Difference between pages "Xfce" and "Clang"

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== About Xfce ==
+
==Introduction==
 +
LLVM can be used as an alternative to GNU's compiler, GCC. The main benefit of using LLVM compilers instead of GCC is their lower memory usage, faster compile time and better diagnostics. There are some Benchmarks on the [http://clang.llvm.org/features.html#performance Clang] and [http://www.phoronix.com/scan.php?page=article&item=llvm3_gcc_open64 Phoronix] homepages.
  
''Xfce is a lightweight desktop environment for UNIX-like operating systems. It aims to be fast and low on system resources, while still being visually appealing and user friendly.
+
It may happen that some programs do not compile (like glibc) because they depend on GCC-specific language extensions [http://gcc.gnu.org/onlinedocs/gcc/C-Extensions.html] (this is why the whole BSD code can be compiled with LLVM but some GNU code cannot) or segfault after successful compilation with LLVM (like xorg-server) but after following this guide, the system will still be able to compile packages with gcc. So if something goes wrong, it can be switched back to gcc for the particular package by uncommenting lines in /etc/make.conf and the bug should be reported.
  
''Xfce embodies the traditional UNIX philosophy of modularity and re-usability. It consists of a number of components that provide the full functionality one can expect of a modern desktop environment. They are packaged separately and you can pick among the available packages to create the optimal personal working environment.
+
LLVM's C/C++ frontends clang and clang++ version 3.0 are stable enough to be self-hosting [http://blog.llvm.org/2010/02/clang-successfully-self-hosts.html] and compile Boost [http://blog.llvm.org/2010/05/clang-builds-boost.html], Qt [http://labs.qt.nokia.com/2010/10/29/compiling-qt-with-clang/], LibreOffice [http://wiki.documentfoundation.org/Development/Building_LibreOffice_with_Clang], FreeBSD [http://wiki.freebsd.org/BuildingFreeBSDWithClang], some parts of the Linux kernel [http://lwn.net/Articles/411654/] and more.
  
''Another priority of Xfce is adhereance to standards, specifically those defined at [http://freedesktop.org/ freedesktop.org]. <ref>http://www.xfce.org/about/</ref>
+
Further, using LLVM 3.0 and up, there is a third way to compile with LLVM: the dragonegg package creates a gcc-plugin, that uses LLVM's optimizers but parses the code and creates binaries with gcc, which means that everything that compiles and works with gcc should work with dragonegg also. This plugin can be enabled by using a single CFLAG. Since LLVM 3.0 the old llvm-gcc package is deprecated and replaced by dragonegg, so it will disappear from portage with llvm version 2.9.
  
== Installation ==
+
==LLVM Frontends==
 +
To be able to compile some sourcecode of a specific language, LLVM needs an appropriate frontend. There are clang, llvm-gcc and dragonegg in portage.
  
Before emerging Xfce you need to:
+
The goal of the Clang project is to create a new C, C++, Objective C and Objective C++ front-end for the LLVM compiler.
  
* Check that you have enabled the USE flags necessary for a desktop environment. You can either [[Funtoo_Linux_First_Steps#Changing_profile|set your profile to "desktop"]] or you can edit your <tt>make.conf</tt>:
+
llvm-gcc is a modified version of gcc that compiles C/ObjC programs into native objects, LLVM bitcode or LLVM assembly language, depending upon the options. As written in the previous section, dragonegg replaced llvm-gcc in version 3.0.
  
{{File
+
So after installing llvm, clang and dragonegg, you will be able to choose between gcc and llvm whenever you like or use them both at the same time.
|/etc/portage/make.conf|<pre>
+
USE="-minimal dbus consolekit jpeg libnotify lock session startup-notification svg thunar udev X"
+
</pre>}}
+
  
* Verify that you have [[Funtoo_Linux_First_Steps#X.Org|X.Org Server]] configured properly.
+
== Install LLVM and its Frontends ==
 +
Simply emerge the packages on ~arch systems. On arch systems you have to unmask some packages first. dragonegg requires gcc's ''lto'' USE-flag to be set and works with gcc 4.5 and gcc 4.6.
  
