From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 66B318494 for ; Thu, 5 Jan 2023 13:08:22 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id C11DFC433EF; Thu, 5 Jan 2023 13:08:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1672924102; bh=ITPOEHyex1SzygDUJvML3Zwe9jJ4CHBY4RADUG6dyNI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=TWqC/nMQ22WH3CVvMc3z/yGcHllbthY/clGSyPVB3M1RKCCodj4NNSOl4mfW04Es7 tktogy631ixSiiPPs0D5lQkVEvya45Ollwx93PvASwxBPoQ1F4Tu5I3xiyYpv4Y33+ SGNHYMRAMWdvVWlf48cuHyOmeGahg43hnXM9dAQM= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Nathan Chancellor , Arnd Bergmann , Nick Desaulniers , "Russell King (Oracle)" Subject: [PATCH 4.9 235/251] ARM: 9256/1: NWFPE: avoid compiler-generated __aeabi_uldivmod Date: Thu, 5 Jan 2023 13:56:12 +0100 Message-Id: <20230105125345.642038433@linuxfoundation.org> X-Mailer: git-send-email 2.39.0 In-Reply-To: <20230105125334.727282894@linuxfoundation.org> References: <20230105125334.727282894@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Nick Desaulniers commit 3220022038b9a3845eea762af85f1c5694b9f861 upstream. clang-15's ability to elide loops completely became more aggressive when it can deduce how a variable is being updated in a loop. Counting down one variable by an increment of another can be replaced by a modulo operation. For 64b variables on 32b ARM EABI targets, this can result in the compiler generating calls to __aeabi_uldivmod, which it does for a do while loop in float64_rem(). For the kernel, we'd generally prefer that developers not open code 64b division via binary / operators and instead use the more explicit helpers from div64.h. On arm-linux-gnuabi targets, failure to do so can result in linkage failures due to undefined references to __aeabi_uldivmod(). While developers can avoid open coding divisions on 64b variables, the compiler doesn't know that the Linux kernel has a partial implementation of a compiler runtime (--rtlib) to enforce this convention. It's also undecidable for the compiler whether the code in question would be faster to execute the loop vs elide it and do the 64b division. While I actively avoid using the internal -mllvm command line flags, I think we get better code than using barrier() here, which will force reloads+spills in the loop for all toolchains. Link: https://github.com/ClangBuiltLinux/linux/issues/1666 Reported-by: Nathan Chancellor Reviewed-by: Arnd Bergmann Signed-off-by: Nick Desaulniers Tested-by: Nathan Chancellor Cc: stable@vger.kernel.org Signed-off-by: Russell King (Oracle) Signed-off-by: Greg Kroah-Hartman --- arch/arm/nwfpe/Makefile | 6 ++++++ 1 file changed, 6 insertions(+) --- a/arch/arm/nwfpe/Makefile +++ b/arch/arm/nwfpe/Makefile @@ -10,3 +10,9 @@ nwfpe-y += fpa11.o fpa11_cpdo.o fpa11 entry.o nwfpe-$(CONFIG_FPE_NWFPE_XP) += extended_cpdo.o + +# Try really hard to avoid generating calls to __aeabi_uldivmod() from +# float64_rem() due to loop elision. +ifdef CONFIG_CC_IS_CLANG +CFLAGS_softfloat.o += -mllvm -replexitval=never +endif