[U-Boot] [patch 0/1] [Experimental] U-boot linking against libgcc

[U-Boot] [patch 0/1] [Experimental] U-boot linking against libgcc

[Remy Bohmer]

Related to the discussion we had earlier today about linking to libgcc I 
have here a experimental patch that removes the linking to libgcc.
I called it experimental because I am not able to test if all architectures
still compile properly, and as such it needs more (build)testing before
integrating. I tested several ARM boards, and they all compile and link fine.

I am convinced that it is the way to go to remove libgcc of the link list,
but if we integrate this patch I cannot guarantee it does not break some builds,
but it makes clear which boards are depending on interfaces from a library it
should not depend on in the first place. Those boards might need additional
patches.

I hope you can find some time to do some more compile testing across different
architectures.


Kind Regards,

Remy B??hmer

[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Remy Bohmer]

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[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Scott Wood]

On Fri, Oct 31, 2008 at 11:37:33PM +0100, Remy Bohmer wrote:
> Currently U-boot is linking against libgcc. This should not be needed
> because the compiler toolchain is usually compiled with a certain OS
> interface in mind, and can even be configured for GNU-EABI interfaces.
> This can cause linking conflicts in U-boot when linking to libgcc.

libgcc should *not* be expecting any OS interface.  It may use a few
basic libc functions such as memcpy, that u-boot provides.

> While looking at compiler includes, the only header used from GCC (I
> have seen) seems to be the stdarg.h header, which is even doubtful to
> include in U-boot, because of the same reasons not to link against
> libgcc. This patch only removes the linking part to libgcc.

Why is it doubtful to remove the only portable way to get variable
arguments?  stdarg.h should *not* contain OS dependencies.

-Scott

[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Remy Bohmer]

Hello Scott (and Wolfgang),

> libgcc should *not* be expecting any OS interface.  It may use a few
> basic libc functions such as memcpy, that u-boot provides.

libgcc could also expect the 'raise()' routine to pass divide-by-zero
execeptions. (if __div0, (or __aeabi_ldiv0) is not overruled)
If not overruled, than libgcc can depend on libc, and thus indirectly
depend on a OS interface.

In the mean time things are getting more clear. In the ARM section of
U-boot a large amount of (division) routines which are usually
provided by libgcc are also implemented in the lib_arm directory. For
many ARM boards it is therefore possible to remove linking to libgcc
at all, and it won't hurt the resulting binary at all (because nothing
is currently used from libgcc on ARM).

At first, I thought 'why not remove libgcc if it is not used', but it
is clear to me now that this is not the route to go. I got confused by
the redundant implementation in lib_arm... (Other architectures than
ARM seem to do it better, removing libgcc would break those for the
wrong implementation which is currently in the ARM tree. )

Some routines are still lacking in the lib_arm tree and if GCC starts
generating code (due to some code change) that
requires those missing routines, we now get link errors if a EABI only
compiler is used. (If for example the eabi compiler toolchain of
CodeSourcery is used (e.g. 2008q1) then the provided libgcc is not
compatible with the compile option -mabi=apcs-gnu, the libgcc of the
non-EABI toolchain of CS however seems to be compatible with this
flag)
Today I even saw patches coming by that moves some division routines
to do_div(), just to workaround these linking issues, and thus
actually to prevent linking against libgcc. This happened before in
the past resulting in the (rather big) lib_arm tree.

So, I agree (now) that libgcc should be used, but this is _not the
case_ currently in the ARM tree.

>> While looking at compiler includes, the only header used from GCC (I
>> have seen) seems to be the stdarg.h header, which is even doubtful to
>> include in U-boot, because of the same reasons not to link against
>> libgcc. This patch only removes the linking part to libgcc.
> Why is it doubtful to remove the only portable way to get variable
> arguments?  stdarg.h should *not* contain OS dependencies.

My _first_ conclusion was that the libraries provided by the compiler
toolchain were not suitable to build a standalone application like
U-boot. From that point of view it was logical to say that if no
libraries from a compiler are used, why use any headers?(regardless
what is in there...)

What I now believe that should be done is to investigate (compile
time) what type of ABI is supported by the libgcc provided by the
compiler, and adjust the compile settings to that ABI.
I was already doing some hacks to check if I could get this working...
(A person must have a hobby :-) )
I have removed a lot of redundant implementations in lib_arm and I
have at least succeeded in linking to the EABI libgcc (no hardware
here to test...)

I hope this explanation sounds much more sane to everybody ;-)


Remy

[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Wolfgang Denk]

Dear "Remy Bohmer",

In message <3efb10970811021218g6d30d611r479c8e800991abf4@mail.gmail.com> you wrote:
> 
> libgcc could also expect the 'raise()' routine to pass divide-by-zero
> execeptions. (if __div0, (or __aeabi_ldiv0) is not overruled)

raise()? exceptions? In C?

