[cfe-dev] How can I make clang/llvm use specific toolchain?

Tue Dec 25 16:52:20 PST 2012

Hello Leonid Borisenko,

>> So, it's not clear, whether I can say that some of my headers and tools are "outside of sb2" or "inside of sb2".

If this is cleared, I believe your approach is very good workaround!
I believe header files and library and runtime must be of the same
toolchain. if it is mixed from several toolchain, the build system can
not be trust worthy.

I do cross compile using a toolchain in the tizen-sdk, which use
chroot environment, similar to the SB environment. The options needed
are

-ccc-host-triple $(CCC_HOST_TRIPLE_ARM) \
--sysroot=$(SYSROOT_ARM) \
-gcc-toolchain $(GCC_TOOLCHAIN)
LDFLAGS = -Wl,-dynamic-linker,/lib/ld-linux.so.3

In my understanding to use clang under SB2 environment, a porting is
needed because clang only works in known environment only. With the
knowledge of the OS clang finds header files and library and runtime.
But your workaround removes the necessity of porting!!

And the third question of mine was actually not a question. But your
answer is so much informative!!

I learned about the configuration triplet. I asked a question about
this on this list earlier but didn't receive any reply yet.

I really appreciate you for your fully informative answers.
I even feel sorry for my questions. __;

Thank you very much.

