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

Wed Dec 19 14:46:08 PST 2018

phosek added a comment.

In D54724#1304986 <https://reviews.llvm.org/D54724#1304986>, @dim wrote:

> 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.

I considered both options, the reason I went with conditionals is because we seem to use the same approach for other runtimes like compoler-rt builtins, sanitizers or XRay, but also because in this case there's likely going to be more duplication, e.g. the `switch` to handle either libstdc++ or libc++ would have to be duplicated in FreeBSD, NetBSD and OpenBSD drivers (unless we introduce a common base class for *BSD drivers). I could try implementing it the other way for comparison.

> 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 )

I'm specifically focusing on static case, for shared case this is not relevant since libc++.so.1 already dynamically links all the dependencies.

> 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

We use that solution for libc++abi as well but that doesn't handle other dependencies.

> 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?

Correct, on other platforms you also need `-ldl`, but on *BSD this is part of libc AFAIK. We could also consider surrounding these dependencies with `--push-state --as-needed` and `--pop-state` to avoid linking them in if they're not needed.

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