llvm-config build order can break build with parallel build

[llvmbot]

Bugzilla Link	983
Resolution	INVALID
Resolved on	Feb 22, 2010 12:50
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor

Extended Description

After the change was made to use llvm-config to generate the set of libraries
for linking a tool, the dependency on those libraries in the makefile was
broken. You can now rebuild a library that a tool needs and not get a new
executable. For example, make a change to the CBackend and llc won't re-link.

[llvmbot]

Apparently, the tools are dependent on make-config and make-config is dependent
on all the libraries. This isn't optimal. It meas that any time any library
changes all the tools must be rebuilt. It also thwarts things like:

make -C lib/Target/CBackend
make -C tools/llc

which should automatically rebuild llc, but it doesn't.

The tools should depend on the libararies they link in, not on the tool used to
generate that list of libraries linked in.

[llvmbot]

This is bogus. I must have been trying to build a tool that didn't actually
depend on the library that changed.

[llvmbot]

There actually is a problem, but its not exactly the one I thought. If you do this:

make -C lib/Target/CBackend
make -j 3 -C tools

Then you can end up with a broken build. The CBackend library gets updated which
means that llvm-config needs to rebuild. However, that rebuild is normally done
at the end of building lib. With a parallel build llvm-config gets rebuilt at
the same time as other things. However the tools depend on llvm-config to build.
If timing is just right, llvm-config will produce no output (error) and the link
command line for one of the tools will not contain any libraries (or worse)
consequently the tool link either fails outright or you end up with unresolved
symbols (lots of them).

[lattner]

The build in general only works if you do a top-level build. This is what the logic in llvm/Makefile is all
about. Use of make -C or 'makellvm' is only safe if you "know what you're doing". Otherwise, do a top-
level build always.

-Chris

[llvmbot]

I strongly disagree. The makefiles should work regardless of the command given
to them. I agree that if people avoid rebuilding something through their use of
make then they are on their own. However, this situation was safe and the
Makefiles do the wrong thing. The current hack to build tools/llvm-config at the
end of lib instead of the beginning of tools is just, well, wrong. The
tools/Makefile should be smart enough to build llvm-config first (without
letting any other directories build) and then proceed in parallel.

However, I'm not going to fight about it this close to a release. I'll clean
this up after the release.

[lattner]

This is a property of recursive make. It's not some simple thing that can be fixed, though I guess some
crude hacks to address special cases.

-Chris