[Mesa-dev] [PATCH] configure: Trust LLVM >= 4.0 llvm-config --libs for shared libraries

[Jan Vesely]

On Mon, 2017-08-14 at 17:11 +0900, Michel Dänzer wrote:
> On 11/08/17 02:13 AM, Jan Vesely wrote:
> > On Thu, 2017-08-10 at 17:45 +0100, Emil Velikov wrote:
> > > 
> > > Topic for another thread:
> > > There's been a handful of issues reported with BUILD_SHARED_LIBS=ON.
> > > Should we consider that unsupported setup and error out?
> 
> I don't really see the point / gain in that.
> 
> 
> > please no.
> > BUILD_SHARED_LIBS=ON is a must for development setup
> > debug build of clang + llvm takes 1.4GB, each. Having a ~2.8GB library,
> > is a really bad idea for testing,
> 
> Not sure what testing you're referring to exactly, but IME application
> startup takes on the order of hundreds of ms longer when linking against
> the individual shared libraries than when linking against the single
> one. It's one reason why distros building with BUILD_SHARED_LIBS=ON is
> just silly.
> 
> 
> > not to mention linking time that goes well into minutes for even a
> > minor change in LLVM.
> 
> It can certainly take a long time, but I don't remember it taking
> minutes. Are you counting the time for linking other libraries / tools
> against libLLVM-*.0*.so as well? You can avoid that by explicitly
> building only the library, e.g.:
> 
>  ninja [...] lib/libLLVM-6.0svn.so
> 
> Also, using the gold or lld linker might help.

The problem is that setting both
BUILD_SHARED_LIBS=ON
LLVM_BUILD_LLVM_DYLIB=ON
Does not work.

BUILD_SHARED_LIBS=OFF results in every component creating its own
static library. These are used be default when linking LLVM tools. This
configuration results in 15G build/lib and another 15G build/bin.

simple test:
touch lib/Target/AMDGPU/R600ISelLowering.cpp
make -j3 (using more jobs exhausts available memory since a linking
process easily takes 7-9GB).
takes ~15mins to complete, and that's without the install step.

using LLVM_LINK_LLVM_DYLIB=ON improves the test time to ~7m (and
reduces build/bin to ~4GB)

but it still does not compare to ~40s that it takes BUILD_SHARED_LIBS
to finish the same test.

Jan


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