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

["Kinney, Michael D" <michael.d.kinney@intel.com>]

Andrew,

The VS compilers available when GLOBAL_REMOVE_IF_UNREFERENCED was added referred to __declspec( selectany ) as putting the symbol into its own comdat, so it was then available to be optimized away with the use of OPT:REF.

I think it is time to re-evaluate the VS optimizers to see if they can optimize away global variables without being decorated with__declspec( selectany ).  If we can remove __declspec( selectany ), then we have a path to use STATIC properly to hide global variables that are not declared as extern in the library class.

I will investigate some more.

Mike

From: afish@apple.com [mailto:afish@apple.com]
Sent: Thursday, May 25, 2017 2:26 PM
To: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; Wu, Hao A <hao.a.wu@intel.com>; edk2-devel@lists.01.org; Felix Poludov <Felixp@ami.com>; Kinney, Michael D <michael.d.kinney@intel.com>; Fan, Jeff <jeff.fan@intel.com>
Subject: Re: [edk2] [Patch] SourceLevelDebugPkg/SecPeiDebugAgentLib: Fix duplicate symbol


On May 25, 2017, at 2:02 PM, Laszlo Ersek <lersek@redhat.com<mailto:lersek@redhat.com>> wrote:

On 05/25/17 22:37, Andrew Fish wrote:



On May 25, 2017, at 1:28 PM, Laszlo Ersek <lersek@redhat.com<mailto: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<mailto: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.

Sorry, my question was imprecise.

Assume there is a public library function ("external linkage") that
calls a static function in the same library instance and uses a static
variable in the same library instance. Then this library instance is
linked into a driver, but the driver never actually calls the extern
function -- so the static variable and the static function too become
useless.

In this case, will /OPT:REF remove the static variable and the static
function too?

It seems counter-intuitive to me that an internal-only function or an
internal-only variable has to be declared extern (via
GLOBAL_REMOVE_IF_UNREFERENCED) just so it can be eliminated at link
time, if it is never referenced (transitively).


Laszlo,

I agree. The LLVM LTO does not have an issue "doing the right thing". Seems like static is also more of a compile time concept vs a link time (global optimization) kind of thing?

Given on VC++ GLOBAL_REMOVE_IF_UNREFERENCED maps to __declspec( selectany ) I would guess maybe it has more to due with supporting old non standard header files that can't change without breaking compatibility.

MSDN on __declspec( selectany ) :
A global data item can normally be initialized only once in an EXE or DLL project. selectany can be used in initializing global data defined by headers, when the same header appears in more than one source file. selectany is available in both the C and C++ compilers.

Thanks,

Andrew Fish



Thanks
Laszlo
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