Re: TOC errors on AIX

[David Edelsohn <dje dot gcc at gmail dot com>]

On Sun, Jul 29, 2012 at 12:48 PM, Perry Smith <pedzsan@gmail.com> wrote:
> Hi,
>
> This is an age old topic but I can't find how to solve it.  I've searched the past few days.
>
> I'm trying to build passenger on AIX 6.1 TL07 SP03 using gcc 4.5.2 that I built myself.  I've used it for a number of months and have built many things.
>
> The short question is how do I get rid of these warnings?
>
>> ld: 0711-768 WARNING: Object ext/apache2/module_libboost_oxt.a[system_calls.o], section 1, function .accept:
>>         The branch at address 0x2ac0 is not followed by a recognized no-op
>>         or TOC-reload instruction. The unrecognized instruction is 0x7C601B78.
>
> The link has many errors like this.  For whatever reason, passenger builds the same file two different ways and puts them in two different places.  Upon closer examination the file that the link is complaining about does in fact have bad code.
>
> The bad compile line is:
>
>> g++ -Iext -fPIC -fvisibility=hidden -DVISIBILITY_ATTRIBUTE_SUPPORTED -U__STR__ -D_THREAD_SAFE -D_LARGEFILE64_SOURCE -I/gsa/ausgsa/projects/r/ruby/apache2/include/apr-1 -I/gsa/ausgsa/projects/r/ruby/apache2/include/apr-1 -I/gsa/ausgsa/projects/r/ruby/apache2/include -D_REENTRANT -I/usr/local/include -DHASH_NAMESPACE="__gnu_cxx" -DHASH_NAMESPACE="__gnu_cxx" -DHASH_FUN_H="<hash_fun.h>" -DOXT_DISABLE_BACKTRACES -DHAS_ALLOCA_H -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wpointer-arith -Wwrite-strings -Wno-long-long -Wno-missing-field-initializers -g -DPASSENGER_DEBUG -DBOOST_DISABLE_ASSERTS -o ext/apache2/module_libboost_oxt/oxt/system_calls.o -c ext/oxt/system_calls.cpp
>
> and produces:
>
>> ...
>>        lwz 5,128(31)
>>         bl .accept
>>         mr 0,3
>> ...
>
> There needs to be a nop after the bl so the linker / loader can stuff in an instruction to restore the toc.
>
> There are many warnings for this compile as well about the visibility.  e.g.
>
>> ext/oxt/system_calls.cpp: In function 'int oxt::syscalls::accept(int, sockaddr*, socklen_t*)':
>> ext/oxt/system_calls.cpp:209:1: warning: visibility attribute not supported in this configuration; ignored
>
>
> The compile line that is good is:
>
>> g++ -Iext  -D_REENTRANT -I/usr/local/include -DHASH_NAMESPACE="__gnu_cxx" -DHASH_NAMESPACE="__gnu_cxx" -DHASH_FUN_H="<hash_fun.h>" -DOXT_DISABLE_BACKTRACES -DHAS_ALLOCA_H -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wpointer-arith -Wwrite-strings -Wno-long-long -Wno-missing-field-initializers -g -DPASSENGER_DEBUG -DBOOST_DISABLE_ASSERTS -o ext/common/libboost_oxt/oxt/system_calls.o -c ext/oxt/system_calls.cpp
>
> which produces good code:
>
>> ...
>>         lwz 5,128(31)
>>         bl .accept
>>         nop
>>         mr 0,3
>> ...
>
> The good compile has no warnings.
>
> I figure it is the -fvisibility that is getting me into trouble but I am not sure if it is safe to just get rid of it.  There are also macros in the boost *.hpp files that muck with the visibility setting.
>
> The link line is (if needed):
>
>> g++ -shared ext/apache2/Configuration.o ext/apache2/Bucket.o ext/apache2/Hooks.o ext/apache2/mod_passenger.o -fPIC -o ext/apache2/mod_passenger.so -fPIC -fvisibility=hidden -DVISIBILITY_ATTRIBUTE_SUPPORTED -U__STR__ -D_THREAD_SAFE -D_LARGEFILE64_SOURCE -I/gsa/ausgsa/projects/r/ruby/apache2/include/apr-1 -I/gsa/ausgsa/projects/r/ruby/apache2/include/apr-1 -I/gsa/ausgsa/projects/r/ruby/apache2/include -D_REENTRANT -I/usr/local/include -DHASH_NAMESPACE="__gnu_cxx" -DHASH_NAMESPACE="__gnu_cxx" -DHASH_FUN_H="<hash_fun.h>" -DOXT_DISABLE_BACKTRACES -DHAS_ALLOCA_H -Wall -Wextra -Wno-unused-parameter -Wno-parentheses -Wpointer-arith -Wwrite-strings -Wno-long-long -Wno-missing-field-initializers -g -DPASSENGER_DEBUG -DBOOST_DISABLE_ASSERTS ext/apache2/module_libpassenger_common.a ext/apache2/module_libboost_oxt.a -fPIC -L/gsa/ausgsa/projects/r/ruby/apache2/lib -lapr-1 -Wl,-G -Wl,-brtl -L/gsa/ausgsa/projects/r/ruby/apache2/lib -laprutil-1 -lpthread

The problem most likely is -fvisibility.  AIX does not support
visibility the way that GNU/Linux does, although GNU/Linux is evolving
toward the symbol visibility model that AIX had all along.

Two questions are:

1) Why does the package think that AIX supports visibility and using
those options?

2) Why is the package linking with -Wl,-G -Wl,-brtl?

Basically, the package is trying to mix-and-match AIX linking behavior
and SVR4 behavior.  It is trying to invoke the AIX linker with SVR4
behavior, but then overriding the default SVR4 semantics and not using
matching flags for AIX.  AIX defaults to not exporting most symbols,
but has options to export more symbols -- matching SVR4 behavior.

The visibility options are trying to override the default SVR4 symbol
export behavior, which asserts to the compiler that the symbols will
be local and not need nops required by external calls.  But then the
AIX linker is invoked with options to export more symbols and it is
finding external definitions of the symbols.

I do not know what you mean by building two files in two different
ways and placing them in two different locations.  Does the link line
link against both copies?  The Makefile also may be confused about
PIC, which is the default on AIX.  The external symbol may be
overriding a local symbol because of AIX linker semantics.

The basic problem is GNU/Linux symbol visibility behavior is becoming
more complicated (and actually evolving toward original AIX behavior),
but it also is using additional syntax and options in Makefiles
without invoking AIX with compatible, matching options.

Removing -fvisibility probably will not cause any additional problems
because the AIX linker already is ignoring the options and intended
behavior, but many more symbols are leaking out than the package
intended.

- David