Messages in this thread | | | Date | Wed, 10 Mar 2021 14:42:09 -0800 | From | Fangrui Song <> | Subject | Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION |
| |
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 .
| |