 +
{{Root|emerge llvm clang dragonegg}}
  
We are now ready to install Xfce. Install the meta-package, which pulls in all of the components you need for a minimal Xfce installation:
+
Note, that for clang++ the C++ headers search path is hardcoded to the active gcc profile.
<console>
+
If you change the active gcc profile, or update gcc to a new version, you will have to remerge clang to update the search path.
# ##i## emerge xfce4-meta
+
</console>
+
  
== Running Xfce ==
+
To use dragonegg, run gcc as usual, with an extra command line argument "-fplugin=/usr/lib/llvm/dragonegg.so"
 +
If you change the active gcc profile, or update gcc to a new version, you will have to remerge dragonegg to update the plugin.
  
There are several different ways to start Xfce:
+
After the installation, check which CPUs are supported by using the command
 +
<pre>llvm-as < /dev/null | llc -mcpu=help</pre>
 +
and then add the following lines to /etc/make.conf (uncommenting the lines you need) to enable compilation via LLVM, adapting the march-option according to the previous command:
  
* Typing the command <tt>startxfce4</tt> at the console.
+
in /etc/make.conf :
* Editing the <tt>.xinitrc</tt> file in your user's home directory.
+
<pre>
* Using a display manager (e.g. SLiM, GDM, and KDM).
+
# LLVM
* Configuring your shell profile to automatically start Xfce upon a successful login.
+
#CC="/usr/bin/clang"
 +
#CXX="/usr/bin/clang++"
  
=== .xinitrc ===
+
# llvm-gcc for C++ code and fortran
 +
# llvm-gcc is deprecated and only used with LLVM 2.9
 +
#CC="/usr/bin/llvm-gcc"
 +
#CXX="/usr/bin/llvm-g++"
 +
#CPP="/usr/bin/llvm-cpp"
 +
#F77="/usr/bin/llvm-gfortran"
  
The most common way to start the environment is to configure <tt>~/.xinitrc</tt> to launch an Xfce session. The proper way to launch Xfce from the terminal is by using <tt>startxfce4</tt>.
+
# Flags for clang: Insert your arch here instead of k8 and have a look at the manpage of clang for flag descriptions.
<console>
+
# Some gcc flags like -pipe and -pthread also work, though they might be ignored by clang.
$ ##i##echo "exec ck-launch-session startxfce4" > ~/.xinitrc
+
#CFLAGS="-march=k8 -O2"
</console>
+
  
Note that <tt>ck-launch-session</tt> (from ConsoleKit) is required by Xfce for important tasks such as shutting down, suspending, and automatically mounting removable storage media. Some applications will not work properly without it. Therefore we need to add ConsoleKit to the <tt>default</tt> runlevel, executing the following command as root:
+
# Flags for dragonegg; just use all the gcc flags you like and append -fplugin=/path/to/dragonegg.so
<console>
+
#CFLAGS="-march=k8 -O2 -fplugin=/usr/lib64/llvm/dragonegg.so"
# ##i##rc-update add consolekit default
+
</pre>
</console>
+
  
Finally, to start the graphical environment:
+
Have a look at clang's manpages for additional information. If you get errors that your compiler cannot produce code, you should check your flags, e.g. don't use -O4 -flto -S or stuff like that; the examples above will work.
<console>
+
$ ##i##startx
+
</console>
+
  
== Display manager ==
+
== Using clang with portage ==
A display manager is a program that manages sessions and acts as a login screen. Here's a short list of a few of the display managers available in Funtoo:
+
  
* XDM (<tt>x11-apps/xdm</tt>): X.Org's standard display manager.
+
Although Gentoo package tree is not designed to be used with compiler other than GCC, clang can be enforced on most of the packages through ''CC'' and ''CXX'' variables.
  
* LightDM (<tt>x11-misc/lightdm</tt>): A lightweight, but highly configurable display manager.
+
Please note, however, that many of Gentoo packages still don't build with clang and a few don't work correctly after being built. That's why we suggest using ''/etc/portage/env'' file to enable the use of clang per-package.
  
