llvm dev - Jan 2013 - [LLVMdev] Building llvm from source with clang instead of gcc

On 1/5/2013 8:01 PM, Justin Holewinski wrote:
>
> On Sat, Jan 5, 2013 at 5:59 PM, Edward Diener
> <eldlistmailingz at tropicsoft.com <mailto:eldlistmailingz at
tropicsoft.com>>
> wrote:
>
>     On 01/05/2013 01:11 PM, Dmitri Gribenko wrote:
>
>         On Sat, Jan 5, 2013 at 7:48 PM, Edward Diener
>         <eldlistmailingz at tropicsoft.__com
>         <mailto:eldlistmailingz at tropicsoft.com>>  wrote:
>
>             I actually found out that llvm/configure will choose clang
>             if it is
>             found in the path. This contradicts what is written at
>             http://llvm.org/docs/__GettingStarted.html#__requirements
>             <http://llvm.org/docs/GettingStarted.html#requirements>,
>             where it says it
>             will use the first GCC it finds. Perhaps that explanation
>             needs to be
>             updated.
>
>
>         Updated docs in r171630.
>
>             Finally I found that if llvm/configure uses clang instead of
>             gcc, it
>             invariably fails on my Linux distros because it is using the
gcc
>             distributed included directories and an earlier version of
>             clang will
>             often fail with these gcc header files. Once I force
>             llvm/configure to
>             use gcc, building llvm/clang succeeds again.
>
>
>         Right.  Maybe we need to skip old clang versions in this case.
>
>
>     I think this should be done until clang can use its own header files
>     and libraries, when it is installed as a binary, rather than the
>     ones that gcc installs. In fact I am a bit amazed that clang does
>     not already have its own header files and libraries for use when one
>     tries llvm/configure and it sets up llvm's build system to use
clang
>     as its compiler. Is this the fault of clang or is it the fault of
>     the llvm/configure system ?
>
>
> There is no complete solution for LLVM-only headers/libraries yet on
> Linux.  For C++, you can use libc++, but you still need some GCC
> libraries (or alternatives).  And you will at least need glibc or
> another compatible C runtime.
Should not what it is needed by clang to work correctly under Linux be 
posted somewhere on the clang Getting Started web page ? Even what you 
posted above requires guesswork on my part.

Am I supposed to install libc++ and glibc in a Linux distro for clang to 
work correctly ?

Which versions do I need ?

How do I tell clang what to use or is this automatically figured out by 
clang ?

Will llvm/configure use these libraries if I let it choose a clang 
binary in order to build the latest llvm/clang from source ?

On Sat, Jan 5, 2013 at 11:08 PM, Edward Diener <
eldlistmailingz at tropicsoft.com> wrote:
> On 1/5/2013 8:01 PM, Justin Holewinski wrote:
>
>>
>> On Sat, Jan 5, 2013 at 5:59 PM, Edward Diener
>> <eldlistmailingz at tropicsoft.**com <eldlistmailingz at
tropicsoft.com><mailto:
>> eldlistmailingz@**tropicsoft.com <eldlistmailingz at
tropicsoft.com>>>
>>
>> wrote:
>>
>>     On 01/05/2013 01:11 PM, Dmitri Gribenko wrote:
>>
>>         On Sat, Jan 5, 2013 at 7:48 PM, Edward Diener
>>         <eldlistmailingz at tropicsoft.__**com
>>         <mailto:eldlistmailingz@**tropicsoft.com<eldlistmailingz
at tropicsoft.com>>>
>>  wrote:
>>
>>
>>             I actually found out that llvm/configure will choose clang
>>             if it is
>>             found in the path. This contradicts what is written at
>>            
http://llvm.org/docs/__**GettingStarted.html#__**requirements<http://llvm.org/docs/__GettingStarted.html#__requirements>
>>            
<http://llvm.org/docs/**GettingStarted.html#**requirements<http://llvm.org/docs/GettingStarted.html#requirements>
>> >,
>>
>>             where it says it
>>             will use the first GCC it finds. Perhaps that explanation
>>             needs to be
>>             updated.
>>
>>
>>         Updated docs in r171630.
>>
>>             Finally I found that if llvm/configure uses clang instead
of
>>             gcc, it
>>             invariably fails on my Linux distros because it is using
the
>> gcc
>>             distributed included directories and an earlier version of
>>             clang will
>>             often fail with these gcc header files. Once I force
>>             llvm/configure to
>>             use gcc, building llvm/clang succeeds again.
>>
>>
>>         Right.  Maybe we need to skip old clang versions in this case.
>>
>>
>>     I think this should be done until clang can use its own header
files
>>     and libraries, when it is installed as a binary, rather than the
>>     ones that gcc installs. In fact I am a bit amazed that clang does
>>     not already have its own header files and libraries for use when
one
>>     tries llvm/configure and it sets up llvm's build system to use
clang
>>     as its compiler. Is this the fault of clang or is it the fault of
>>     the llvm/configure system ?
>>
>>
>> There is no complete solution for LLVM-only headers/libraries yet on
>> Linux.  For C++, you can use libc++, but you still need some GCC
>> libraries (or alternatives).  And you will at least need glibc or
>> another compatible C runtime.
>>
>
> Should not what it is needed by clang to work correctly under Linux be
> posted somewhere on the clang Getting Started web page ? Even what you
> posted above requires guesswork on my part.
>
Clang will attempt to use your installed system libraries, usually
libstdc++ and glibc on a Linux distribution.  We could probably maintain a
libstdc++ compatibility table that says clang x.y is compatible with
libstdc++ up to z.w.  In the vast majority of cases, this just works if you
install a somewhat-newer version of clang.  The problems arise when you use
an older clang with a newer libstdc++ (e.g. newer gcc and/or linux
distribution).

