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

["Laszlo Ersek" <lersek@redhat.com>]

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).

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