* SLiM (<tt>x11-misc/slim</tt>): Simple Login Manager -- a lightweight display manager. Not very configurable.
+
In order to do that, first create a new environment override to use:
  
Any of these would make a fine choice. After you've chosen which display manager to use, install it:
+
in /etc/portage/env/clang :
<console>
+
<pre>
# ##i##emerge lightdm
+
CC=clang
</console>
+
CXX=clang++
 +
</pre>
  
All that's left to do now is to add the name of the display manager to <tt>/etc/conf.d/xdm</tt>, add the <tt>xdm</tt> service to the <tt>default</tt> runlevel, and start the service:
+
Then you can enable use of clang for packages using ''[[:/etc/portage/env|/etc/portage/package.env]]'' file:
  
{{File
+
in /etc/portage/package.env
|/etc/conf.d/xdm|<pre>
+
<pre>
DISPLAYMANAGER="lightdm"
+
app-foo/bar clang
</pre>}}
+
app-bar/baz clang
 +
</pre>
  
<console>
+
If you want to use clang by default you can and need specify some core packages. Here is small list of core packages that currently failing on clang, but not that could be outdated:
# ##i##rc-update add xdm default
+
# ##i##/etc/init.d/xdm start
+
</console>
+
  
[[Category:Desktop]]
+
You need to add /etc/portage/env/gcc :
 +
<pre>
 +
CC=gcc
 +
CXX=g++
 +
</pre>
 +
 
 +
if addition I recommend to add compiler flags there
 +
<pre>
 +
CFLAGS="-O2 -march=native -mtune=native -pipe"
 +
CXXFLAGS="-O2 -march=native -mtune=native -pipe"
 +
LDFLAGS="-Wl,--as-needed"
 +
#You can disable gold link here
 +
#EXTRA_ECONF="--enable-gold=default"
 +
</pre>
 +
 
 +
And in /etc/portage/package.env
 +
<pre>
 +
#---------------CORE PACKAGES TO BUILD WITH GCC:
 +
sys-apps/which gcc
 +
sys-fs/reiserfsprogs gcc
 +
sys-libs/ncurses gcc
 +
sys-libs/zlib gcc
 +
sys-apps/busybox gcc
 +
sys-fs/e2fsprogs gcc
 +
sys-devel/binutils gcc
 +
sys-libs/glibc gcc
 +
sys-devel/dragonegg gcc
 +
dev-libs/openssl gcc
 +
sys-boot/grub gcc
 +
#---------------USER PACKAGES TO BUILD WITH GCC:
 +
sys-apps/pacman gcc
 +
www-client/firefox gcc
 +
x11-libs/cairo gcc
 +
media-libs/mesa gcc
 +
</pre>
 +
 
 +
If you have app-portage/flaggie installed, the ''/etc/portage/package.env'' file could be modified using:
 +
 
 +
{{Root|flaggie app-foo/bar app-bar/baz +clang}}
 +
 
 +
== Enabling link-time optimizations ==
 +
 
 +
The ''link-time optimization'' feature defers optimizing the resulting executables to linking phase. This can result in better optimization of packages but is unsupported in Gentoo yet, and many packages simply fail to build.
 +
 
 +
When using LTO, clang compiles units into LLVM byte-code rather than machine code. In order to support linking such object files, the [[gold]] linker must be installed and set as the default linker, as it does support plugins.
 +
 
 +
Similarly, ''ar'' needs plugin support as well. Sadly, binutils ar doesn't support passing '--plugin'' option before the actual command. Thus, we need to create a wrapper for it:
 +
 
 +
in /usr/local/bin/clang-ar:
 +
<pre>
 +
#!/bin/sh
 +
firstarg=${1}
 +
shift
 +
 
 +
exec /usr/bin/ar "${firstarg}" --plugin /usr/lib/llvm/LLVMgold.so "${@}"
 +
</pre>
 +
 
 +
If that's done, you can create a new environment override profile for LTO-enabled clang:
 +
 
 +
in /etc/portage/env/clang-lt:
 +
<pre>
 +
CC='clang'
 +
CXX='clang++'
 +
CFLAGS="${CFLAGS} -O4"
 +
CXXFLAGS="${CXXFLAGS} -O4"
 +
LDFLAGS="${LDFLAGS} -O4 -Wl,-plugin,/usr/lib/llvm/LLVMgold.so"
 +
AR='/usr/local/bin/clang-ar'
 +
RANLIB=':'
 +
NM='nm --plugin /usr/lib64/llvm/LLVMgold.so'
 +
</pre>
 +
 