>
> Am I supposed to install libc++ and glibc in a Linux distro for clang to
> work correctly ?
>
glibc will already be there.  libc++ is purely optional.  Clang will
happily use libstdc++ (which comes with your linux distribution), but newer
libstdc++ versions may use new GCC extensions that clang does not yet
support.  This is an unfortunate condition that arises from depending on
GNU libraries that assume GCC.  At this time, libc++ is not entirely
self-contained on Linux and still requires some GCC libraries for some
external functions.

> Which versions do I need ?
>
I'm guessing you are running into issues with the C++ standard library
headers.  In that case, it depends on which clang version you are using.
 For compatibility reasons, it is best to always use the latest released
clang (if not trunk).  If you try again with 3.2, I bet your problems go
away.

>
> How do I tell clang what to use or is this automatically figured out by
> clang ?
>
There are ways to override the location for the headers/libraries of
libstdc++, but I would not recommend this.  Upgrading to a newer clang
should be easier in the long run.

>
> Will llvm/configure use these libraries if I let it choose a clang binary
> in order to build the latest llvm/clang from source ?

Which libraries?  An older libstdc++?  You can use CXXFLAGS and LDFLAGS to
adjust the library search paths, just as you use CXX to set the compiler to
use.


Generally, it is recommended to bootstrap clang:

1. Build llvm/clang with system gcc
2. Build llvm/clang with new clang
3. Install new clang somewhere (which will automatically use system
libstdc++/glibc).

This should solve your problem.

>
>
>
>
> ______________________________**_________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>
http://lists.cs.uiuc.edu/**mailman/listinfo/llvmdev<http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev>
>


-- 

Thanks,

Justin Holewinski
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On 1/6/2013 9:47 AM, Justin Holewinski wrote:
> On Sat, Jan 5, 2013 at 11:08 PM, Edward Diener
> <eldlistmailingz at tropicsoft.com <mailto:eldlistmailingz at
tropicsoft.com>>
> wrote:
>
>     On 1/5/2013 8:01 PM, Justin Holewinski wrote:
>
>
>         On Sat, Jan 5, 2013 at 5:59 PM, Edward Diener
>         <eldlistmailingz at tropicsoft.__com
>         <mailto:eldlistmailingz at tropicsoft.com>
>         <mailto:eldlistmailingz at __tropicsoft.com
>         <mailto:eldlistmailingz at tropicsoft.com>>>
>
>         wrote:
>
>              On 01/05/2013 01:11 PM, Dmitri Gribenko wrote:
>
>                  On Sat, Jan 5, 2013 at 7:48 PM, Edward Diener
>                  <eldlistmailingz at tropicsoft.____com
>                  <mailto:eldlistmailingz at __tropicsoft.com
>         <mailto:eldlistmailingz at tropicsoft.com>>>  wrote:
>
>
>                      I actually found out that llvm/configure will
>         choose clang
>                      if it is
>                      found in the path. This contradicts what is written at
>         http://llvm.org/docs/____GettingStarted.html#____requirements
>         <http://llvm.org/docs/__GettingStarted.html#__requirements>
>
>         <http://llvm.org/docs/__GettingStarted.html#__requirements
>         <http://llvm.org/docs/GettingStarted.html#requirements>>,
>
>                      where it says it
>                      will use the first GCC it finds. Perhaps that
>         explanation
>                      needs to be
>                      updated.
>
>
>                  Updated docs in r171630.
>
>                      Finally I found that if llvm/configure uses clang
>         instead of
>                      gcc, it
>                      invariably fails on my Linux distros because it is
>         using the gcc
>                      distributed included directories and an earlier
>         version of
>                      clang will
>                      often fail with these gcc header files. Once I force
>                      llvm/configure to
>                      use gcc, building llvm/clang succeeds again.
>
>
>                  Right.  Maybe we need to skip old clang versions in
>         this case.
>
>
>              I think this should be done until clang can use its own
>         header files
>              and libraries, when it is installed as a binary, rather
>         than the
>              ones that gcc installs. In fact I am a bit amazed that
>         clang does
>              not already have its own header files and libraries for use
>         when one
>              tries llvm/configure and it sets up llvm's build system to
>         use clang
>              as its compiler. Is this the fault of clang or is it the
>         fault of
>              the llvm/configure system ?
>
>
>         There is no complete solution for LLVM-only headers/libraries yet
on
>         Linux.  For C++, you can use libc++, but you still need some GCC
>         libraries (or alternatives).  And you will at least need glibc or
>         another compatible C runtime.
>
>
>     Should not what it is needed by clang to work correctly under Linux
>     be posted somewhere on the clang Getting Started web page ? Even
>     what you posted above requires guesswork on my part.
>
>
> Clang will attempt to use your installed system libraries, usually
> libstdc++ and glibc on a Linux distribution.  We could probably maintain
> a libstdc++ compatibility table that says clang x.y is compatible with
> libstdc++ up to z.w.  In the vast majority of cases, this just works if
> you install a somewhat-newer version of clang.  The problems arise when
> you use an older clang with a newer libstdc++ (e.g. newer gcc and/or
> linux distribution).
This is what evidently happens. A clang binary distribution gets created 
for a particular Linux distro but there is no compatibility in the 
package which states that it only works with some version of libstdc++ 
or glibc or earlier. Then a later version of gcc gets installed and with 
it a later version of libstdc++ and glibc as a dependency. Now the 
binary version of clang no longer works on that system because of the 
updates to the C/C++ standard library.

