public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "kspalaiologos at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/105429] Unnecessary moves generated with _mm_crc32_u64
Date: Fri, 29 Apr 2022 12:38:35 +0000	[thread overview]
Message-ID: <bug-105429-4-TizKkXyALF@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-105429-4@http.gcc.gnu.org/bugzilla/>

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105429

--- Comment #2 from Palaiologos <kspalaiologos at gmail dot com> ---
I have observed the same behaviour with and without `mov eax, eax`. CRC32 is a
32-bit checksum, so I'd presume that the high bits aren't considered by the
instruction.

To support my claim, Vol. 2A 3-257 of Intel Software Development Manual gives
the following operation for 2 REX.W 0F 38 F1 /r:

>>>
TEMP1[63-0] := BIT_REFLECT64 (SRC[63-0])
TEMP2[31-0] := BIT_REFLECT32 (DEST[31-0])
TEMP3[95-0] := TEMP1[63-0] « 32
TEMP4[95-0] := TEMP2[31-0] « 64
TEMP5[95-0] := TEMP3[95-0] XOR TEMP4[95-0]
TEMP6[31-0] := TEMP5[95-0] MOD2 11EDC6F41H
DEST[31-0] := BIT_REFLECT (TEMP6[31-0])
DEST[63-32] := 00000000H
<<<

      parent reply	other threads:[~2022-04-29 12:38 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-04-28 20:02 [Bug other/105429] New: Unnecessary moves generated by the compiler mareksz1958 at wp dot pl
2022-04-28 20:04 ` [Bug target/105429] Unnecessary moves generated with _mm_crc32_u64 pinskia at gcc dot gnu.org
2022-04-29 12:30 ` ubizjak at gmail dot com
2022-04-29 12:38 ` kspalaiologos at gmail dot com [this message]

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-105429-4-TizKkXyALF@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).