 +
Note that the link-time optimizations were indirectly enabled here via ''-O4''. If you don't want to enable other optimizations enforced by ''-O3'', please use ''-flto'' instead. You need to also pass optimization flags when linking because that's where clang needs them.
 +
 
 +
You may also need to adjust the libdir path to plugin. Newer (live) versions of clang add `-plugin` when linking automatically, so `-Wl,-plugin`… is no longer necessary.
 +
 
 +
== Using clang with distcc ==
 +
 
 +
In order to use clang on distcc client, additional symlinks have to be created in ''/usr/lib*/distcc/bin'':
 +
 
 +
{{Root|ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang
 +
ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang++}}
 +
 
 +
{{GLW|src=http://en.gentoo-wiki.com/wiki/Llvm}}
 +
 
 +
[[Category:HOWTO]]

Revision as of 03:11, 31 December 2012

Introduction

LLVM can be used as an alternative to GNU's compiler, GCC. The main benefit of using LLVM compilers instead of GCC is their lower memory usage, faster compile time and better diagnostics. There are some Benchmarks on the Clang and Phoronix homepages.

It may happen that some programs do not compile (like glibc) because they depend on GCC-specific language extensions [1] (this is why the whole BSD code can be compiled with LLVM but some GNU code cannot) or segfault after successful compilation with LLVM (like xorg-server) but after following this guide, the system will still be able to compile packages with gcc. So if something goes wrong, it can be switched back to gcc for the particular package by uncommenting lines in /etc/make.conf and the bug should be reported.

LLVM's C/C++ frontends clang and clang++ version 3.0 are stable enough to be self-hosting [2] and compile Boost [3], Qt [4], LibreOffice [5], FreeBSD [6], some parts of the Linux kernel [7] and more.

Further, using LLVM 3.0 and up, there is a third way to compile with LLVM: the dragonegg package creates a gcc-plugin, that uses LLVM's optimizers but parses the code and creates binaries with gcc, which means that everything that compiles and works with gcc should work with dragonegg also. This plugin can be enabled by using a single CFLAG. Since LLVM 3.0 the old llvm-gcc package is deprecated and replaced by dragonegg, so it will disappear from portage with llvm version 2.9.

LLVM Frontends

To be able to compile some sourcecode of a specific language, LLVM needs an appropriate frontend. There are clang, llvm-gcc and dragonegg in portage.

The goal of the Clang project is to create a new C, C++, Objective C and Objective C++ front-end for the LLVM compiler.

llvm-gcc is a modified version of gcc that compiles C/ObjC programs into native objects, LLVM bitcode or LLVM assembly language, depending upon the options. As written in the previous section, dragonegg replaced llvm-gcc in version 3.0.

So after installing llvm, clang and dragonegg, you will be able to choose between gcc and llvm whenever you like or use them both at the same time.

Install LLVM and its Frontends

Simply emerge the packages on ~arch systems. On arch systems you have to unmask some packages first. dragonegg requires gcc's lto USE-flag to be set and works with gcc 4.5 and gcc 4.6.

emerge llvm clang dragonegg


Note, that for clang++ the C++ headers search path is hardcoded to the active gcc profile. If you change the active gcc profile, or update gcc to a new version, you will have to remerge clang to update the search path.

To use dragonegg, run gcc as usual, with an extra command line argument "-fplugin=/usr/lib/llvm/dragonegg.so" If you change the active gcc profile, or update gcc to a new version, you will have to remerge dragonegg to update the plugin.

After the installation, check which CPUs are supported by using the command

llvm-as < /dev/null | llc -mcpu=help

and then add the following lines to /etc/make.conf (uncommenting the lines you need) to enable compilation via LLVM, adapting the march-option according to the previous command:

in /etc/make.conf :

# LLVM
#CC="/usr/bin/clang"
#CXX="/usr/bin/clang++"

# llvm-gcc for C++ code and fortran
# llvm-gcc is deprecated and only used with LLVM 2.9
#CC="/usr/bin/llvm-gcc"
#CXX="/usr/bin/llvm-g++"
#CPP="/usr/bin/llvm-cpp"
#F77="/usr/bin/llvm-gfortran"

# Flags for clang: Insert your arch here instead of k8 and have a look at the manpage of clang for flag descriptions.
# Some gcc flags like -pipe and -pthread also work, though they might be ignored by clang.
#CFLAGS="-march=k8 -O2"

# Flags for dragonegg; just use all the gcc flags you like and append -fplugin=/path/to/dragonegg.so
#CFLAGS="-march=k8 -O2 -fplugin=/usr/lib64/llvm/dragonegg.so"

Have a look at clang's manpages for additional information. If you get errors that your compiler cannot produce code, you should check your flags, e.g. don't use -O4 -flto -S or stuff like that; the examples above will work.

