Apple might not make money directly with its compiler, but for its 3rd party developers to be stuck with a mediocre compiler is definitely a disadvantage for Apple.

Hypothetical: The hardware under the hood is identical to that of a Dell. Compare an application written for both, but the one running on the Dell uses Intel's highly optimized compiler while Apple uses one that's not tuned for any particular platform, and therefore performs somewhat poorly on x86.

Result: Apple has a 20% performance deficit.

Now say that application is Photorealistic RenderMan or Cinema4D or Modo.

That 20% performance deficit, that has nothing to do with the hardware, just created a significant cost disadvantage for a mac user -- to implement a renderfarm, the mac adds either 20% in rendering time, or 20% in extra hardware costs to offset the rendering time (which ends up costing more still in logistics).

Result? Apple would lose a potentially non-trivial sale.

Having an optimized in-house compiler would probably also help to improve OpenGL performance on the mac -- because odds are the entire OS is compiled with the same compiler...

Though it may not be a significant direct revenue generator, having a good compiler would probably still be very beneficial for Apple AND its customers, not to mention developers.

Not only that, but making tweaks and optimizations (remember that Apple's compiler has to target a very narrow subset of what general Clang, or GCC, target) to an internal version even while releasing the sources for the "base" version would also give Apple a competitive advantage.

Since they own the product, the licence would not bother them much - they could just relicense it - but I assume that by using GPL they would be educating their customers in subjects they would rather keep them ignorant.

edit: Apple doesn't own Clang or LLVM. Copyright belongs to individual contributors. However, since it's BSD, Apple's ability to tune a "secret-sauce" version to its hardware is unhindered.

64-bit processors allow similar assumptions on Windows, too.

LGPL would be fine for that.

Looks a lot like a 3-clause BSD, but it's a cross between MIT and BSD. The point on LGPL is that if GCC were LGPL, Apple would be able to link it to Xcode.