linux-kernel - Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for LLVM=1 LLVM

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Message-ID: <CAHk-=wjDdDPDa6mfoC-QM=NZULsmQfcUbF2RdwMq0J4Ztm+UAw@mail.gmail.com>
Date:   Tue, 20 Jul 2021 14:11:16 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christoph Hellwig <hch@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH v2 2/2] Makefile: infer CROSS_COMPILE from SRCARCH for
 LLVM=1 LLVM_IAS=1

On Tue, Jul 20, 2021 at 1:52 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> >     -CC = $(CROSS_COMPILE)gcc
> >     +CC = $(CROSS_COMPILE)clang
>
> So you haven't been using LLD... :( (imagine using more than one
> thread to link, and being faster than ld.gold)

I started out just building my own clang - nothing else.

And now that I'm using the distro clang, the linker isn't even _close_
to a performance issue. Since clang is a big, slow, bloated pig.

I posted profiles and performance numbers of clang being three times
slower than gcc, and it was all just clang itself, not the linker. All
due to insanely expensive startup costs.

I suspect a lot of clang users build bloated C++ code where the time
to compile a single object file is so big that the startup costs don't
even show up. But for the (fairly) lean C kernel header files that
don't bring in millions of lines of headers for some template library,
startup costs are a big deal.

So honestly, I don't care at all about "imagine being faster than ld.gold".

I can only _dream_ of clang itself not being 3x slower than gcc.

              Linus