Re: [PATCH 01/20] Hexagon HVX (target/hexagon) README

[Rob Landley]

On 7/5/21 6:34 PM, Taylor Simpson wrote:
> Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>
> ---
>  target/hexagon/README | 83 
> ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 82 insertions(+), 1 deletion(-)

I'm poking at the hexagon toolchain build script you checked into the test
directory, which boils down to (starting with):

git clone https://github.com/llvm/llvm-project
mkdir build-llvm
cd build-llvm
cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_INSTALL_PREFIX=$(readlink -f ../llvm) -DLLVM_ENABLE_LLD=ON \
  -DLLVM_TARGETS_TO_BUILD="Hexagon" -DLLVM_ENABLE_PROJECTS="clang;lld" \
  $(readlink -f ../llvm-project/llvm)

Except the LLVM_ENABLE_LLD part breaks with a standard debian/devuan x86-64 host
toolchain because it ONLY works with host llvm, and apparently only a pretty
current one at that:

  https://github.com/tensorflow/mlir-hlo/issues/4

(Devuan Beowulf only packages lld-7, not lld-10.)

I'm currently building:

cmake -G Ninja -DCMAKE_BUILD_TYPE=Release \
  -DCMAKE_INSTALL_PREFIX=$(readlink -f ../llvm) -DLLVM_ENABLE_PROJECTS="lld" \
  $(readlink -f ../llvm-project/llvm)
ninja all install

On the theory that should give me an lld-git I can play $PATH games and re-run
the other build with, but my QUESTION is what does the LLVM_ENABLE_LLD=potato
actually accomplish here? Is it a sanitizing step or is there something about
building with gcc's lld that's known to break the hexagon toolchain? If I just
omit it (to avoid building lld _twice_) will I (probably) get a working hexagon
toolchain? (Assuming I do the musl and headers-install builds and so on?)

What's the _issue_ here that this config symbol addresses?

Thanks,

Rob