[PATCH] Fix unresolved linker symbols from misplaced extern definitions

[Chuck Atkins via llvm-commits]

Hi David,
Thanks for taking a look.  Yes, explicitly namespacing the externs
fixes the issue in the same way. The failure is a link error with
undefined references for LoopSimplifyID and LCSSAID in that object
file.

../../lib/libLLVMTransformUtils.a(LoopUtils.cpp.o): In function
`llvm::getLoopAnalysisUsage(llvm::AnalysisUsage&)':
LoopUtils.cpp:(.text+0x33dd): undefined reference to `LoopSimplifyID'
LoopUtils.cpp:(.text+0x3410): undefined reference to `LCSSAID'

 Is there a reason to use a locally defined extern definition instead
of pulling it in from the header it already resides in?

- Chuck

On Tue, Sep 20, 2016 at 7:16 PM, David Majnemer
<david.majnemer at gmail.com> wrote:
>
>
> On Tue, Sep 20, 2016 at 3:55 PM, David Majnemer <david.majnemer at gmail.com>
> wrote:
>>
>>
>>
>> On Fri, Sep 16, 2016 at 8:15 AM, Chuck Atkins via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>>>
>>> ping?
>>>
>>> Anybody mind taking a look at this patch?  It's fairly trivial; but
>>> without it, builds with the Intel compiler fail.
>>
>>
>> What is the failure mode?
>
>
>
> Out of curiosity, does the following make ICC any happier:
>
> diff --git a/lib/Transforms/Utils/LoopUtils.cpp
> b/lib/Transforms/Utils/LoopUtils.cpp
> index 8878689..1b2a2d6 100644
> --- a/lib/Transforms/Utils/LoopUtils.cpp
> +++ b/lib/Transforms/Utils/LoopUtils.cpp
> @@ -929,6 +929,13 @@ SmallVector<Instruction *, 8>
> llvm::findDefsUsedOutsideOfLoop(Loop *L) {
>    return UsedOutside;
>  }
>
> +// We locally access their IDs here because users shouldn't directly get
> them
> +// from this header.
> +namespace llvm {
> +extern char &LoopSimplifyID;
> +extern char &LCSSAID;
> +}
> +
>  void llvm::getLoopAnalysisUsage(AnalysisUsage &AU) {
>    // By definition, all loop passes need the LoopInfo analysis and the
>    // Dominator tree it depends on. Because they all participate in the loop
> @@ -938,10 +945,7 @@ void llvm::getLoopAnalysisUsage(AnalysisUsage &AU) {
>    AU.addRequired<LoopInfoWrapperPass>();
>    AU.addPreserved<LoopInfoWrapperPass>();
>
> -  // We must also preserve LoopSimplify and LCSSA. We locally access their
> IDs
> -  // here because users shouldn't directly get them from this header.
> -  extern char &LoopSimplifyID;
> -  extern char &LCSSAID;
> +  // We must also preserve LoopSimplify and LCSSA.
>    AU.addRequiredID(LoopSimplifyID);
>    AU.addPreservedID(LoopSimplifyID);
>    AU.addRequiredID(LCSSAID);
>
>
>>
>>
>>>
>>>
>>> On Tue, Sep 13, 2016 at 11:38 AM, Chuck Atkins <chuck.atkins at kitware.com>
>>> wrote:
>>>>
>>>> When LCSSAID and LoopSimplifyID are declared extern inside a function
>>>> body,
>>>> some linkers will fail to resolve the actual symbols defined in the llvm
>>>> namespace.  By removing the function local extern declarations and
>>>> relying
>>>> on the header definitions in Scalar.h, the symbols can be properly
>>>> reolved.
>>>> This is specifically an issue when building with the Intel compiler.
>>>> ---
>>>>  lib/Transforms/Utils/LoopUtils.cpp | 3 +--
>>>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>>>
>>>> diff --git a/lib/Transforms/Utils/LoopUtils.cpp
>>>> b/lib/Transforms/Utils/LoopUtils.cpp
>>>> index 3902c67..9a1cd0b 100644
>>>> --- a/lib/Transforms/Utils/LoopUtils.cpp
>>>> +++ b/lib/Transforms/Utils/LoopUtils.cpp
>>>> @@ -26,6 +26,7 @@
>>>>  #include "llvm/IR/ValueHandle.h"
>>>>  #include "llvm/Pass.h"
>>>>  #include "llvm/Support/Debug.h"
>>>> +#include "llvm/Transforms/Scalar.h"
>>>>  #include "llvm/Transforms/Utils/LoopUtils.h"
>>>>
>>>>  using namespace llvm;
>>>> @@ -845,8 +846,6 @@ void llvm::getLoopAnalysisUsage(AnalysisUsage &AU) {
>>>>
>>>>    // We must also preserve LoopSimplify and LCSSA. We locally access
>>>> their IDs
>>>>    // here because users shouldn't directly get them from this header.
>>>> -  extern char &LoopSimplifyID;
>>>> -  extern char &LCSSAID;
>>>>    AU.addRequiredID(LoopSimplifyID);
>>>>    AU.addPreservedID(LoopSimplifyID);
>>>>    AU.addRequiredID(LCSSAID);
>>>> --
>>>> 2.7.4
>>>>
>>>
>>>
>>> _______________________________________________
>>> llvm-commits mailing list
>>> llvm-commits at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>
>>
>