

xmaencoder.lib(aresample.obj) : warning LNK4099: PDB 'audiosrc.pdb' was not found with 'C:\Program Files (x86)\Microsoft Xbox 360 SDK\lib\win32\vs2008\xmaencoder.lib' or at 'C:\projects\Tachyon\_Lib\DebugWin32\audiosrc.pdb'; linking object as if no debug info

xmaencoder.lib(gresample.obj) : warning LNK4099: PDB 'audiosrc.pdb' was not found with 'C:\Program Files (x86)\Microsoft Xbox 360 SDK\lib\win32\vs2008\xmaencoder.lib' or at 'C:\projects\Tachyon\_Lib\DebugWin32\audiosrc.pdb'; linking object as if no debug info

Hi

1. "(global c++ object)You can use /INCLUDE switch to force a symbol available in the final image."

I don't want to /INCLUDE anything by hand, the main idea with static c++ objects, that I take into an object file and works (at least should work)...

To be more clear, the project has 1200+ cpp files, and we do such (small) objects by macro to register "services".

I would really need a force link switch to the linker like this: --force-all-objects-on mylib.lib --force-all-objects-off

2. "(#pragma comment)Yes, linker will not search in LIBPATH if a directory is specified for security reasons. For example, if user specifies ..\..\aa.lib, this might lead to some private libraries that are linked."

Ok, I have never said to do that. I would like to use like at headers (#include "lib/header.h"): #pragma comment(lib, "dir/file.lib").

There is a "workaround" now for this:

#pragma comment(lib, __FILE__ "/../../../mylibdir/mylib.lib")

It doesn't seems to be nicer...

3. "(LNK4099)Sorry, linker does not provide an option to ignore any warnings silently."

Yes, and it's quite silly, that I can't turn off the warnings of the Microsoft's own libraries... :)

Avoid the java 1 class per file scheme.  It is quite slow over the long run considering it takes a few extra seconds over and over throughout the life of the project.

Avoid making hundreds of extra 1 liner classes just for completeness.  They can be wrapped into a large catch all class (e.g., a large class that does data type conversions from machine specific to not machine specific basic data types).

Avoid having 10 different overloaded versions of the same method.  Make 1 method that does all of the work and have optional pareters to provide the equivalent signature to the caller.  (NB: I've stopped using optional arguments in the last 2 years since I've seen offshore written code that only supplies the minimal set of parameters to a function instead of calling the function the right way for the time it is needed.  I think this is the . intellisense lazy method.)

Avoid creating unnecessary names and extra function calls just for coding style.

.NET, java, reflection and modern tools promote an explosion of names and identifiers but that, while simple in the narror scope, is detrimental in the long run.  More names/symbols means more costly and harder to progress the project in the future.

"Most users normally specify /debug when linking, irrespective of whether the application is built under “debug” or “release” configuration."

Are you sure that's actually intensional or is it more likely that "most users" just didn't notice that MS added /debug to the default project templates a few versions ago?

@CMWoods,

It is standard practice in most professional development shops to store PDBs for all released binaries, even if the PDBs themselves are not released, for help in debugging. Adding /debug to the default template was an effect, not a cause.

A few observations:

1. PDB vs no-PDB difference tells that the binary write is much faster than PDB write. Why? I suppose you just have to dump the tables already built in memory. Parallelising them would save a pittance. A reason for longer PDB write could be that the current linker creates the PDB file as compressed. If you issue writes in pieces smaller than the compression unit (64K or so?) it could cause multiple compress-decompress rounds. I hope the linker is not calling WriteFile per each PDB record; that would just kill the performance.

2 Typically, it makes sense to avoid writing two large files in parallel in small pieces. The most effective approach is to build the data in memory and write it in one shot. With the modern memory sizes, it's feasible. And NO, a file mapping doesn't beat that; forget it. Especially a mapped compressed file (PDB).

3. If you deal with a lot of small files, it makes sense to read them to memory, as many as possible. To avoid disk thrashing, open a bunch of them at once, and then read; don't do one by one.