[llvm] [AArch64, ELF] Allow implicit $d/$x at section beginning (PR #99718)

[Simon Tatham via llvm-commits]

statham-arm wrote:

> Can we alleviate these concerns by marking object files that use this mapping symbol convention. That way we could fix them up later if needed.

That was my first thought, too.

As far as I can see, this option is essentially inventing a nonstandard variation on the Arm ELF spec, which is not fully compatible with the standard one, as the PR description mentions:

> This scheme works as long as the user can rule out some error scenarios:
> 
> * .text.1 assembled using the traditional behavior is combined with .text.2 using the new behavior

Currently, as far as I can see, the only way that a user _can_ rule out this error scenario is to use their own memory and self-discipline to keep track of which objects (or which libraries, or which whole projects) they're building using this nonstandard flag.

Surely it would be better to mark object files to indicate explicitly that they follow this nonstandard ABI. Then at the very least a linker can detect a mixture of the two and complain, even if it can't automatically compensate. That way the user doesn't have to do it all by self-discipline.

https://github.com/llvm/llvm-project/pull/99718