libgcc: On AIX, increase chances to find landing pads for exceptions

David Edelsohn dje.gcc@gmail.com
Wed Feb 10 13:27:00 GMT 2016


On Wed, Feb 10, 2016 at 1:52 AM, Michael Haubenwallner
<michael.haubenwallner@ssi-schaefer.com> wrote:
>
> On 02/08/2016 02:59 PM, David Edelsohn wrote:
>> Runtime linking is disabled by default on AIX, and I disabled it for libstdc++.
>
> For large applications mainly developed on/for Linux I do prefer/need
> runtime linking even on AIX. Still I do believe there is no AIX-based
> reason to leave runtime linking disabled, but build-/linktime issues
> instead that cause things to fail with runtime linking enabled.

What do you mean by the term "runtime linking"?  Runtime linking means
runtime overloading of symbols -- preloading -- not dynamic linking
and loading.  dlopen does not require runtime linking.  There also are
issues with searching for shared objects with .a or .so file
extension, but that can be addressed separately.

Runtime linking causes every global, exported function call to be
invoked through indirect glue code.  And each function must be
inserted into the TOC.  The indirect call overhead is very expensive,
and potential TOC overflow can cause even more performance
degradation.

Your statement of no AIX-based reason to leave runtime linking
disabled is fundamentally flawed.

>
>> There are two remaining issues:
>>
>> 1) FDEs with overlapping ranges causing problems with exceptions.  I'm
>> not sure of the best way to work around this.  Your patch is one
>> possible solution.
>
> This patch is not meant as a final solution, but to improve current
> situation with broken build systems exporting even _GLOBAL__ symbols.
> I'm about to prepare another libtool patch to fix that one.
>
>> 2) AIX linker garbage collection conflicting with scanning for
>> symbols.  collect2 scanning needs to better emulate SVR4 linker
>> semantics for object files and archives.
>
> Probably collect2 should filter the symbol list originating in either
> an explicit -bexport:file or the -bexpall/-bexpfull flags and pass the
> resulting symbol list as explicit -bexport:file only to the AIX linker?

-bexpall and -bexpfull cause numerous problem by re-exporting symbols.

All of the suggestions will produce programs that function, but have
severe performance impacts and unintended consequences that you seem
to be ignoring.

- David

>
> /haubi/
>
>>
>> Thanks, David
>>
>>
>> On Mon, Feb 8, 2016 at 7:14 AM, Michael Haubenwallner
>> <michael.haubenwallner@ssi-schaefer.com> wrote:
>>> Hi David,
>>>
>>> still experiencing exception-not-caught problems with gcc-4.2.4 on AIX
>>> leads me to some patch proposed in http://gcc.gnu.org/PR13878 back in
>>> 2004 already, ought to be fixed by some different commit since 3.4.0.
>>>
>>> As long as build systems (even libtool right now) on AIX do export these
>>> _GLOBAL__* symbols from shared libraries, overlapping frame-base address
>>> ranges may become registered, even if newer gcc (seen with 4.8) does name
>>> the FDE symbols more complex to reduce these chances.
>>>
>>> But still, just think of linking some static library into multiple shared
>>> libraries and/or the main executable. Or sometimes there is just need for
>>> some hackery to override a shared object's implementation detail and rely
>>> on runtime linking to do the override at runtime.
>>>
>>> Agreed both is "wrong" to some degree, but the larger an application is,
>>> the higher is the chance for this to happen.
>>>
>>> Thoughts?
>>>
>>> Thanks!
>>> /haubi/



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