[LLVMdev] Fwd: [windows) how to use weak references with llvm 3.4 and windows?

[Reid Kleckner]

You'd have to use extern_weak linkage.  Clang compiles the foo declaration
to:
$ clang -cc1 -emit-llvm -o - t.c | grep declare.*@foo
declare extern_weak i32 @foo(...) #1



On Tue, Feb 11, 2014 at 4:16 AM, Carl <name.is.carl at gmail.com> wrote:

> Thanks for your clear answer. Do you know what modifier should I use to
> declare such weak symbols in my llvm intermediate code?
>
> So that it can be compiled to the .o file with the weak attribute ?
> Le 10 févr. 2014 19:44, "Reid Kleckner" <rnk at google.com> a écrit :
>
> COFF doesn't support the same kind of concept of 'weak' that ELF does.
>>  This is the issue that users brought up with mingw:
>> https://sourceware.org/bugzilla/show_bug.cgi?id=9687
>>
>> Instead, COFF supports weak externals, which you can use like:
>>
>> $ cat t.c
>> int foo() __attribute__((weak));
>> int main() {
>>   if (foo)
>>     return foo();
>>   return 13;
>> }
>>
>> $ cat t2.c
>> int foo() {
>>   return 42;
>> }
>>
>> $ clang t.c t2.c -o t && ./t ; echo $?
>> 42
>>
>> So, we got the definition of foo from t2.c.  If t2.c hadn't been linked,
>> foo would be null.
>>
>>
>> On Mon, Feb 10, 2014 at 6:28 AM, Carl <name.is.carl at gmail.com> wrote:
>>
>>>
>>> Hello,
>>>
>>> I'm generating C code (and the resulting obj files) using llvm 3.4 for
>>> both unix and windows.
>>> And I use the dreaded weak references, that, for windows, are not too
>>> widely supported.
>>>
>>> When I link my application on linux, I have no issue.
>>> But when I'm doing the same on windows using mingw I got a duplicate
>>> symbol error :
>>>
>>> ..\robovm-0.0.8\lib\robovm-rt.jar\dalvik\system\BlockGuard$BlockGuardPolicyException.class.o:(.text$dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicy__I_lookup[_dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicy__I_lookup]+0x0):
>>> multiple definition of
>>> `dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicy__I_lookup'
>>> ..\Temp\robovm3774596063679264132.tmp\linker.o:(.text+0x340): first
>>> defined here
>>> ..\robovm-0.0.8\lib\robovm-rt.jar\dalvik\system\BlockGuard$BlockGuardPolicyException.class.o:(.text$dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicyViolation__I_lookup[_dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicyViolation__I_lookup]+0x0):
>>> multiple definition of
>>> `dalvik_system_BlockGuard$24BlockGuardPolicyException_getPolicyViolation__I_lookup'
>>> ..\Temp\robovm3774596063679264132.tmp\linker.o:(.text+0x350): first
>>> defined here
>>> ..\robovm-0.0.8\lib\robovm-rt.jar\dalvik\system\BlockGuard$BlockGuardPolicyException.class.o:(.text$dalvik_system_BlockGuard$24BlockGuardPolicyException_getMessage__Ljava_lang_String$3B_lookup[_dalvik_system_BlockGuard$24BlockGuardPolicyException_getMessage__Ljava_lang_String$3B_lookup]+0x0):
>>> multiple definition of
>>> `dalvik_system_BlockGuard$24BlockGuardPolicyException_getMessage__Ljava_lang_String$3B_lookup'
>>>
>>> Because those symbols are declared as weak.
>>>
>>> What is the status about the support of weak symbols on windows?
>>> Are they supposed to work? Are they supposed to *never *work ?
>>>
>>> Do you know any way to fix this (or work around it). I use weak symbols
>>> everywhere in my code generation, so I would prefer a fix that doesn't
>>> involve to rewrite my code.
>>>
>>> I'm considering rewriting the obj files using objcopy.
>>> Has this any chance to work ?
>>> Here are the o.
>>>
>>> Cheers,
>>> Carl.
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>>>
>>
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