Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751875AbZA1T1d (ORCPT ); Wed, 28 Jan 2009 14:27:33 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751433AbZA1T1Y (ORCPT ); Wed, 28 Jan 2009 14:27:24 -0500 Received: from rn-out-0910.google.com ([64.233.170.187]:7723 "EHLO rn-out-0910.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751348AbZA1T1X (ORCPT ); Wed, 28 Jan 2009 14:27:23 -0500 MIME-Version: 1.0 In-Reply-To: <498095F2.4060502@zytor.com> References: <497A0500.3080706@gmail.com> <497B408C.20802@gmail.com> <20090124172758.GA31699@elte.hu> <200901272042.57272.baldrick@free.fr> <497F7BBE.4070500@zytor.com> <497FBB30.3020804@zytor.com> <498095F2.4060502@zytor.com> Date: Wed, 28 Jan 2009 14:27:22 -0500 Message-ID: Subject: Re: [LLVMdev] inline asm semantics: output constraint width smaller than input From: Kyle Moffett To: "H. Peter Anvin" Cc: Duncan Sands , llvmdev@cs.uiuc.edu, Ingo Molnar , =?UTF-8?B?VMO2csO2ayBFZHdpbg==?= , Thomas Gleixner , Linux Kernel Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1659 Lines: 31 On Wed, Jan 28, 2009 at 12:29 PM, H. Peter Anvin wrote: > Kyle Moffett wrote: >> Even in the 64-bit-integer on 32-bit-CPU case, you still end up with >> the lower 32-bits in a standard integer GPR, and it's trivial to just >> ignore the "upper" register. You also would not need to do any kind >> of bit-shift, so long as your inline assembly initializes both GPRs >> and puts the halves of the result where they belong. > > In this case, we're talking about what happens when the assembly takes a > 64-bit input operand in the same register as a 32-bit output operand > (with a "0" constraint.) Is the output operand the same register number > as the high register or the low register? On an LE machine the answer > is trivial and obvious -- the low register; on a BE machine both > interpretations are possible (I actually suspect gcc will assign the > high register, just based on how gcc internals work in this case.) On a BE 32-bit machine, the "output register" technically ought to be "64-bit" anyways, since it's constrained to be the same as the 64-bit "input register". That means that you ought to make sure to set *both* output registers appropriately, one of them being 0 and the other being the 32-bit number. I think that's the only answer that actually makes any sense from a holistic code-generation sense. So it seems we are in violent agreement :-D. Cheers, Kyle Moffett -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/