public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "unlvsur at live dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/99434] std::bit_cast generates more instructions than __builtin_bit_cast and memcpy with -march=native
Date: Sat, 06 Mar 2021 21:36:42 +0000 [thread overview]
Message-ID: <bug-99434-4-ynoxHMbpdn@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-99434-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99434
--- Comment #2 from cqwrteur <unlvsur at live dot com> ---
(In reply to Andrew Pinski from comment #1)
> This is just a register allocation issue dealing with mulx and TImode.
>
> If mulq was used instead (that is without -march=native), all of the
> functions are done correctly.
I do not think so. I think GCC generally did things like this wrong. I have
even found out how to produce different wrong results deterministically.
For example like this
https://godbolt.org/z/PbobYG
Any time it deals with things like >>32 or >>64, it produces a slower result.
This even compiles without -march=native.
While clang generates exactly the same assembly which means my result is
correct. GCC does things for this wrong.
It looks like we need more optimizations on trees for these patterns.
next prev parent reply other threads:[~2021-03-06 21:36 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-06 20:12 [Bug tree-optimization/99434] New: " unlvsur at live dot com
2021-03-06 21:25 ` [Bug target/99434] " pinskia at gcc dot gnu.org
2021-03-06 21:36 ` unlvsur at live dot com [this message]
2021-03-06 22:50 ` pinskia at gcc dot gnu.org
2021-03-08 16:49 ` jakub at gcc dot gnu.org
2021-03-08 16:50 ` jakub at gcc dot gnu.org
2021-03-08 20:40 ` unlvsur at live dot com
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-99434-4-ynoxHMbpdn@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).