[Patch] ld plugin: Fix known_used with CLAIM_FILE_HOOK_V2 for elf_link_add_archive_symbols [GCC PR122432]

[Tobias Burnus]

This is about CLAIM_FILE_HOOK_V2's known_used flag, its effect
on GCC's offloading side and common symbols processing.

Originally, for any call to the hook, the compiler processed
the offloading code - even for unused common symbols. The fix
was to add a 'known_used' to the call (_v2) and only do so when
true.

The current scheme fails, however, if a common symbol comes
in a file before any actually used symbol - as the common
symbol already triggers a call to the plugin - with 'known_used'
set to false - and never calls it again afterward.

The solution proposed in this email/patch is to move the
common symbol handling until after all other symbols have
been handled - such that the hook will either be called
later with known_used = true and only, if none of those
are used, a call to the hook with false can happen.

* * *

The original change that introduce known_used is
commit b21318bd2c2 "Add LDPT_REGISTER_CLAIM_FILE_HOOK_V2
linker plugin hook [GCC PR109128]" with associated GCC
lto-plugin modifying commit r14-677-gc49d51fa8134f6.

* * *

Comments, concerns? If not, OK for mainline?

* * *

That there is some problem was known for longer, but it
was hard to debug - especially for users and code sharing
was not always possible. It also tended to disappear when
reducing it, which made it difficult debug or to create a
reproducer. Fortunately, this issue is now understood.

Tobias

PS: As I never had to push to Sourceware, I only have a
GCC account. Thus, I either need an account or someone to
push it for me :-)

PPS: I have tested this by building Binutils and using it
with the respective code. I have not extensively exercised
a lot of code and I fail to see how to add a testcase for this.
(Commit b21318bd2c2 did only do so for the new plugin interface.)
If you have suggestions what to tested in addition, please tell me.
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