public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "law at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/106585] RISC-V: Mis-optimized code gen for zbs
Date: Wed, 07 Dec 2022 22:45:19 +0000	[thread overview]
Message-ID: <bug-106585-4-igy0daO86J@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-106585-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106585

--- Comment #7 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Raphael and I are poking at this a bit.  I can't convince myself that it's
actually safe to use GPR for the bit manipulation patterns.

For rv64 I'm pretty sure the b* instructions are operating on 64bit quantities
only.  Meaning they might twiddle the SI sign bit without extending.  If we
were to change these patterns to use GPR and the result then fed an addw (for
example) then we would have inconsistent register state as operand twiddled by
the prior b* pattern wouldn't have been sign extended.

To be clear, I think this is a limitation imposed by the ISA docs, not GCC
where this will be reasonably well defined.

With that in mind I think the only path forward is new patterns that (sadly)
use explicit subregs for sources, but still set a DImode destination.

I'm the newbie here, so if I've misinterpreted the ISA docs incorrectly, don't
hesitate to let me know.

  parent reply	other threads:[~2022-12-07 22:45 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-11 15:36 [Bug target/106585] New: " kito at gcc dot gnu.org
2022-08-11 15:45 ` [Bug target/106585] " pinskia at gcc dot gnu.org
2022-08-11 15:46 ` pinskia at gcc dot gnu.org
2022-08-11 15:52 ` pinskia at gcc dot gnu.org
2022-08-11 15:54 ` pinskia at gcc dot gnu.org
2022-08-11 16:10 ` kito at gcc dot gnu.org
2022-08-11 16:23 ` kito at gcc dot gnu.org
2022-08-11 16:27 ` pinskia at gcc dot gnu.org
2022-12-07 22:45 ` law at gcc dot gnu.org [this message]
2022-12-08  5:02 ` palmer at gcc dot gnu.org
2022-12-08  6:25   ` Andrew Waterman
2022-12-08  6:25 ` andrew at sifive dot com
2022-12-08 16:16 ` palmer at gcc dot gnu.org
2023-04-28 22:46 ` law at gcc dot gnu.org

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=bug-106585-4-igy0daO86J@http.gcc.gnu.org/bugzilla/ \
    --to=gcc-bugzilla@gcc.gnu.org \
    --cc=gcc-bugs@gcc.gnu.org \
    /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).