Re: [Patch libitm] [V2] config-based assessment of weakref capability

[Jack Howarth <howarth at bromo dot med dot uc dot edu>]

On Fri, Jan 06, 2012 at 06:07:37PM +0000, Iain Sandoe wrote:
>
> On 6 Jan 2012, at 18:04, Patrick Marlier wrote:
>
>> On 01/06/2012 12:30 PM, Iain Sandoe wrote:
>>>
>>> On 6 Jan 2012, at 16:42, Jack Howarth wrote:
>>>
>>>> On Fri, Jan 06, 2012 at 10:58:51AM -0500, Patrick Marlier wrote:
>>>>> On 01/06/2012 10:38 AM, Mike Stump wrote:
>>>>>> On Jan 6, 2012, at 12:29 AM, Iain Sandoe wrote:
>>>>>>>> I think something is wrong in the configure. Indeed, on
>>>>>>>> apple-darwin10.8.0, weak ref works but it is configure as "no".
>>>>>>>> I guess it should not try to run it but only compile it (or the
>>>>>>>> symbol should exists).
>>>>>>>
>>>>>>> Which tool-chain are you using ? (XCode version) - the problem is
>>>>>>> tool-chain related.
>>>>>>> thanks
>>>>>>
>>>>>> 3.2.6 on x86_64-apple-darwin10.8.0 works for me with either 
>>>>>> -m32 or
>>>>>> -m64. If you have an older Xcode 3.2.x, try updating.
>>>>>
>>>>> 4.0 (build 4A304a) on x86_64-apple-darwin10.8.0. I will try to  
>>>>> update
>>>>> it.
>>>>>
>>>>> Thanks.
>>>>
>>>> Using Xcode 4.2 on Snow Leopard, I see the same runtime failure for
>>>> the configure test of
>>>> "checking whether weak refs work like ELF"...
>>>
>>> Note the "like ELF"
>>> .. the purpose of the test is to find out if we need to have the  
>>> dummy
>>> routines.
>>>
>>> (weak refs actually work at runtime for all current versions of  
>>> Darwin -
>>> - the tricky bit is getting the link phase right)
>>>
>>> Is the resulting libitm working?
>>
>> libitm compiles but the dummy function are used at runtime (eh-1.C  
>> testcase fails)...
>> Removing the dummy definitions in eh_cpp.cc makes it compile and work.
>
> OK. could you file a PR please?
> thanks

Iain,
   We have radar Problem ID: 10466868, "-undefined dynamic_lookup linker bug" open with
your testcase (which I appended below). Do you recall if Nick ever indicated that this
was recognized a real regression in Xcode 4's linker or if it was unsupported  behavior on
darwin that happened to work with the linker on prior Xcode releases? Also, perhaps we
can find someone with access to the Xcode 4.3 pre-release so they can check if this is
fixed yet.
                 Jack

17-Nov-2011 08:22 PM Jack Howarth:
Summary: The following weak.c test case reveals a linker bug in -undefined dynamic_lookup in Xcode 4.2/4.2.1 under both darwin10 and darwin11.

--------------- weak.c ---------------------
#include <stdio.h>

char *myweakfunc(void) __attribute__((weak)) ;

int main(int argc, char **argv)
{
  if (myweakfunc)
    printf ("found myweakfunc %s\n", myweakfunc());
  else
    printf("Weak func not found\n");
  return 0;
}
-------------------------------------------

Steps to Reproduce:
1) Install Xcode 4.2 on darwin10 or darwin11.
2) Compile the weak.c test case with...

/usr/bin/llvm-gcc weak.c -o wk -undefined dynamic_lookup

3) Execute the resulting "wk" executable

Expected Results:

I expected the same results as is seen when the weak.c test case is compiled with llvm-gcc from Xcode 3.2.6...

howarth% /Developer-3.2.6/usr/bin/llvm-gcc weak.c -o wk -undefined dynamic_lookup
howarth% ./wk
Weak func not found

Actual Results:

The resulting binary fails to execute with the runtime error...

dyld: Symbol not found: _myweakfunc
  Referenced from: /Users/howarth/./wk
  Expected in: flat namespace
 in /Users/howarth/./wk
Trace/BPT trap

Regression:

This can be shown to be a regression in Xcode 4.2's ld since...

howarth% /usr/bin/llvm-gcc weak.c -o wk -undefined dynamic_lookup --save-temps
howarth% /Developer-3.2.6//usr/bin/ld -dynamic -arch x86_64 -macosx_version_min 10.6.8 -undefined dynamic_lookup -weak_reference_mismatches non-weak -undefined dynamic_lookup -o wk -lcrt1.10.6.o -L/Developer-3.2.6/usr/llvm-gcc-4.2/bin/../lib/gcc/i686-apple-darwin10/4.2.1/x86_64 -L/Developer/usr/llvm-gcc-4.2/lib/gcc/i686-apple-darwin10/4.2.1/x86_64 -L/usr/lib/gcc/i686-apple-darwin10/4.2.1/x86_64 -L/usr/lib/i686-apple-darwin10/4.2.1 -L/Developer-3.2.6/usr/llvm-gcc-4.2/bin/../lib/gcc/i686-apple-darwin10/4.2.1 -L/Developer-3.2.6/usr/llvm-gcc-4.2/bin/../lib/gcc -L/Developer/usr/llvm-gcc-4.2/lib/gcc/i686-apple-darwin10/4.2.1 -L/usr/lib/gcc/i686-apple-darwin10/4.2.1 -L/Developer-3.2.6/usr/llvm-gcc-4.2/bin/../lib/gcc/i686-apple-darwin10/4.2.1/../../.. -L/Developer/usr/llvm-gcc-4.2/lib/gcc/i686-apple-darwin10/4.2.1/../../.. weak.o -lSystem -lgcc -lSystem
howarth% ./wk
Weak func not found

works when the Xcode 3.2.6 linker is used instead of the Xcode 4.2 linker.

Notes:


> Iain