public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "bugdal at aerifal dot cx" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/94643] New: [x86_64] gratuitous sign extension of nonnegative value from 32 to 64 bits
Date: Sat, 18 Apr 2020 00:50:08 +0000	[thread overview]
Message-ID: <bug-94643-4@http.gcc.gnu.org/bugzilla/> (raw)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94643

            Bug ID: 94643
           Summary: [x86_64] gratuitous sign extension of nonnegative
                    value from 32 to 64 bits
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bugdal at aerifal dot cx
  Target Milestone: ---

Test case:

#include <stdint.h>
uint16_t a[];
uint64_t f(int i)
{
        return a[i]*16;
}

Produces:

        movslq  %edi, %rdi
        movzwl  a(%rdi,%rdi), %eax
        sall    $4, %eax
        cltq
        ret

The value is necessarily in the range [0,1M) (in particular, nonnegative) and
operation on eax has already cleared the upper bits of rax, so cltq is
completely gratuitous. I've observed the same in nontrivial examples where
movslq gets used.

             reply	other threads:[~2020-04-18  0:50 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-18  0:50 bugdal at aerifal dot cx [this message]
2020-04-18  1:49 ` [Bug tree-optimization/94643] [sign " pinskia 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-94643-4@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).