Liming, I agree with /Gw. That works for newer versions of VS. We will need to adjust the behavior of GLOBAL_REMOVE_IF_UNREFERENCED based on VS version as well.
We can not define GLOBAL_REMOVE_IF_UNREFERENCED to static. We also use this macro for globals that are required to be exported from a library. So static should be added to the globals that are not exported. The challenge is that older versions of VS require GLOBAL_REMOVE_IF_UNREFERENCED to be mapped to __declspec(selectany) and static can not be combined with __declspec(selectany). Mike > -----Original Message----- > From: Gao, Liming > Sent: Thursday, May 25, 2017 10:21 PM > To: Kinney, Michael D <[email protected]>; [email protected]; Laszlo > Ersek > <[email protected]>; Kinney, Michael D <[email protected]> > Cc: Wu, Hao A <[email protected]>; [email protected]; Fan, Jeff > <[email protected]>; Felix Poludov <[email protected]>; Ard Biesheuvel > <[email protected]> > Subject: RE: [edk2] [Patch] SourceLevelDebugPkg/SecPeiDebugAgentLib: Fix > duplicate symbol > > Mike: > I remember community suggests to use VS /Gw option to remove the global > data, > and then can define GLOBAL_REMOVE_IF_UNREFERENCED as empty or static. > > Thanks > Liming > >-----Original Message----- > >From: edk2-devel [mailto:[email protected]] On Behalf Of > >Kinney, Michael D > >Sent: Friday, May 26, 2017 6:42 AM > >To: [email protected]; Laszlo Ersek <[email protected]>; Kinney, Michael D > ><[email protected]> > >Cc: Wu, Hao A <[email protected]>; [email protected]; Fan, Jeff > ><[email protected]>; Felix Poludov <[email protected]>; Ard Biesheuvel > ><[email protected]> > >Subject: Re: [edk2] [Patch] SourceLevelDebugPkg/SecPeiDebugAgentLib: Fix > >duplicate symbol > > > >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: [email protected] [mailto:[email protected]] > >Sent: Thursday, May 25, 2017 2:26 PM > >To: Laszlo Ersek <[email protected]> > >Cc: Ard Biesheuvel <[email protected]>; Wu, Hao A > ><[email protected]>; [email protected]; Felix Poludov > ><[email protected]>; Kinney, Michael D <[email protected]>; Fan, > >Jeff <[email protected]> > >Subject: Re: [edk2] [Patch] SourceLevelDebugPkg/SecPeiDebugAgentLib: Fix > >duplicate symbol > > > > > >On May 25, 2017, at 2:02 PM, Laszlo Ersek > ><[email protected]<mailto:[email protected]>> wrote: > > > >On 05/25/17 22:37, Andrew Fish wrote: > > > > > > > >On May 25, 2017, at 1:28 PM, Laszlo Ersek > ><[email protected]<mailto:[email protected]>> wrote: > > > >On 05/25/17 22:11, Ard Biesheuvel wrote: > > > >On 25 May 2017 at 13:06, Kinney, Michael D > ><[email protected]<mailto:[email protected]>> 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 > >_______________________________________________ > >edk2-devel mailing list > >[email protected]<mailto:[email protected]> > >https://lists.01.org/mailman/listinfo/edk2-devel > > > >_______________________________________________ > >edk2-devel mailing list > >[email protected] > >https://lists.01.org/mailman/listinfo/edk2-devel _______________________________________________ edk2-devel mailing list [email protected] https://lists.01.org/mailman/listinfo/edk2-devel
