[lld] [PAC][lld] Use braa instr in PAC PLT sequence with valid PAuth core info (PR #113945)

[Daniil Kovalev via llvm-commits]

kovdan01 wrote:

> > Assume PAC instructions being supported with PAuth core info different from (0,0). Given that, autia1716; br x17 can be replaced with braa x17, x16; nop.
> 
> It is worth explaining what braa is.
> 
> "Assume PAC instructions being supported with PAuth core info different from (0,0)" seems difficult to follow for AArch64 PAuth outsiders

Thanks for suggestion! I changed the PR description to the following:

> Assume PAC instructions being supported with PAuth core info different from (0,0). The (0,0) value means that an ELF file is incompatible with PAuth - see https://github.com/ARM-software/abi-aa/blob/2024Q3/pauthabielf64/pauthabielf64.rst#core-information. With PAC non-hint instructions supported, `autia1716; br x17` can be replaced with `braa x17, x16; nop`, where `braa` is an authenticated branch instruction using IA key, discriminator from x16 and signed target address from x17.


https://github.com/llvm/llvm-project/pull/113945