linux-kernel - Re: [v2,12/18] kbuild: add support for clang LTO

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Date:   Tue, 21 Nov 2017 11:01:52 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Alex Matveev <alxmtvv@...il.com>, Andi Kleen <ak@...ux.intel.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Maxim Kuvyrkov <maxim.kuvyrkov@...aro.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Yury Norov <ynorov@...iumnetworks.com>,
        Matthias Kaehlcke <mka@...omium.org>
Subject: Re: [v2,12/18] kbuild: add support for clang LTO

On Mon, 20 Nov 2017 12:21:52 -0800
Sami Tolvanen <samitolvanen@...gle.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