From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from list by lists.gnu.org with archive (Exim 4.71) id 1YOsVE-0008Hq-Sx for mharc-grub-devel@gnu.org; Fri, 20 Feb 2015 13:40:41 -0500 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37235) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YOpf2-0007zd-Ct for grub-devel@gnu.org; Fri, 20 Feb 2015 10:38:37 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YOpey-0006pb-Ff for grub-devel@gnu.org; Fri, 20 Feb 2015 10:38:36 -0500 Received: from casper.infradead.org ([85.118.1.10]:56800) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YOpey-0006iT-8B for grub-devel@gnu.org; Fri, 20 Feb 2015 10:38:32 -0500 Received: from i7.infradead.org ([2001:8b0:10b:1:225:64ff:fee8:e9df]) by casper.infradead.org with esmtpsa (Exim 4.80.1 #2 (Red Hat Linux)) id 1YOpeY-0008JE-Uq; Fri, 20 Feb 2015 15:38:07 +0000 Message-ID: <1424446685.5437.52.camel@infradead.org> Subject: Re: clang .code16 with -Os producing larger code that it needs to From: David Woodhouse To: Vladimir =?UTF-8?Q?=27=CF=86-coder/phcoder=27?= Serbinenko Date: Fri, 20 Feb 2015 15:38:05 +0000 In-Reply-To: <54E74B9D.8090605@gmail.com> References: <54E74B9D.8090605@gmail.com> Content-Type: multipart/signed; micalg="sha-1"; protocol="application/x-pkcs7-signature"; boundary="=-pgPS5qqcYZuaRMMuo6hD" X-Mailer: Evolution 3.12.10 (3.12.10-1.fc21) Mime-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org See http://www.infradead.org/rpr.html X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 85.118.1.10 X-Mailman-Approved-At: Fri, 20 Feb 2015 13:40:39 -0500 Cc: The development of GRUB 2 , llvmdev@cs.uiuc.edu X-BeenThere: grub-devel@gnu.org X-Mailman-Version: 2.1.14 Precedence: list Reply-To: The development of GNU GRUB List-Id: The development of GNU GRUB List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 20 Feb 2015 15:38:37 -0000 --=-pgPS5qqcYZuaRMMuo6hD Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, 2015-02-20 at 15:58 +0100, Vladimir '=CF=86-coder/phcoder' Serbinen= ko wrote: > When experimenting with compiling GRUB2 with clang using integrated as, > I found out that it generates a 16-bit code bigger than gas counterpart > and result gets too big for size constraints of bootsector. This was > traced mainly to 2 problems. ... > 32-bit access to 16-bit addresses. > clang: > 7cbc: 67 66 8b 1d 5c 7c 00 00 addr32 mov 0x7c5c,%ebx > gas: > 7cbc: 66 8b 1e 5c 7c mov 0x7c5c,%ebx > 32-bit jump. > clang: > + 7cb5: 66 0f 83 07 01 00 00 jae 7dc3 > gas: > - 7cb5: 0f 83 0a 01 jae 7dc3 To a large extent, those are the *same* problem. We don't know that it's eventually going to fit into a 16-bit offset, so we emit it with a fixup record which can cope with 32 bits. Arguably, the jump is *particularly* gratuitous in many cases... but in 'big real' mode is the IP *really* limited to 16 bits? We could make it default to 16-bit, as gas does. But then we'd be screwed in the cases where we really *do* need 32-bit. What we actually need to do is implement handling for the explicit addr32 prefix. 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