Using clang with portage

Although Gentoo package tree is not designed to be used with compiler other than GCC, clang can be enforced on most of the packages through CC and CXX variables.

Please note, however, that many of Gentoo packages still don't build with clang and a few don't work correctly after being built. That's why we suggest using /etc/portage/env file to enable the use of clang per-package.

In order to do that, first create a new environment override to use:

in /etc/portage/env/clang :

CC=clang
CXX=clang++

Then you can enable use of clang for packages using /etc/portage/package.env file:

in /etc/portage/package.env

app-foo/bar clang
app-bar/baz clang

If you want to use clang by default you can and need specify some core packages. Here is small list of core packages that currently failing on clang, but not that could be outdated:

You need to add /etc/portage/env/gcc :

CC=gcc
CXX=g++

if addition I recommend to add compiler flags there

CFLAGS="-O2 -march=native -mtune=native -pipe"
CXXFLAGS="-O2 -march=native -mtune=native -pipe"
LDFLAGS="-Wl,--as-needed"
#You can disable gold link here
#EXTRA_ECONF="--enable-gold=default"

And in /etc/portage/package.env

#---------------CORE PACKAGES TO BUILD WITH GCC:
sys-apps/which gcc
sys-fs/reiserfsprogs gcc
sys-libs/ncurses gcc
sys-libs/zlib gcc
sys-apps/busybox gcc
sys-fs/e2fsprogs gcc
sys-devel/binutils gcc
sys-libs/glibc gcc
sys-devel/dragonegg gcc
dev-libs/openssl gcc
sys-boot/grub gcc
#---------------USER PACKAGES TO BUILD WITH GCC:
sys-apps/pacman gcc
www-client/firefox gcc
x11-libs/cairo gcc
media-libs/mesa gcc

If you have app-portage/flaggie installed, the /etc/portage/package.env file could be modified using:

flaggie app-foo/bar app-bar/baz +clang


Enabling link-time optimizations

The link-time optimization feature defers optimizing the resulting executables to linking phase. This can result in better optimization of packages but is unsupported in Gentoo yet, and many packages simply fail to build.

When using LTO, clang compiles units into LLVM byte-code rather than machine code. In order to support linking such object files, the gold linker must be installed and set as the default linker, as it does support plugins.

Similarly, ar needs plugin support as well. Sadly, binutils ar doesn't support passing '--plugin option before the actual command. Thus, we need to create a wrapper for it:

in /usr/local/bin/clang-ar:

#!/bin/sh
firstarg=${1}
shift

exec /usr/bin/ar "${firstarg}" --plugin /usr/lib/llvm/LLVMgold.so "${@}"

If that's done, you can create a new environment override profile for LTO-enabled clang:

in /etc/portage/env/clang-lt:

CC='clang'
CXX='clang++'
CFLAGS="${CFLAGS} -O4"
CXXFLAGS="${CXXFLAGS} -O4"
LDFLAGS="${LDFLAGS} -O4 -Wl,-plugin,/usr/lib/llvm/LLVMgold.so"
AR='/usr/local/bin/clang-ar'
RANLIB=':'
NM='nm --plugin /usr/lib64/llvm/LLVMgold.so'

Note that the link-time optimizations were indirectly enabled here via -O4. If you don't want to enable other optimizations enforced by -O3, please use -flto instead. You need to also pass optimization flags when linking because that's where clang needs them.

You may also need to adjust the libdir path to plugin. Newer (live) versions of clang add `-plugin` when linking automatically, so `-Wl,-plugin`… is no longer necessary.

Using clang with distcc

In order to use clang on distcc client, additional symlinks have to be created in /usr/lib*/distcc/bin:

ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang++



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