Sincerely
Journeyer

2012/12/26 Leonid Borisenko <leo.borisenko at gmail.com>:
> Journeyer J. Joh wrote:
>>>> Workaround: I was lucky enough to have cross-linker installed into
>>>> /usr/bin/arm-linux-gnueabi-ld. So I've passed following options to clang:
>>>>
>>>>   -target arm-linux-gnueabi -march=armv7-a -mthumb -mtune=cortex-a8
>>>>
>>>> As -target is defined, clang will try to find arm-linux-gnueabi-ld first and
>>>> it will be found. -march, -mthumb and -mtune options are overriding defaults
>>>> set after -target value.
>>>>
>>>> And 'sb2 clang ...' with these options invokes cross-linker.
>>
>> - This means that you used cross-binutil to link ELFs. But (the
>> library and?) header files are of SB2?
>> - Did you execute clang under SB2 environment(context)? and used
>> cross-binutil out of SB2?
>>
>> Thank you for your sharing of your knowledge.
>> The usage of the options above would be helpful.
>
> First of all, I'd like to note that I've installed scratchbox2 separately of
> cross-toolchain and target rootfs and made sb2 target by myself with
> 'sb2-init ...'. So, it's not clear, whether I can say that some of my headers
> and tools are "outside of sb2" or "inside of sb2". Certainly, my cross-tools
> (gcc, as, ld etc.) are "outside of sb2" as they aren't belonged to
> scratchbox2 installation or to target rootfs. On the other hand, they are
> accessible from sb2 command context/shell, as scratchox2 automatically mapped
> some paths (including paths to cross-tools) to real, not target rootfs. So
> my cross-tools are "inside of s2". The same could be said about standard
> headers provided by cross-toolchain.
>
> Answering to your first question: yes, clang/clang++ uses the same
> headers/libraries as gcc would use when invoked by 'sb2 gcc ...' (i.e.
> headers/libraries "of sb2"), but by various reasons it's not handled
> automatically by scratchbox2, so it should be set manually.
>
> As a prerequisite, I had to pass to clang location of standard
> cross-toolchain headers. By default clang uses headers from toolchain used
> in it's own compilation process. As my clang was built for x86 architecture,
> it uses host toolchain headers, not cross-toolchain headers. It could be
> fixed with -isystem and -cxx-isystem options. Locations of cross-toolchain
> standard headers could be found with the following commands:
>
>   sb2 sh -c "echo| gcc -xc -E -v -" 2>&1 | awk -e '/<...>/,/^End/ {print}'
>   sb2 sh -c "echo| gcc -xc++ -E -v -" 2>&1 | awk -e '/<...>/,/^End/ {print}'
>
> First command shows paths to standard headers used by cross-toolchain in
> compilation of C programs. Second command shows paths to standard headers
> used by cross-toolchain in compilation of C++ programs.
>
> I've used 'sb2 ...' just for convenience. Instead, full path to
> cross-toolchain's gcc could be used, like:
>
>   echo | /opt/cross/bin/gcc -xc -E -v -" 2>&1 | awk -e '/<...>/,/^End/ {print}
>
> On my machine, first command outputs:
> #include <...> search starts here:
>  /usr/local/include
>  /usr/include
>  /usr/lib/gcc/arm-linux-gnueabi/4.7/include
>  /usr/lib/gcc/arm-linux-gnueabi/4.7/include-fixed
>  /usr/arm-linux-gnueabi/include
> End of search list.
>
> And second command outputs:
> #include <...> search starts here:
>  /usr/local/include
>  /usr/include
>  /usr/arm-linux-gnueabi/include/c++/4.7.2
>  /usr/arm-linux-gnueabi/include/c++/4.7.2/arm-linux-gnueabi
>  /usr/arm-linux-gnueabi/include/c++/4.7.2/backward
>  /usr/lib/gcc/arm-linux-gnueabi/4.7/include
>  /usr/lib/gcc/arm-linux-gnueabi/4.7/include-fixed
>  /usr/arm-linux-gnueabi/include
> End of search list.
>
> So, required options for clang are:
>
>   -isystem /sb2/target/rootfs/usr/local/include
>   -isystem /sb2/target/rootfs/usr/include
>   -isystem /usr/lib/gcc/arm-linux-gnueabi/4.7/include
>   -isystem /usr/lib/gcc/arm-linux-gnueabi/4.7/include-fixed
>   -isystem /usr/arm-linux-gnueabi/include
>   -cxx-isystem /usr/arm-linux-gnueabi/include/c++/4.7.2
>   -cxx-isystem /usr/arm-linux-gnueabi/include/c++/4.7.2/arm-linux-gnueabi
>   -cxx-isystem /usr/arm-linux-gnueabi/include/c++/4.7.2/backward
>
> Paths from output of first command should be values of -isystem. Paths
> from output of second command, that aren't already appeared in output of
> first command, should be values of -cxx-isystem. I've also prepended
> path to scratchbox2 target rootfs to /usr/local/include and /usr/include
> paths, as mapping of these paths couldn't be handled by scratchbox2 because
> clang driver calls clang compiler with posix_spawn (on my machine), so
> compiler "escapes" from scratchbox2 monitoring.
>
> To use libraries in target rootfs, absolute paths to libraries locations
> should be passed to clang in following options:
>
>   -L /sb2/target/rootfs/usr/lib
>   -L /sb2/target/rootfs/lib
>   -Wl,-rpath-link,/sb2/target/rootfs/usr/lib
>   -Wl,-rpath-link,/sb2/target/rootfs/lib
>
> -L and -rpath-link are described in 'man ld'. sb2-init automatically sets
> applying of these options for each invocation of 'sb2 gcc ...', but for
> clang these options should be applied manually.
>
> Answering your second question: yes, I did execute clang under scratchbox2
> environment (context) and used cross-binutil out of scratchbox2.
>
> Answering your question about usage of -target and following options:
> -target is a renamed -ccc-host-triple [1]. It's used for cross-compiling.
> My clang had been built on x86 machine and, by default, produces code for
> x86. As I want to cross-compile, I must provide clang with target
> architecture by using -target option. You can read about target format in
> [2] [3].
>
> Other options (-march etc.) are target-specific options (in my case --
> ARM-specific [4]).
>
> [1] http://lists.cs.uiuc.edu/pipermail/cfe-dev/2012-January/019442.html
> [2] http://www.sourceware.org/autobook/autobook/autobook_17.html
> [3] http://airs.com/ian/configure/configure_4.html
> [4] http://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html
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-- 
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Journeyer J. Joh
o o s a p r o g r a m m e r
a t
g m a i l  d o t  c o m
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