Re: [Patch] SourceLevelDebugPkg/SecPeiDebugAgentLib: Fix duplicate symbol

[Andrew Fish <afish@apple.com>]

> On May 25, 2017, at 1:28 PM, Laszlo Ersek <lersek@redhat.com> wrote:
> 
> On 05/25/17 22:11, Ard Biesheuvel wrote:
>> On 25 May 2017 at 13:06, Kinney, Michael D <michael.d.kinney@intel.com> wrote:
>>> Laszlo and Andrew,
>>> 
>>> With the information that has been collected on this thread, I
>>> still think this patch in its original form is a good change
>>> to resolve the this one specific duplicate symbol issue for all
>>> tool chains.  'static' can not be mixed with
>>> GLOBAL_REMOVE_IF_UNREFERENCED for MSFT tool chains, so renaming
>>> the global variable is the easiest way to remove the duplicate.
>>> 
>> 
>> GLOBAL_REMOVE_IF_UNREFERENCED itself is problematic imo. I think it
>> was Felix who reported on this recently?
>> 
>> STATIC is really the only sensible way to deal with this for symbols
>> that are only referenced by a single compilation unit.
>> 
>>> I will continue to work on ways to detect duplicate symbols for
>>> all tool chains and will enter a Bugzilla issue to for that
>>> feature.
>>> 
>>> In addition, the idea of detecting if a library is exporting more
>>> than the library class defines is another good feature to consider
>>> and I will enter a Bugzilla issue for that one as well.
>>> 
>>> If we can find ways to both restrict the symbols exported by a
>>> library and strip all symbols that are unused, then we can have
>>> additional Bugzilla issues to perform that clean up on each
>>> library instance that is exporting more than the library class.
>>> 
>> 
>> A static library is nothing more than an archive containing a
>> collection of object files. Sadly, that implies that we cannot
>> distinguish between symbols that may only be referenced by other
>> objects in the same static library and symbols that are exported to
>> the library client.
> 
> Do we know for a fact that, with /OPT:REF, VS does not strip unused
> *static* variables and functions?
> 
> I mean, is it certain that *replacing* GLOBAL_REMOVE_IF_UNREFERENCED
> with STATIC in this case would lead to a size increase?
> 
> If that's the case, then I'm fine if we go ahead with this patch, I'd
> just like to request that Mike please file some of those BZs, and please
> reference them from the commit message (as the longer term solution),
> before committing the patch.
> 

Clang will warn if you have unused static variables when warnings are cranked up.

~/work/Compiler>cat static.c
static unsigned char gTest[] = { 42 };

static int test ()
{
  return 1;
}

int main ()
{
  return 0;
}
~/work/Compiler>clang -Os static.c -Wall
static.c:1:22: warning: unused variable 'gTest' [-Wunused-variable]
static unsigned char gTest[] = { 42 };
                     ^
static.c:3:12: warning: unused function 'test' [-Wunused-function]
static int test ()
           ^
2 warnings generated.


Thanks,

Andrew Fish

> Thanks
> Laszlo