Re: [edk2-devel] [Patch 11/12] OvmfPkg: Enable CLANG9 tool chain

["Liming Gao" <liming.gao@intel.com>]

Laszlo:

> -----Original Message-----
> From: Laszlo Ersek <lersek@redhat.com>
> Sent: Wednesday, October 9, 2019 9:44 PM
> To: Andrew Fish <afish@apple.com>; devel@edk2.groups.io
> Cc: Gao, Liming <liming.gao@intel.com>
> Subject: Re: [edk2-devel] [Patch 11/12] OvmfPkg: Enable CLANG9 tool chain
> 
> On 10/09/19 01:08, Andrew Fish wrote:
> 
> > So I guess the way to describe it is XCODE inherits GCC and only needs to override when it is different.
> 
> Thank you for the explanation!
> 
> I've been trying to figure out why this inheritance bothers me so much.
> I guess the reason is the following:
> 
> I'm a user of the actual GCCxx toolchains (such as GCC48, GCC49, GCC5).
> Assume that I realize that a gcc-specific change is needed to some part
> of the code. (It could be build options, or gcc specific source code,
> etc.) Then, I go to the gcc documentation, and verify whether the change
> is indeed supported by all the relevant gcc versions.
> 
> If the change is possible with only some gcc versions, then I likely
> write the change for a specific toolchain only, such as GCC5. Fine.
> 
> Now assume that my documentation review proves that *all* GCCxx
> toolchains, supported by edk2, should receive the update. This could be
> a #pragma, a command line flag, some __attribute__, etc. I'd very likely
> express the change for the whole GCC toolchain *family*, and not
> enumerate it for GCC48, GCC49, GCC5 individually.
> 
> And now I learn that said approach would be wrong, because such a change
> would be inherited by XCODExx and CLANGxx too (via FAMILY), fully
> against my intent, unless XCODExx and CLANGxx overrode the particular
> aspect (via BUILDRULEFAMILY).

The difference between XCODE/CLANG and GCCXX is the linker. Current patches are
introduced for the different linker. Clang supports most usage of GCC compiler.
So, CLANG and XCODE uses GCC family. When I enable XCODE or CLANG tool chain
in the platform, I don't find other incompatible behavior with GCC compiler.
So, I think it is safe to let CLANG inherit GCC option except for these two cases.
When you make new changes, and verify them with GCC compiler, that's enough.
You don't need to specially verify them with XCODE or CLANG. In most case, GCC 
compiler pass, XCODE or CLANG can also pass.

Thanks
Liming
> 
> This means that, after checking the gcc documentation meticulously
> (multiple releases), and possibly even testing multiple gcc
> installations, I could still regress edk2 (CLANGxx and/or XCODExx
> specific parts), because they could inherit my GCC-oriented changes that
> I never meant for CLANGxx and/or XCODExx.
> 
> I don't use XCODE or CLANG, I don't test on them, and now it seems I can
> restrict changes to the *actual* gcc compilers only if I keep spelling
> out the individual toolchain names, such as GCC48, GCC49, GCC5.
> 
> This makes the toolchain family concept totally useless to me (and to
> other users that only care about actual gcc compilers). There is no
> *family* facility available that allows people to restrict settings to
> actual gcc compilers. In other words, we can't express "all GCCxx
> toolchains, regardless of toolchain version, but not XCODE, and also not
> CLANG".
> 
> Thanks
> Laszlo