[compiler-rt] [compiler-rt][AArch64] Enable libc-free builtins Linux build (PR #125922)

[Peter Waller via llvm-commits]

peterwaller-arm wrote:

Thanks for the hint! I agree it would be better if nixpkgs moved to a supported configuration.

A problem for nixpkgs is 'how to get the libc headers' for an arbitrary libc. I'm currently under the impression it may be necessary to build the libc before you can get your hands on the installed headers, as some headers may be generated. Is that right, and if so, does it create a cycle with compiler-rt which needs breaking? Is there a convention for acquiring the headers for a libc, I wonder, or is this bespoke for each libc.

https://github.com/llvm/llvm-project/pull/125922