Gentoolto: clang as system compiler

[Althorion]

I will be building Gentoo from scratch soon, and I though about trying to use clang + LTO. Will you be interested in my results?

[InBetweenNames]

Sure! You could post them on this issue if you'd like.

On Sun, Sep 16, 2018, 9:09 AM Althorion, [e-mail] wrote:

I will be building Gentoo from scratch soon, and I though about trying
to use clang + LTO. Will you be interested in my results?

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[Supded]

ThinLTO?

[InBetweenNames]

Yup, same as GCC

On Sun, Sep 16, 2018, 5:32 PM Supded, [e-mail] wrote:

ThinLTO?

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[Supded]

https://clang.llvm.org/docs/ThinLTO.html
Is it same? -flto=thin

[InBetweenNames]

Ahh, I confused this with GCCs own nomenclature. Still, I think it's
interesting and worth trying.

On Mon, Sep 17, 2018, 1:44 AM Supded, [e-mail] wrote:

https://clang.llvm.org/docs/ThinLTO.html
Is it same? -flto=thin

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[Supded]

I think, where must be a per compiler flags, ThinLTO for clang and LTO for gcc.

[Althorion]

So, for now I have successfully build the basic system, with following settings:

make.conf:

CBUILD="x86_64-pc-linux-gnu"
CHOST="x86_64-pc-linux-gnu"
CC="clang"
CXX="clang++"
CFLAGS="-march=native -O3 -pipe -flto=thin"
CXXFLAGS="${CFLAGS}"
LDFLAGS="-Wl,-O3 -Wl,--as-needed"
AR="llvm-ar"
NM="llvm-nm"
RANLIB="llvm-ranlib"
STRIP_MASK="*.a"

USE="-pch bindist clang mmx sse sse2 pgo"
VIDEO_CARDS="amdgpu radeonsi"
INPUT_DEVICES="libinput"
LINGUAS="en en_GB"
L10N="en en-GB"

ACCEPT_KEYWORDS="~amd64"
ACCEPT_LICENSE="*"
ABI_X86="64 32"

MAKEOPTS="-j32 -l32"
EMERGE_DEFAULT_OPTS="--jobs=16  --load-average 32"
PORTAGE_NICENESS=19
PORTAGE_IONICE_COMMAND="ionice -c 3 -p \${PID}"
FEATURES="cgroup parallel-install"

PORTDIR="/usr/portage"
DISTDIR="${PORTDIR}/distfiles"
PKGDIR="${PORTDIR}/packages"

package.env:

# Clang without LTO
app-arch/bzip2 clang-no-lto
app-arch/unzip clang-no-lto
sys-libs/readline clang-no-lto
sys-libs/e2fsprogs-libs clang-no-lto
sys-fs/e2fsprogs clang-no-lto
sys-apps/less clang-no-lto
dev-libs/openssl clang-no-lto
net-firewall/iptables clang-no-lto
sys-apps/busybox clang-no-lto
dev-lang/perl clang-no-lto
dev-lang/python clang-no-lto
dev-libs/popt clang-no-lto
net-misc/openssh clang-no-lto
sys-devel/gettext clang-no-lto
app-crypt/rhash clang-no-lto
sys-apps/texinfo clang-no-lto
sys-libs/binutils-libs clang-no-lto
sys-apps/iproute2 clang-no-lto
sys-apps/pciutils clang-no-lto
app-admin/mcelog clang-no-lto
app-arch/zstd clang-no-lto
sys-fs/btrfs-progs clang-no-lto

# GCC
sys-devel/m4 gcc-standard
dev-libs/mpfr gcc-standard
dev-libs/libgcrypt gcc-standard
sys-libs/glibc gcc-standard
mail-mta/nullmailer gcc-standard

(clang-no-lto is, you’ve guessed it, Clang with -O3 and no LTO and gcc-standard is GCC with -O3 and no LTO—I’m not using LTO on GCC at all at the moment)

Plus, I’ve failed to build sys-apps/man-db on any settings, including GCC with -O0.

Tomorrow hopefully I’ll build the rest of my system (Plasma with systemd on 17.1 profile).

[darkbasic]

It would be interesting to use gcc as the main compiler and use clang for packages like Firefox which fail when using GCC+LTO (but not with clang+LTO)

[Althorion]

So, the short version: it cannot be reasonably done.

Longer version: I had to downgrade the binutils to get the system to even fully build (https://bugzilla.redhat.com/show_bug.cgi?id=1599521), and then I got caught up with a plethora of QObject::connect: signal not found bugs, which seem to be connected to https://bugreports.qt.io/browse/QTBUG-43556. Tracking down each and every one last place where things build fine, but are unresponsive doesn’t seem like a thing I have a time to do, so I’m giving up.

[InBetweenNames]

Ouch! Thanks for taking the time to try it out. It may be best that we recommend clang for specific packages only.