[v2,12/18] kbuild: add support for clang LTO

[Sami Tolvanen]

On Tue, Nov 21, 2017 at 11:01:52AM +1000, Nicholas Piggin wrote:
> I just wonder are you doing this because there is some worthwhile
> performance gain? Or to enable more testing and development of LTO?
> Any clues for why a user would want to enable it.

I'm primarily interested in CFI, which with clang requires LTO; not
for the optimizations, but for source visibility. We do expect to see
performance improvements with LTO though, especially if combined with
PGO.

> Thanks, if you could. Possibly file a request with LLVMgold too, it
> seems to be that toolchain support for archives is quite strong, so it
> will be good to keep pushing for that.

It turns out LLVMgold is fine with mixed IR/object archives, but we need
to use llvm-ar to generate symbol tables for them, and there are some
compatibility issues with objdump that I had to work around. I'll send
v3 for review once I receive some feedback for the clang/gold patches we
need first.

Sami