From: Fangrui Song <i@maskray.me>
To: Alan Modra <amodra@gmail.com>
Cc: binutils@sourceware.org
Subject: Re: [PowerPC] Garbage collecting .toc entries in a non-adhoc way (section group)
Date: Mon, 02 Mar 2020 18:03:00 -0000 [thread overview]
Message-ID: <BY5PR07MB73164FF0F21128820D5DB002CBE70@BY5PR07MB7316.namprd07.prod.outlook.com> (raw)
In-Reply-To: <20200302105939.GI5384@bubble.grove.modra.org>
On Mon, Mar 2, 2020 at 2:59 AM Alan Modra <amodra@gmail.com> wrote:
>
> 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.
Yes, there is inventing the GOT. But neither GCC nor clang produces
foo@got@ha and foo@got@l pairs. Is there some hidden wisdom I am
missing?
next prev parent reply other threads:[~2020-03-02 18:03 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-02 6:56 Fangrui Song
2020-03-02 10:59 ` Alan Modra
2020-03-02 18:03 ` Fangrui Song [this message]
2020-03-02 22:50 ` Alan Modra
2020-03-03 2:03 ` Fangrui Song
2020-03-04 1:25 ` Alan Modra
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