[cmake] Append LLVM_VERSION_SUFFIX to SOVERSION

[mgorny]

Summary

Append LLVM_VERSION_SUFFIX to SOVERSION. This makes it possible
to use the suffix to differentiate binary-incompatible versions
of LLVM built via BUILD_SHARED_LIBS.

We are planning to use this to temporarily preserve ABI-incompatible
variants of LLVM while switching the system between them, e.g. when
rebuilding the system to use libc++. Normally this would mean that once
LLVM is rebuilt using libc++ all the reverse dependencies become
immediately broken. Using a distinct SOVERSION allows us to preserve
the ABI compatibility before all the packages are rebuilt.

[beanz]

Who is the "we" you mention in your commit message? This seems to me like a patch very specific to a single downstream user, and you describe it as temporary, so I'm not convinced we should take it upstream.

Please elaborate on who will use it, what for, and how long you foresee needing it.

[mgorny]

We = Gentoo. What for is already described = to install libstdc++ and libc++ versions of LLVM simultaneously for rebuild period. How long = for the time being, or until we find a better working solution.

[beanz]

Ok. Rather than adding a new option I think you should just add LLVM_VERSION_SUFFIX to the SOVERSION property, then that should work for you.

[mgorny]

Hmm, that's an interesting option too. I was thinking of using LLVM_VERSION_SUFFIX originally but it obviously did not go to SOVERSION. Can this have any unforeseen consequences? Should VERSION become *major+suffix-0-minor* then?

[beanz]

I think leaving VERSION as it is today, but changing SOVERSION to be ${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX} makes the most sense to me. That should meet your needs.

[mgorny]

I'm worried that having a different first component of VERSION and SOVERSION could confuse some tools like ldconfig. I'm going to see if we can solve it via different install prefixes with RPATHs first.

[mgorny]

@beanz, updated as requested.

[mgorny]

Ping.

[mgorny]

Erm, louder ping?

[beanz]

Looks good. Sorry for the extended delay in review.

[mgorny]

No problem. Thanks for reviewing both of those patches!

llvm/trunk/cmake/modules/AddLLVM.cmake

if(ARG_SHARED)
     endif()
 
     # Set SOVERSION on shared libraries that lack explicit SONAME
     # specifier, on *nix systems that are not Darwin.
     if(UNIX AND NOT APPLE AND NOT ARG_SONAME)
       set_target_properties(${name}
         PROPERTIES
         # Since 4.0.0, the ABI version is indicated by the major version
-        SOVERSION ${LLVM_VERSION_MAJOR}
+        SOVERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX}
         VERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_SUFFIX})
     endif()
   endif()
 
   if(ARG_MODULE OR ARG_SHARED)
     # Do not add -Dname_EXPORTS to the command-line when building files in this
     # target. Doing so is actively harmful for the modules build because it
     # creates extra module variants, and not useful because we don't use these