From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 13065 invoked by alias); 10 Jun 2012 23:32:32 -0000 Received: (qmail 13056 invoked by uid 22791); 10 Jun 2012 23:32:31 -0000 X-SWARE-Spam-Status: No, hits=-4.3 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00,KHOP_THREADED X-Spam-Check-By: sourceware.org Received: from localhost (HELO gcc.gnu.org) (127.0.0.1) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Sun, 10 Jun 2012 23:32:19 +0000 From: "adam at consulting dot net.nz" To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/53623] [4.7/4.8 Regression] sign extension is effectively split into two x86-64 instructions Date: Sun, 10 Jun 2012 23:32:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: middle-end X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: adam at consulting dot net.nz X-Bugzilla-Status: NEW X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org X-SW-Source: 2012-06/txt/msg00546.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53623 --- Comment #3 from Adam Warner 2012-06-10 23:32:19 UTC --- (Off topic "Note" correction) In my previous note I suggested the instruction "movsbq dh -> rdx". There is no such instruction! One cannot encode register ah/bh/ch/dh in an instruction requiring a REX prefix. A REX prefix would be required for sign extending ah/bh/ch/dh to 64 bits. My note does apply to zero extending dh to 64 bits. "movzbl dh -> edx" achieves this since edx is implicitly zero extended to rdx.