"NOCROSSREFS_TO" doesn't supported by LLVM's ld.lld

[P1119r1m]

Bugzilla Link	42480
Version	unspecified
OS	Linux
CC	@P1119r1m,@smithp35

Extended Description

Build fails on the use of "NOCROSSREFS_TO" command in linker script.

Example:

• funnyos.lds.S:

...
NOCROSSREFS_TO (.init .text .data)
...
ENTRY(_start)
...

Reported build/link error:

ld.lld: error: core/funnyos.lds:11: unknown directive: NOCROSSREFS_TO
>>> NOCROSSREFS_TO (.init .text .data)
>>> ^

P.S. Problem also reported on https://stackoverflow.com/questions/56235666/is-it-possible-to-use-nocrossrefs-to-in-linker-script-for-clangs-ld-lld

[smithp35]

Looking at the documentation for that particular feature in:
https://sourceware.org/binutils/docs/ld/Miscellaneous-Commands.html#Miscellaneous-Commands

It looks like that particular command makes the linker diagnose certain cross-references between sections but doesn't change the section layout. As a work around you should be able to remove the command and get the same results.

I understand that the additional checks enabled by that command are there for a good reason. For embedded systems it would be helpful to add this feature at some point.

[P1119r1m]

Thank you!
Yes, this command is just for compile and link-time, but it is important sometimes (embedded development, as you've already mentioned).
Support of this command provides, at list, portability when moving project from binutil's ld to the llvm's lld.

[nickdesaulniers]

It looks like NOCROSSREFS is also not supported yet by LLD.

This is breaking builds of 32b ARM with recent Spectre-BHB mitigations from linking with LLD.
ClangBuiltLinux/linux#1609

[P1119r1m]

@nickdesaulniers IMHO this feature can be very useful in some cases. It's surprising that this hasn't been implemented in LLD yet.

[nickdesaulniers]

cc @stuij

[pskrgag]

Seems like this issue is kinda stale for a while, so I decided to try implementing it myself.
I am new to llvm world, so let see how it will go

See #95714