I do not believe this should ever happen. I am not focusing blame but 
simply saying that from an end-user's POV this is not acceptable if one 
wants to use clang without having to build it oneself.
>
>
>     Am I supposed to install libc++ and glibc in a Linux distro for
>     clang to work correctly ?
>
>
> glibc will already be there.  libc++ is purely optional.  Clang will
> happily use libstdc++ (which comes with your linux distribution), but
> newer libstdc++ versions may use new GCC extensions that clang does not
> yet support.  This is an unfortunate condition that arises from
> depending on GNU libraries that assume GCC.  At this time, libc++ is not
> entirely self-contained on Linux and still requires some GCC libraries
> for some external functions.
>
>
>     Which versions do I need ?
>
>
> I'm guessing you are running into issues with the C++ standard library
> headers.  In that case, it depends on which clang version you are using.
>   For compatibility reasons, it is best to always use the latest
> released clang (if not trunk).  If you try again with 3.2, I bet your
> problems go away.
>
>
>     How do I tell clang what to use or is this automatically figured out
>     by clang ?
>
>
> There are ways to override the location for the headers/libraries of
> libstdc++, but I would not recommend this.  Upgrading to a newer clang
> should be easier in the long run.
>
>
>     Will llvm/configure use these libraries if I let it choose a clang
>     binary in order to build the latest llvm/clang from source ?
>
>
> Which libraries?  An older libstdc++?  You can use CXXFLAGS and LDFLAGS
> to adjust the library search paths, just as you use CXX to set the
> compiler to use.
>
>
> Generally, it is recommended to bootstrap clang:
>
> 1. Build llvm/clang with system gcc
> 2. Build llvm/clang with new clang
> 3. Install new clang somewhere (which will automatically use system
> libstdc++/glibc).
>
> This should solve your problem.
OK, this sounds good. Can this process not be explained on the clang 
Getting Started page for those who are building clang from source. I am 
not sure how step 3) is supposed to go. Do I just copy the built bin 
directory somewhere else and then put it first in my PATH, before 
running llvm/configure ? Or is "Install new clang" more complicated
than
that ?

Originally I thought I could have both an earlier binary version of 
clang as well as the latest built version of clang in a Linux distro so 
that I could test code on both for compatibility of my library for 
end-users. But with the fairly common breakage of the binary version 
package of clang when libstdc++/glibc gets updated, this has become much 
less of a reality.

On 1/6/2013 8:47 AM, Justin Holewinski wrote:
>
> Generally, it is recommended to bootstrap clang:
>
> 1. Build llvm/clang with system gcc
> 2. Build llvm/clang with new clang
> 3. Install new clang somewhere (which will automatically use system
> libstdc++/glibc).
>
> This should solve your problem.
Do you mean that it is recommended to bootstrap clang (i.e. steps 1, 2, 
3), as opposed to just building it with whatever compiler that is 
available (steps 1, 3)?  If so, why?

-Krzysztof

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