From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753037AbbA1ULy (ORCPT ); Wed, 28 Jan 2015 15:11:54 -0500 Received: from mail-ie0-f175.google.com ([209.85.223.175]:49302 "EHLO mail-ie0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751606AbbA1ULb (ORCPT ); Wed, 28 Jan 2015 15:11:31 -0500 MIME-Version: 1.0 In-Reply-To: References: <1422422306-27473-1-git-send-email-behanw@converseincode.com> <54C8EE04.6060809@linaro.org> <54C91791.9030508@linaro.org> <54C9383D.10901@linaro.org> Date: Wed, 28 Jan 2015 20:11:30 +0000 Message-ID: Subject: Re: [PATCH] bcm: address clang inline asm incompatibility From: Ard Biesheuvel To: Alex Elder Cc: Behan Webster , bcm@fixthebug.org, Florian Fainelli , Russell King - ARM Linux , Matt Porter , bcm-kernel-feedback-list@broadcom.com, "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28 January 2015 at 19:38, Ard Biesheuvel wrote: > On 28 January 2015 at 19:27, Alex Elder wrote: >> On 01/28/2015 01:17 PM, Ard Biesheuvel wrote: >>> On 28 January 2015 at 17:20, Ard Biesheuvel wrote: >>>> On 28 January 2015 at 17:08, Alex Elder wrote: >>>>> On 01/28/2015 10:17 AM, Ard Biesheuvel wrote: >>>>>> On 28 January 2015 at 14:11, Alex Elder wrote: >>>>>>> On 01/28/2015 05:15 AM, Ard Biesheuvel wrote: >>>>>>>> On 28 January 2015 at 05:18, Behan Webster wrote: >>>>>>>>> From: Alex Elder >>>>>>>>> >>>>>>>>> My GCC-based build environment likes to call register r12 by the >>>>>>>>> name "ip" in inline asm. Behan Webster informed me that his Clang- >>>>>>>>> based build environment likes "r12" instead. >>>>>>>>> >>>>>>>>> Try to make them both happy. >>>>>>>>> >>>>>>>>> Signed-off-by: Alex Elder >>>>>>>>> Signed-off-by: Behan Webster >>>>>>>>> --- >>>>>>>>> arch/arm/mach-bcm/bcm_kona_smc.c | 9 +++++++-- >>>>>>>>> 1 file changed, 7 insertions(+), 2 deletions(-) >>>>>>>>> >>>>>>>>> diff --git a/arch/arm/mach-bcm/bcm_kona_smc.c b/arch/arm/mach-bcm/bcm_kona_smc.c >>>>>>>>> index a55a7ec..3937bd5 100644 >>>>>>>>> --- a/arch/arm/mach-bcm/bcm_kona_smc.c >>>>>>>>> +++ b/arch/arm/mach-bcm/bcm_kona_smc.c >>>>>>>>> @@ -106,9 +106,14 @@ int __init bcm_kona_smc_init(void) >>>>>>>>> * request result appropriately. This result value is found in r0 >>>>>>>>> * when the "smc" request completes. >>>>>>>>> */ >>>>>>>>> +#ifdef __clang__ >>>>>>>>> +#define R12 "r12" >>>>>>>>> +#else /* !__clang__ */ >>>>>>>>> +#define R12 "ip" /* gcc calls r12 "ip" */ >>>>>>>>> +#endif /* !__clang__ */ >>>>>>>> >>>>>>>> Why not just use r12 for both? >>>>>>> >>>>>>> Yes, that would have been an obvious fix. But the >>>>>>> assembler (in the GCC environment) doesn't accept that. >>>>>>> >>>>>> >>>>>> Mine has no problems with it at all >>>>>> >>>>>> $ echo 'mov r12, #0' | arm-linux-gnueabihf-gcc -c -x assembler-with-cpp - >>>>>> >>>>>> and grepping for r12 under arch/arm suggests the same >>>>> >>>>> The use of "r12" is fine. But it's not just the assembler, >>>>> I believe it also involves gcc. >>>>> >>>>> The problem is with the use of the __asmeq(x, y) macro. >>>>> >>>> >>>> Ah right. Apologies for assuming that you had missed something obvious here. >>>> But __asmeq is not the toolchain, it is a local construct #define'd in >>>> compiler.h >>>> >>>>> If I assign the "ip" variable with "r12": >>>>> register u32 ip asm("r12"); /* Also called ip */ >>>>> >>>>> Then that's fine. However, this line then causes an error: >>>>> __asmeq("%0", "r12") >>>>> >>>>> Apparently gcc uses register "ip" when it sees asm("r12"). So >>>>> attempting to verify the desired register got used with __asmeq() >>>>> causes a string mismatch--"ip" is not equal to "r12". >>>>> >>>>> So I could use: >>>>> >>>>> register u32 ip asm("r12"); /* Also called ip */ >>>>> ... >>>>> __asmeq("%0", "ip") >>>>> >>>>> And that will build. But it's a little non-intuitive, and >>>>> I suspect that clang might (rightfully) have a failure in >>>>> this __asmeq() call. >>>>> >>>> >>>> In that case, I would strongly suggest fixing the __asmeq () macro >>>> instead, and teach it that ("r12","ip") and ("ip","r12") are fine too. >>>> >>>> The thing is, inline asm is a dodgy area to begin with in terms of >>>> clang-to-gcc compatibility. On arm64, we have been seeing issues where >>>> the width of the register -which is fixed on gcc- is selected based on >>>> the size of that variable, i.e., an int32 gets a w# register and int64 >>>> gets a x# register. Imagine debugging that, e.g., a str %0, [xx] that >>>> writes 8 bytes on GCC suddenly only writing 4 bytes when built with >>>> clang. >>>> >>>> If we also start using the preprocessor to conditionalise what is >>>> emitted by inline asm, the waters get even murkier and it becomes even >>>> harder to claim parity between the two. >>>> >>> >>> Something like this perhaps? >> >> So __asmeq() yields true if the register names (strings) are >> equal, or if one is "ip" and the other is "r12" (in either order). >> >> I can't comment on whether it's right in all build environments but >> this looks OK to me, to handle this special case. >> >> I would much rather you generate that patch. Is that OK? >> > > Sure, I can cook up a patch if you guys can confirm that it fixes your > use case. (I tested GCC myself but I don't have clang installed) > Actually, if clang is guaranteed to emit the correct register name inside the inline asm for register asm variables used in input or output constraints, I think it makes sense to #define __asmeq as a nop if __clang__ is defined. (Note that __asmeq only exists to work around a specific GCC bug) -- Ard.