> In the mean time things are getting more clear. In the ARM section of
> U-boot a large amount of (division) routines which are usually
> provided by libgcc are also implemented in the lib_arm directory. For
> many ARM boards it is therefore possible to remove linking to libgcc
> at all, and it won't hurt the resulting binary at all (because nothing
> is currently used from libgcc on ARM).

...unless you use some (probably older) tool chains  where  this  may
break.  I guess there was a readon to add these functions. We did not
do this for the fun of it.

> Some routines are still lacking in the lib_arm tree and if GCC starts
> generating code (due to some code change) that
> requires those missing routines, we now get link errors if a EABI only
> compiler is used. (If for example the eabi compiler toolchain of
> CodeSourcery is used (e.g. 2008q1) then the provided libgcc is not

> So, I agree (now) that libgcc should be used, but this is _not the
> case_ currently in the ARM tree.

I think I saw libgcc on the linker command line ?

> What I now believe that should be done is to investigate (compile
> time) what type of ABI is supported by the libgcc provided by the
> compiler, and adjust the compile settings to that ABI.

If such a thing needs to be done, it is a compiler-issue, and we do
not have to know or care about that.

Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
Life is a game. Money is how we keep score.              - Ted Turner

[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Remy Bohmer]

Hello Wolfgang,

>> libgcc could also expect the 'raise()' routine to pass divide-by-zero
>> execeptions. (if __div0, (or __aeabi_ldiv0) is not overruled)
> raise()? exceptions? In C?

Please, do not get pikky about choice of words... You can call it an
exception, signal, or whatever. (If I meant C++-exception handling, I
would call it that way)
According to the docs, overrule __div0 (as done in U-boot) or
implement a raise() routine that does not return, in a OS it is
probably redirected to a signal handler (via libc).

>> In the mean time things are getting more clear. In the ARM section of
>> U-boot a large amount of (division) routines which are usually
>> provided by libgcc are also implemented in the lib_arm directory. For
>> many ARM boards it is therefore possible to remove linking to libgcc
>> at all, and it won't hurt the resulting binary at all (because nothing
>> is currently used from libgcc on ARM).
>
> ...unless you use some (probably older) tool chains  where  this  may
> break.  I guess there was a readon to add these functions. We did not
> do this for the fun of it.

I never told it was added for fun...
Even in the case if these functions are added for an old toolchain,
maybe it is preferable to link first to libgcc, and finally to
lib_arm.a to make sure that the most of libgcc functions are used. Or
make it somewhat compiler version related

>> Some routines are still lacking in the lib_arm tree and if GCC starts
>> generating code (due to some code change) that
>> requires those missing routines, we now get link errors if a EABI only
>> compiler is used. (If for example the eabi compiler toolchain of
>> CodeSourcery is used (e.g. 2008q1) then the provided libgcc is not
>
>> So, I agree (now) that libgcc should be used, but this is _not the
>> case_ currently in the ARM tree.
> I think I saw libgcc on the linker command line ?

Yep.. it is there on the commandline... but it is useless! No
functions of this lib are used thus libgcc is not used. (on a modern
GCC 4.2.3 toolchain to be precise) On ARM, you can remove it and still
it works...
You can probably link a 100 more libraries, if it won't resolve any
unresolved external, it will not be used.

>> What I now believe that should be done is to investigate (compile
>> time) what type of ABI is supported by the libgcc provided by the
>> compiler, and adjust the compile settings to that ABI.
>
> If such a thing needs to be done, it is a compiler-issue, and we do
> not have to know or care about that.

Okay, now I really lost you, first you have a very strong opinion that
libgcc _must_ be used. I follow you (I admit I was wrong in the
beginning), so I start investigating for a solution to actually start
using libgcc on ARM, and now you tell me you do not want that libgcc
is actually being used on ARM... (Besides, if you are consequent about
the 'not caring' part, you should also not approve the do_div()
patches which are used to get around linking problems with EABI
toolchains..., or any other compiler-bug workarounds...)


Kind Regards,

Remy

[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Wolfgang Denk]

Dear Remy,

In message <3efb10970811021323w71c480fdq507814c01b834441@mail.gmail.com> you wrote:
> 
> >> What I now believe that should be done is to investigate (compile
> >> time) what type of ABI is supported by the libgcc provided by the
> >> compiler, and adjust the compile settings to that ABI.
> >
> > If such a thing needs to be done, it is a compiler-issue, and we do
> > not have to know or care about that.
> 
> Okay, now I really lost you, first you have a very strong opinion that
> libgcc _must_ be used. I follow you (I admit I was wrong in the
> beginning), so I start investigating for a solution to actually start
> using libgcc on ARM, and now you tell me you do not want that libgcc
> is actually being used on ARM... (Besides, if you are consequent about
> the 'not caring' part, you should also not approve the do_div()

I did not say that.

