From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mailman by lists.gnu.org with tmda-scanned (Exim 4.43) id 1NSNKG-0007ik-Gb for qemu-devel@nongnu.org; Tue, 05 Jan 2010 23:16:52 -0500 Received: from exim by lists.gnu.org with spam-scanned (Exim 4.43) id 1NSNKF-0007hL-4V for qemu-devel@nongnu.org; Tue, 05 Jan 2010 23:16:51 -0500 Received: from [199.232.76.173] (port=33255 helo=monty-python.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1NSNKE-0007h8-TI for qemu-devel@nongnu.org; Tue, 05 Jan 2010 23:16:50 -0500 Received: from are.twiddle.net ([75.149.56.221]:58923) by monty-python.gnu.org with esmtp (Exim 4.60) (envelope-from ) id 1NSNKE-0000w1-Jz for qemu-devel@nongnu.org; Tue, 05 Jan 2010 23:16:50 -0500 Message-ID: <4B440EA3.9040502@twiddle.net> Date: Tue, 05 Jan 2010 20:16:35 -0800 From: Richard Henderson MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH 2/2] tcg-x86_64: Avoid unnecessary REX.B prefixes. References: <20100106010537.9C9D2CBB@are.twiddle.net> In-Reply-To: <20100106010537.9C9D2CBB@are.twiddle.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: laurent.desnogues@gmail.com, aurelien@aurel32.net On 01/05/2010 04:31 PM, Richard Henderson wrote: > A while ago Laurent pointed out that the setcc opcode emitted by > the setcond patch had unnecessary REX prefixes. > > The existing P_REXB internal opcode flag unconditionally emits > the REX prefix. Technically it's not needed if the register in > question is %al, %bl, %cl, %dl. I should perhaps mention that this patch doesn't help out all that much. Only EBX is free for use by the code generator, so more often than not we're using %bpl, which does require the REX prefix. r~