Difference between pages "User talk:Lo0na" and "Clang"

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(thanks for the help :): new section)
 
(Using clang with portage)
 
Line 1: Line 1:
Looking into the issue you described...
+
==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.
  
OK, based on my initial research into this issue, and I could be wrong... I am leaning towards calling this a quirk in how the new "spam diff" works, which is a new feature in 1.17 or 1.18 of mediawiki. When it is showing you all those lines that it wants to remove, it is on the spam protection page, right? So it is not actually erasing stuff. It just looks like it wants to erase stuff. But this may be a quirk in my spam code with the new mediawiki feature.
+
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.
  
So quick summary - I think the wiki is safe, unless you actually *see* the wiki erase lots of content like the spam protection page is making you think it wants to do. I think I just need to fix the output of the spam protection page.
+
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.
  
If you actually see data being destroyed, let me know. But even then, the wiki has history. So don't be afraid of editing and don't be scared by the spam protection warning at this time.
+
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.
  
UPDATE: I have added some URLs to the whitelist so the installation tutorial isn't set up to trigger the spam filter out of the gates. You should now be able to save changes to this page.
+
==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.
  
UPDATE: OK, the strange thing is that you should not have been able to make any changes to the Installation Tutorial on the wiki like you did, since this page had links in it that triggered the spam filter. Did you do anything special to get by the spam filter? I will look at the code and make sure it is applying to you.
+
The goal of the Clang project is to create a new C, C++, Objective C and Objective C++ front-end for the LLVM compiler.
  
Reply: Ah, ok, that makes perfect sense re: you editing sections and not the whole document. You may want to try switching your UI to English and seeing if that makes things work correctly when the spam protection filter does trigger. Yes, the spam protection filter seems to check against the entire contents of the new page (or if editing just a section, the section). So any existing links in the page or section will trigger the spam filter. This is explained in the English spam filter message but I don't have a German variant :)
+
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.
  
== thanks for the help :) ==
+
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.
  
Hi Lo0na, I certainly would appreciate editing help with the Wiki. English is our official language -- we made a decision early on that it is easier to target one language rather than many, since most people know English to some degree anyway. Translated articles tend to get out-of-date. But I would love to have you help with the wiki in any way you can, including editing or adding new good content to the wiki.
+
== 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.
 +
<console>
 +
###i## emerge llvm clang dragonegg
 +
</console>
 +
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.
  
I don't know if you use wireless on Linux or not, but I think we could use some information in our networking guide about setting up <tt>wicd</tt>. I feel we are a bit light on the wireless side of things. New users often pop on to freenode to ask wireless setup questions. Maybe all they need are a few pointers. <tt>wicd</tt> seems to be the preferred thing to use because it is so reliable.
+
To use dragonegg, run gcc as usual, with an extra command line argument <tt>-fplugin=/usr/lib/llvm/dragonegg.so</tt>
 +
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
 +
<console>
 +
###i## llvm-as < /dev/null | llc -mcpu=help
 +
</console>
 +
and then add the following lines to <code>/etc/make.conf</code> (uncommenting the lines you need) to enable compilation via LLVM, adapting the march-option according to the previous command:
 +
 
 +
in <code>/etc/portage/make.conf</code>:
 +
{{File
 +
|/etc/portage/make.conf|<pre>
 +
# 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"
 +
</pre>}}
 +
 
 +
{{Note}} 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 <tt>-O4 -flto -S</tt> 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 <tt>/etc/portage/env</tt> file to enable the use of clang per-package.
 +
 
 +
In order to do that, first create a new environment override to use in <code>/etc/portage/env/clang</code>:
 +
{{File
 +
|/etc/portage/env/clang|<pre>
 +
CC=clang
 +
CXX=clang++
 +
</pre>}}
 +
 
 +
Then you can enable use of clang for packages using the [[:/etc/portage/env|/etc/portage/package.env]] file:
 +
|/etc/portage/package.env|<pre>
 +
app-foo/bar clang
 +
app-bar/baz clang
 +
</pre>}}
 +
 
 +
If you want to use clang by default you can and need to specify some core packages. Here is small list of core packages that are currently failing on clang, but not that could be outdated:
 +
{{File
 +
|/etc/portage/env/gcc|<pre>
 +
CC=gcc
 +
CXX=g++
 +
</pre>}}
 +
 
 +
in addition, it is recommend to add compiler flags there:
 +
{{File
 +
|/etc/portage/env/gcc|<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 <code>/etc/portage/package.env</code>:
 +
{{File
 +
|/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 {{Package|app-portage/flaggie}} installed, you can modify <code>/etc/portage/package.env</code> by running the following:
 +
<console>
 +
###i## flaggie app-foo/bar app-bar/baz +clang
 +
</console>
 +
 
 +
== 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, 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 ''<code>/usr/local/bin/clang-ar</code>'':
 +
<console>
 +
###i## nano /usr/local/bin/clang-ar
 +
#!/bin/sh
 +
firstarg=${1}
 +
shift
 +
 
 +
exec /usr/bin/ar "${firstarg}" --plugin /usr/lib/llvm/LLVMgold.so "${@}"
 +
</console>
 +
 
 +
If that's done, you can create a new environment override profile for LTO-enabled clang:
 +
 
 +
in ''<code>/etc/portage/env/clang-lt</code>'':
 +
<console>
 +
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'
 +
</console>
 +
 
 +
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 ''<code>/usr/lib*/distcc/bin</code>'':
 +
<console>
 +
###i## ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang
 +
###i## ln -s /usr/bin/distcc /usr/lib/distcc/bin/clang++
 +
</console>
 +
 
 +
{{GLW|src=http://wiki.gentoo.org/wiki/Clang}}
 +
 
 +
[[Category:HOWTO]]

Revision as of 14:27, 28 January 2014

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/portage/make.conf:

{{{name}}}
{{{body}}}

Note Note: 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:

{{{name}}}
{{{body}}}

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

|/etc/portage/package.env|
app-foo/bar clang
app-bar/baz clang
}}

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

{{{name}}}
{{{body}}}

in addition, it is recommend to add compiler flags there:

{{{name}}}
{{{body}}}

And in /etc/portage/package.env:

{{{name}}}
{{{body}}}

If you have app-portage/flaggie installed, you can modify /etc/portage/package.env by running the following:

# 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, 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:

# nano /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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