[PATCH] D145131: [Arm][AArch64] Setting IsX18ReservedByDefault() to true for Unknown OSes

[Lucas Prates via Phabricator via llvm-commits]

pratlucas added a comment.

I agree with @dmgreen, this doesn't seem to be the right default behaviour according to the ABI. I understand it can be useful for building cross-platform code, but "unknown" also affects other use cases such as bare metal targets.

As @DavidSpickett mentioned above, there are existing command line options that allow the user to opt-in to this behaviour, as recommended by the AAPCS64 document when creating platform-independent code:

> Software developers creating platform-independent code are advised to avoid using r18 if at all possible. Most compilers provide a mechanism to prevent specific registers from being used for general allocation;


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https://reviews.llvm.org/D145131