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Date: Tue, 21 Nov 2017 11:01:52 +1000
From: Nicholas Piggin <npiggin@gmail.com>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Alex Matveev <alxmtvv@gmail.com>, Andi Kleen <ak@linux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Greg Hackmann <ghackmann@google.com>,
        Kees Cook <keescook@chromium.org>,
        linux-arm-kernel@lists.infradead.org, linux-kbuild@vger.kernel.org,
        linux-kernel@vger.kernel.org, Mark Rutland <mark.rutland@arm.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Yury Norov <ynorov@caviumnetworks.com>,
        Matthias Kaehlcke <mka@chromium.org>
Subject: Re: [v2,12/18] kbuild: add support for clang LTO
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On Mon, 20 Nov 2017 12:21:52 -0800
Sami Tolvanen <samitolvanen@google.com> wrote:

> On Sat, Nov 18, 2017 at 01:21:39PM +1000, Nicholas Piggin wrote:
> > Do you have any kind of numbers for this, out of curiosity? Binary
> > size, performance, build time?  
> 
> I don't have performance numbers to share. Are there any specific
> benchmarks you'd be interested in seeing? Build time typically
> increases with LTO and in my experience, binary size tends to increase
> by ~10-15% as well.

By deduction, then you must see some performance improvement? :) 

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.

> 
> > Why is this needed? It would have been nice to get rid of the
> > !THIN_ARCHIVES option if you can make the patches work with the thin
> > archives paths.  
> 
> I believe LLVMgold doesn't know how to deal with an archive of LLVM IR
> files, but I can certainly use thin archives as an index and extract
> the path names for linking. I'll look into it.

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.

Thanks,
Nick