I said if any compiler options must  be  adapted  to  match  the  ABI
implemented by the tool chain, then this is an issue that is internal
to  this  tool chain, and nothing we should bother about. If the tool
chain  wants  to  produce  FOO-comptible  code  it  must  make   sure
(internally)  to  set  all  required options and provide all required
library routines for the FOO ABI - whatever that might be.

Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
Never worry about theory as long as  the  machinery  does  what  it's
supposed to do.                                      - R. A. Heinlein

[U-Boot] [patch 1/1] Remove linking to libgcc while compiling U-boot

[Wolfgang Denk]

Dear Remy,

In message <20081031224856.847997187@bohmer.net> you wrote:
> Currently U-boot is linking against libgcc. This should not be needed because

And where would the GCC compiler gets its builtin functions from,
then?

> the compiler toolchain is usually compiled with a certain OS interface in mind,
> and can even be configured for GNU-EABI interfaces.

The toolchain is not OS dependent. It implements a cetrain  ABI.  And
libgcc provides the needed support functions.

> This can cause linking conflicts in U-boot when linking to libgcc.

I have an idea what you mean, but as stated here it's wrong.

> It usually becomes really visible that these conflicts are there when some piece
> of code requires an external routine that is not available in U-boot itself.

This must never happen. U-Boot  is  supposed  to  be  self-contained,
except  for any compiler-speeific functions which are provided by the
compiler itself - in libgcc.a (assuming you use GCC).

> Such an unresolved external are finally searched in the libgcc library, because
> U-boot is told to link against. If the compiler happened to be a EABI compiler, linking
> will definately fail. These are not a compiler problems, but U-boot problems, because

No, it will not, unless it is misconfigured.

> U-boot needs to keep its own pants up (It is not linking to any OS, it is standalone)

Correct. But we cannot know what the compiler may need or provide.

> If the compiler is _not_ a EABI compiler toolchain, linking might succeed, but its
> behaviour will be undefined, because it is unknown what the external dependencies
> of such libraries will be. (syscalls required?)

Huch? I cannot make any sense of this.

> While looking at compiler includes, the only header used from GCC (I have seen) seems to
> be the stdarg.h header, which is even doubtful to include in U-boot, because of
> the same reasons not to link against libgcc. This patch only removes the linking
> part to libgcc.

Oops? How would you implement support for variable number of arguments
then, when you remove the only standard way to do it?

> I tested it on several ARM boards, and linking still works fine...

And, do they also run fine?

> Several older mailthreads that show similar issues (just a simple grep):
> http://www.mail-archive.com/u-boot-users at lists.sourceforge.net/msg03176.html

I think the code would not run (because it uses an incompatible [with
the compiler used] ABI version) if you omitted linking against libgcc
here.

> http://lists.denx.de/pipermail/u-boot/2008-August/039526.html

Different problem, has nothing to do with linking against libgcc per
se.

> http://lists.denx.de/pipermail/u-boot/2007-July/022881.html

Has nothing to do with libgcc.a


Your patch will break a lot of PPC boards, for example:

Configuring for BC3450 board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'
make: *** [u-boot] Error 1

Configuring for cm5200 board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'
make: *** [u-boot] Error 1

Configuring for fo300 board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'

Configuring for TB5200 board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'

Configuring for TQM5200 board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'
make: *** [u-boot] Error 1

Configuring for TQM5200_B board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'
make: *** [u-boot] Error 1

Configuring for TQM5200S board...
post/libpost.a(post.o): In function `post_time_ms':
/work/wd/u-boot/post/post.c:451: undefined reference to `__udivdi3'
make: *** [u-boot] Error 1

and so on.


NAK.

Best regards,

Wolfgang Denk

-- 
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd at denx.de
Computers make excellent and efficient servants, but I have  no  wish
to  serve under them. Captain, a starship also runs on loyalty to one
man. And nothing can replace it or him.
	-- Spock, "The Ultimate Computer", stardate 4729.4

[U-Boot] [patch 0/1] [Experimental] U-boot linking against libgcc

[Mike Frysinger]

On Friday 31 October 2008, Remy Bohmer wrote:
> Related to the discussion we had earlier today about linking to libgcc I
> have here a experimental patch that removes the linking to libgcc.
> I called it experimental because I am not able to test if all architectures
> still compile properly, and as such it needs more (build)testing before
> integrating. I tested several ARM boards, and they all compile and link
> fine.
>
> I am convinced that it is the way to go to remove libgcc of the link list,
> but if we integrate this patch I cannot guarantee it does not break some
> builds, but it makes clear which boards are depending on interfaces from a
> library it should not depend on in the first place. Those boards might need
> additional patches.
>
> I hope you can find some time to do some more compile testing across
> different architectures.

it will break Blackfin builds as there are a bunch of places where the 
compiler generates calls to libgcc

i cant see why you'd want to remove libgcc or why you think this is a correct 
long term goal, but Wolfgang already made a much more detailed list ...
-mike
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