From: Patrick O'Neill <patrick@rivosinc.com>
To: Palmer Dabbelt <palmer@dabbelt.com>
Cc: jbeulich@suse.com, binutils@sourceware.org, gnu-toolchain@rivosinc.com
Subject: Re: [RFC] RISCV: Align using RVC insns
Date: Mon, 28 Mar 2022 17:45:38 -0700 [thread overview]
Message-ID: <426ca774-7652-c394-bb5d-aabfd1a39527@rivosinc.com> (raw)
In-Reply-To: <mhng-c493e791-de89-40c3-85a7-3936683fe5df@palmer-ri-x1c9>
On 3/28/22 17:17, Palmer Dabbelt wrote:
> On Mon, 28 Mar 2022 15:19:41 PDT (-0700), Patrick O'Neill wrote:
>>
>> On 3/28/22 00:54, Jan Beulich wrote:
>>> On 26.03.2022 00:15, Patrick O'Neill wrote:
>>>> Currently, .align and .balign directives only use nops to achieve
>>>> alignment. On RVC targets, the linker can selectively compress
>>>> instructions to achieve alignment without introducing nops.
>>>>
>>>> This increases the code size of unlinked binaries since instruction
>>>> compression is deferred to the linker. Linked binaries with align
>>>> directives may be smaller/run faster due to less nops.
>>>>
>>>> Binaries without align directives should be unaffected by this change.
>>>>
>>>> This change requires adding a reloc for compressable instructions. The
>>>> addend of that reloc stores the compressed instruction's opcode.
>>>>
>>>> Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>
>>> May I ask where the specification for this lives? I've just pulled down
>>> the latest version of the psABI, but that doesn't even mention the new
>>> relocation type you use here. Going over
>>> https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues also didn't
>>> turn up anything obvious.
>>>
>>> Jan
>>>
>> Jan,
>>
>> Thank you for the link, I just opened a pull request proposing the
>> addition of this relocation type.
>
> IMO it's really too early to have a meaningful discussion about
> putting this in the spec. There's a handful of complicated
> interactions this has with the rest of the stack, and we need to sort
> these out before we know if this is a good idea (or exactly what
> should be in the spec). Specifically:
>
> * This is going to massively increase link time, as there'll be a lot
> more relaxed relocations. We've got a quadratic time implementation,
> so things will get bad (and things that are already bad will get very
> bad). I think it's possible to make linker relaxation linear time,
> but that has yet to be proved out.
> * We likely want some compiler assistance in picking which instructions
> to compress. There's a bunch of ways to do that, but it'll
> probably require some assembler syntax and may change the relocation
> definition.
> * We're probably not describing the right alignment constraints: for
> some of these systems we're really trying to say "avoid crossing
> a cache line boundary", which is inefficient to encode as an alignment
> constraint (it's really "don't misalign", not "align"). That might
> not be entirely required, as some systems do just need regular
> alignment, but we should at least make sure this new scheme avoids
> obviously breaking anything.
>
> That said, this idea has been bounced around a bunch of times before
> without anything making it to the lists so I figured it'd be best to
> just send the RFC -- at least it's a start, and all those other
> problems are sort of their own concerns. Maybe we'll get lucky and
> one of the other implementations floating around had some smart
> solutions to these problems, but I've got a feeling this is going to
> be more than a bit of work.
I'll start work on the linear link time implementation.
Thanks,
Patrick
next prev parent reply other threads:[~2022-03-29 0:45 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-25 23:15 Patrick O'Neill
2022-03-28 7:54 ` Jan Beulich
2022-03-28 16:08 ` Patrick O'Neill
2022-03-28 22:19 ` Patrick O'Neill
2022-03-29 0:17 ` Palmer Dabbelt
2022-03-29 0:45 ` Patrick O'Neill [this message]
2022-03-29 6:11 ` Andrew Waterman
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=426ca774-7652-c394-bb5d-aabfd1a39527@rivosinc.com \
--to=patrick@rivosinc.com \
--cc=binutils@sourceware.org \
--cc=gnu-toolchain@rivosinc.com \
--cc=jbeulich@suse.com \
--cc=palmer@dabbelt.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).