[PowerPC] Garbage collecting .toc entries in a non-adhoc way (section group)

[Alan Modra]

On Sun, Mar 01, 2020 at 10:56:02PM -0800, Fangrui Song wrote:
> Now I am thinking about whether we can produce .toc in section groups to obtain garbage collection ability for free.
> 
> .text (not in a section group) references `foo` defined in .data.rel.ro.foo (in a section group) via .toc .
> ```
> .text
> addis 3,2,.LC0@toc@ha
> 
> .section .data.rel.ro.foo,"aGw",foo,comdat
> .globl foo
> foo:
> 
> .section .toc,"aGw",@progbits,foo,comdat
> .LC0:
> .tc foo[TC],foo
> ```
> 
> When `.data.rel.ro.foo` and the `.toc` are discarded due to the section
> group rule, the relocation from `.text` to the `.toc` becomes dangling.

Yes, that breaks the ELF gABI which says: "A symbol table entry with
STB_LOCAL binding that is defined relative to one of a group's
sections, and that is contained in a symbol table section that is not
part of the group, must be discarded if the group members are
discarded.  References to this symbol table entry from outside the
group are not allowed."

I think what you're trying to do is reinvent the GOT.  There isn't any
reason why powerpc64 can't use a GOT and GOT relocations.

-- 
Alan Modra
Australia Development Lab, IBM