Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

[Fangrui Song]

On 2021-03-10, Nicolas Pitre wrote:
> On Mon, 1 Mar 2021, Nicholas Piggin wrote:
>
> > Excerpts from Arnd Bergmann's message of February 27, 2021 7:49 pm:
> > > Unlike what Nick expected in his submission, I now think the annotations
> > > will be needed for LTO just like they are for --gc-sections.
> >
> > Yeah I wasn't sure exactly what LTO looks like or how it would work.
> > I thought perhaps LTO might be able to find dead code with circular /
> > back references, we could put references from the code back to these
> > tables or something so they would be kept without KEEP. I don't know, I
> > was handwaving!
> >
> > I managed to get powerpc (and IIRC x86?) working with gc sections with
> > those KEEP annotations, but effectiveness of course is far worse than
> > what Nicolas was able to achieve with all his techniques and tricks.
> >
> > But yes unless there is some other mechanism to handle these tables,
> > then KEEP probably has to stay. I suggest this wants a very explicit and
> > systematic way to handle it (maybe with some toolchain support) rather
> > than trying to just remove things case by case and see what breaks.
> >
> > I don't know if Nicolas is still been working on his shrinking patches
> > recenty but he probably knows more than anyone about this stuff.
>
> Looks like not much has changed since last time I played with this stuff.
>
> There is a way to omit the KEEP() on tables, but something must create a
> dependency from the code being pointed to by those tables to the table
> entries themselves. I did write my findings in the following article
> (just skip over the introductory blurb):
>
> https://lwn.net/Articles/741494/

Hey, this article taught me R_*_NONE which motivated me to add various R_*_NONE
support to LLVM 9!

In the weekend I noticed that with binutils>=2.26, one can use
.reloc ., BFD_RELOC_NONE, target
(https://sourceware.org/bugzilla/show_bug.cgi?id=27530 ).
I implemented it for many targets in LLVM, but that will require 13.0.0.

> Once that dependency is there, then the KEEP() may go and
> garbage-collecting a function will also garbage-collect the table entry
> automatically.
>
> OTOH this trickery is not needed with LTO as garbage collection happens
> at the source code optimization level. The KEEP() may remain in that
> case as unneeded table entries will simply not be created in the first
> place.

For Thin LTO, --gc-sections is still very useful.
I have more notes in https://maskray.me/blog/2021-02-28-linker-garbage-collection#link-time-optimization .