[PATCH] D54724: [Driver] Automatically include C++ library dependencies

Tue Nov 20 23:00:22 PST 2018

dim added a comment.

In https://reviews.llvm.org/D54724#1303844, @phosek wrote:

> In https://reviews.llvm.org/D54724#1303809, @dim wrote:
>
> > I think this is the wrong direction, placing "common" code in `addCXXStdlibLinkDeps`, which then has all kinds of ugly ifs and switches for different OSes?  Let different OSes handle this in their own way, maybe.
>
>
> You mean handling this in individual `ToolChain` subclasses? I've considered doing that. The reason I went with a single method is because we already use this approach for other types of runtimes like sanitizers, XRay, etc. There'd be still some duplication for handling different C++ libraries, but I'd be fine changing the implementation to use that approach if there's a strong preference for doing so.

I think I understand the motivation, it's just that the toolchain classes were split up specifically to avoid `if(OS==foo) ... else if(OS==bar) ... else if(OS==baz)` mazes.  So this feels a little like a regression in that sense, putting back the OS-specific ifs in a common function.  But I guess it is a pattern that pops up more often.

>> Until now I have not encountered an issue where I needed to add -lpthread to the link command line for a static C++ application; I think that is only needed when you actually use threading primitives?  But still, I would rather see this in the OS-dependent handling functions.
> 
> Have you been using libc++ or just libstdc++? libstdc++ is cheating a bit, because it relies on threading functions that are provided by libgcc (which in turn call into pthread), so -lpthread is not needed when using libstdc++, even when you link it statically. libc++ doesn't rely on these functions because it supports different builtin libraries (like compiler-rt) hence having to specify -lpthread explicitly.

We've been using libc++ by default since FreeBSD 10.x, and all older versions are now EOLed.  That said, we have always used a linker script to link in libc++.so:

  $ cat /usr/lib/libc++.so
  /* $FreeBSD: projects/clang700-import/lib/libc++/libc++.ldscript 305102 2016-08-31 00:11:35Z bapt $ */
  GROUP ( /usr/lib/libc++.so.1 /usr/lib/libcxxrt.so )

and for the static case, we've always just added the relevant .o files from libcxxrt into libc++.a:

  $ ar tv /usr/lib/libc++.a
  rw-r--r--       0/0         21720 Jan  1 01:00 1970 variant.o
  rw-r--r--       0/0         61648 Jan  1 01:00 1970 valarray.o
  rw-r--r--       0/0         18224 Jan  1 01:00 1970 utility.o
  rw-r--r--       0/0         10656 Jan  1 01:00 1970 typeinfo.o
  rw-r--r--       0/0        198080 Jan  1 01:00 1970 strstream.o
  rw-r--r--       0/0         59264 Jan  1 01:00 1970 shared_mutex.o
  rw-r--r--       0/0        231552 Jan  1 01:00 1970 regex.o
  rw-r--r--       0/0         96200 Jan  1 01:00 1970 random.o
  rw-r--r--       0/0         25240 Jan  1 01:00 1970 optional.o
  rw-r--r--       0/0        239552 Jan  1 01:00 1970 iostream.o
  rw-r--r--       0/0         17728 Jan  1 01:00 1970 functional.o
  rw-r--r--       0/0        219984 Jan  1 01:00 1970 debug.o
  rw-r--r--       0/0         58256 Jan  1 01:00 1970 chrono.o
  rw-r--r--       0/0         35672 Jan  1 01:00 1970 charconv.o
  rw-r--r--       0/0         21280 Jan  1 01:00 1970 bind.o
  rw-r--r--       0/0         24312 Jan  1 01:00 1970 any.o
  rw-r--r--       0/0       1010288 Jan  1 01:00 1970 algorithm.o
  rw-r--r--       0/0         42944 Jan  1 01:00 1970 hash.o
  rw-r--r--       0/0         45152 Jan  1 01:00 1970 cxxrt_typeinfo.o
  rw-r--r--       0/0        278784 Jan  1 01:00 1970 cxxrt_libelftc_dem_gnu3.o
  rw-r--r--       0/0         17704 Jan  1 01:00 1970 cxxrt_stdexcept.o
  rw-r--r--       0/0        224664 Jan  1 01:00 1970 thread.o
  rw-r--r--       0/0        218392 Jan  1 01:00 1970 future.o
  rw-r--r--       0/0         31456 Jan  1 01:00 1970 exception.o
  rw-r--r--       0/0       4604424 Jan  1 01:00 1970 locale.o
  rw-r--r--       0/0         27216 Jan  1 01:00 1970 vector.o
  rw-r--r--       0/0        102712 Jan  1 01:00 1970 mutex.o
  rw-r--r--       0/0         63872 Jan  1 01:00 1970 memory.o
  rw-r--r--       0/0       1261888 Jan  1 01:00 1970 ios.o
  rw-r--r--       0/0         69528 Jan  1 01:00 1970 condition_variable.o
  rw-r--r--       0/0        181368 Jan  1 01:00 1970 system_error.o
  rw-r--r--       0/0       2006648 Jan  1 01:00 1970 string.o
  rw-r--r--       0/0        105408 Jan  1 01:00 1970 stdexcept.o
  rw-r--r--       0/0         37552 Jan  1 01:00 1970 new.o
  rw-r--r--       0/0         10280 Jan  1 01:00 1970 cxxrt_memory.o
  rw-r--r--       0/0          4880 Jan  1 01:00 1970 cxxrt_auxhelper.o
  rw-r--r--       0/0          2872 Jan  1 01:00 1970 cxxrt_terminate.o
  rw-r--r--       0/0        123504 Jan  1 01:00 1970 cxxrt_exception.o
  rw-r--r--       0/0         18136 Jan  1 01:00 1970 cxxrt_dynamic_cast.o
  rw-r--r--       0/0          5696 Jan  1 01:00 1970 cxxrt_guard.o

But I can link almost all C++ programs statically without having to add `-lpthread`.  I guess you only really need it if you start using the multithreading primitives?  I'm sort of hesitant to always add if, if it is not always needed.  @emaste, what's your opinion on this?

Repository:
  rC Clang

https://reviews.llvm.org/D54724