Questions about ld h->pointer_equality_needed on x86-64

[Jens Remus]

I am working on fixing Binutils PR ld/29655 ("s390x gas generates
PC32DBL instead of PLT32DBL for function call") [1].  Part of that is
to port logic from x86-64 revolving around h->pointer_equality_needed
to s390x.

IIUC and (over)simplified scan_relocs sets h->pointer_equality_needed
on x86-64 [2] in case of the following relocation types.  Presumably
the intention is that these are assumed to be used in address taking
context, requiring pointer equality, whereas e.g. R_X86_64_PLT32
would be assumed to be used in function call context, which does
not require pointer equality.

- R_X86_64_8
- R_X86_64_16
- R_X86_64_32
- R_X86_64_32S
- R_X86_64_64, iff in SEC_READONLY
- R_X86_64_PC8
- R_X86_64_PC16
- R_X86_64_PC32, iff not in SEC_CODE

Why are/were R_X86_64_PC64 and previously also R_X86_64_PC32 explicitly
excluded?  I could not find any explanation in the Git history.

See:
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=47a9f7b34f7a3
https://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=d6ab8113e328

Wouldn't lea with R_X86_64_PC32 instead of movl with R_X86_64_32S also
be valid?  Then restricting R_X86_64_PC32 to non-SEC_CODE would be wrong.

For example:
lea     shlib_shlibvar1(%rip), %edi
instead of:
movl    $shlib_shlibvar1, %edi

Why are R_X86_64_8, R_X86_64_16, R_X86_64_32 included?  How are
addresses of functions taken using these?  Or is that a simplification
that it does not hurt to have h->pointer_equality_needed set for these?

[1]: s390x gas generates PC32DBL instead of PLT32DBL for function call,
      https://sourceware.org/bugzilla/show_bug.cgi?id=29655

[2]: Binutils, bfd/elf64-x86-64.c, scan_relocs(),
      https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elf64-x86-64.c;h=1e08f2e7125424f6409bcfdaabcda8761e4f558a;hb=refs/heads/master#l2869

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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