Subject: Re: webkitgtk fails to build on i686-linux; possibly a clang issue

From: Mark H Weaver <[email-masked]>
Date: Thu, 04 Nov 2021 19:15:20 -0400

Liliana Marie Prikler <[email-masked]> writes:

> Am Mittwoch, den 03.11.2021, 17:04 -0400 schrieb Mark H Weaver:
>> Earlier, I wrote:
>> > libwebkit2gtk-4.0.so fails to link on i686-linux, due to an
>> > undefined reference to '__mulodi4'.
>> 
>> Here are some relevant links:
>> 
>>   https://bugs.webkit.org/show_bug.cgi?id=190208
>>   https://trac.webkit.org/changeset/272140/webkit
>>   https://github.com/android/ndk/issues/506
> This error does not occur when compiling with GCC [1].

Right.  As mentioned in the first link above:

  "This is because clang generates code using the __mulodi4 symbol for
   __builtin_mul_overflow.  But this symbol is available only in
   compiler-rt, and not in the libgcc runtime used by most Linux
   distributions of clang."

So, one possible solution might be to link with compiler-rt, which is
the 'clang-runtime-11' package in Guix.  However, it's possible that
this might cause other complications.

A more conservative approach would be to apply a patch to
trunk/Source/WTF/wtf/CheckedArithmetic.h analogous to the one in the
second link I cited above, namely this one:

  https://trac.webkit.org/changeset/272140/webkit

However, it would need to be changed slightly.  The patch above arranges
to avoid using __builtin_mul_overflow on 32-bit ARM systems.  We would
need to do the same for 32-bit x86 as well.  So, where the patch above
has this:

--8<---------------cut here---------------start------------->8---
/* On Linux with clang, libgcc is usually used instead of compiler-rt, and it does    
 * not provide the __mulodi4 symbol used by clang for __builtin_mul_overflow    
 */    
#if COMPILER(GCC) || (COMPILER(CLANG) && !(CPU(ARM) && OS(LINUX)))    
#define USE_MUL_OVERFLOW 1    
#endif    
--8<---------------cut here---------------end--------------->8---

We would need to change "CPU(ARM)" to "(CPU(ARM) || CPU(XXX))", where
XXX is the appropriate symbol for 32-bit x86.  Or maybe there's another
solution.

I won't be able to look at this in the next couple of days, so hopefully
someone else can pick this up.

> However, now dependant packages fail to link Webkit [2].  We might have
> to add GCC 11 to all of them -- or at least to a fair number.  I've
> verified that gnome-online-accounts builds with GCC 11 added, we might
> want to make sure we check the rest of the gnome package as well.

I'm not sure about this approach.  Maybe it's feasible, but there might
be problems if *any* C++ library built using GCC 7 is linked together
with WebKitGTK.

> On that note, which GCC will be the standard once core-updates-frozen
> is merged?  If it's not GCC 11 – say GCC 10 – we might want to try to
> get Webkit building with that instead, so that at least after the merge
> we're clean on that front.

The standard compiler on 'core-updates-frozen' is GCC 10.  As I wrote
elsewhere, I think it's quite likely that these workarounds will not be
needed on 'core-updates-frozen'.

>> Also, I just noticed that "-mfpmath=sse -msse2" is being passed on
>> the compile command line.  Historically, we've chosen not to assume
>> the availability of SSE or SSE2 on i686-linux, so it would be good to
>> inhibit those flags.
> This is still true for the GCC build.  Could you add the necessary
> flags to disable them?

I don't know when I'll be able to look into it.  It's a busy time for me.

      Regards,
        Mark

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