ELF: Ignore --long-plt flag.

[pcc]

Summary

This flag causes bfd and gold to use a longer PLT format on ARM that
allows for the offset between the PLT and the GOT to exceed 256MB.
It seems unlikely that we would need to implement long PLTs because
by default we lay out .plt at the end of the executable segment (as
opposed to the other linkers, which lay it out at the beginning),
which means that the size of the non-relro part of the r/w segment
would need to exceed 256MB before a long PLT becomes necessary.

Even if we did implement long PLTs, I think it should be done by
recalculating the size of .plt as part of our convergent address
assignment loop, rather than requiring the user to request a long
PLT explicitly.

In any event, we would end up ignoring the flag, so this patch does so.

[smeenai]

LLD actually doesn't need --long-plt because its PLT entries already always use the long format. See https://bugs.llvm.org/show_bug.cgi?id=34734#c2

[ruiu]

LGTM

[pcc]

In D41025#949866, @smeenai wrote:

LLD actually doesn't need --long-plt because its PLT entries already always use the long format. See https://bugs.llvm.org/show_bug.cgi?id=34734#c2

I see, thanks. I will reword the commit message.

lld/ELF/Options.td

 def plugin: S<"plugin">;
 def plugin_eq: J<"plugin=">;
 
 // Options listed below are silently ignored for now for compatibility.
 def allow_shlib_undefined: F<"allow-shlib-undefined">;
 def cref: F<"cref">;
 def detect_odr_violations: F<"detect-odr-violations">;
 def g: Flag<["-"], "g">;
+def long_plt: F<"long-plt">;
 def no_add_needed: F<"no-add-needed">;
 def no_allow_shlib_undefined: F<"no-allow-shlib-undefined">;
 def no_copy_dt_needed_entries: F<"no-copy-dt-needed-entries">;
 def no_ctors_in_init_array: F<"no-ctors-in-init-array">;
 def no_keep_memory: F<"no-keep-memory">;
 def no_mmap_output_file: F<"no-mmap-output-file">;
 def no_warn_common: F<"no-warn-common">;
 def no_warn_mismatch: F<"no-warn-mismatch">;