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

[Reid Kleckner]

Yep, llvm::GlobalValue::ExternalWeakLinkage.


On Wed, Feb 12, 2014 at 2:31 AM, Carl <name.is.carl at gmail.com> wrote:

> So instead of using linkagetypes.weak I should use externalweaklinkage ?
> Le 11 févr. 2014 18:29, "Reid Kleckner" <rnk at google.com> a écrit :
>
> 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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