From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757463AbbJHNAd (ORCPT ); Thu, 8 Oct 2015 09:00:33 -0400 Received: from foss.arm.com ([217.140.101.70]:34261 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756027AbbJHNAb (ORCPT ); Thu, 8 Oct 2015 09:00:31 -0400 Date: Thu, 8 Oct 2015 14:00:26 +0100 From: Catalin Marinas To: Russell King - ARM Linux Cc: mark.rutland@arm.com, Vladimir.Murzin@arm.com, steve.capper@linaro.org, ard.biesheuvel@linaro.org, marc.zyngier@arm.com, andre.przywara@arm.com, "Suzuki K. Poulose" , will.deacon@arm.com, linux-kernel@vger.kernel.org, edward.nevill@linaro.org, aph@redhat.com, james.morse@arm.com, dave.martin@arm.com, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling Message-ID: <20151008130026.GI17192@e104818-lin.cambridge.arm.com> References: <1444064531-25607-1-git-send-email-suzuki.poulose@arm.com> <1444064531-25607-15-git-send-email-suzuki.poulose@arm.com> <20151008111000.GG17192@e104818-lin.cambridge.arm.com> <20151008111709.GE32532@n2100.arm.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151008111709.GE32532@n2100.arm.linux.org.uk> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 08, 2015 at 12:17:09PM +0100, Russell King - ARM Linux wrote: > On Thu, Oct 08, 2015 at 12:10:00PM +0100, Catalin Marinas wrote: > > On Mon, Oct 05, 2015 at 06:02:03PM +0100, Suzuki K. Poulose wrote: > > > +static bool cpus_have_hwcap(const struct arm64_cpu_capabilities *cap) > > > +{ > > > + switch(cap->hwcap_type) { > > > + case CAP_HWCAP: > > > + return !!(elf_hwcap & cap->hwcap); > > > +#ifdef CONFIG_COMPAT > > > + case CAP_COMPAT_HWCAP: > > > + return !!(compat_elf_hwcap & (u32)cap->hwcap); > > > + case CAP_COMPAT_HWCAP2: > > > + return !!(compat_elf_hwcap2 & (u32)cap->hwcap); > > > +#endif > > > + default: > > > + BUG(); > > > + return false; > > > + } > > > +} > > > > Apart from the multiple returns, you don't really need !! since the > > return type is bool already. > > That's wrong. a & b doesn't return 0 or 1, but the bitwise-and result. a & b is indeed a bitwise operation and, in this particular case, its type is an unsigned long. However, because the return type of the function is a bool, the result of the bitwise operation (unsigned long) is converted to a bool. The above may be true only for gcc, I haven't checked other compilers, nor the standard (AFAIK, it appeared in C99). On AArch64, the compiler generates something like: tst x0, x1 cset w0, ne ret On AArch32, Thumb-2, I get: tst r2, r3 ite ne movne r0, #1 moveq r0, #0 bx lr So a bool type function always returns 0 or 1 and does the appropriate conversion. > http://yarchive.net/comp/linux/bool.html > > especially hpa's response. This seems to be more about a union of int and bool rather than automatic type conversion. But I can see in the simple test that Linus did towards the end of the thread that x86 does something similar with converting a char to a bool: testb %al, %al setne %al ret I stand by my original comment. -- Catalin