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 4AA0D2206BF; Wed, 28 May 2025 19:56:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748462196; cv=none; b=pHlNoitthsssrlWasyp2AXtKQDLvtowiXBiWgN0IDspPMF1h1hZa83x82JpJtLTIr5h3db5MFZeMOFR6OdMsMOGgM+Ib9BVGjcBkEeATzwGMhJicQuH6LScZUqrJMTv7U93gzYPn4CXg6V4Nqk+fARKknf59xSvvTRX+J5sDqKg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748462196; c=relaxed/simple; bh=pnfecmKSpQYkvxOkvpTwWbsi+mXim4QALt0Lzr4hQOI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=NpjAPKSQ3iTyPi4+k6HliPzPUKW5Lsp21HgofVlBkRpvbEYB3va2UWpwL7vbcyDfPar6Alw0fya1cBJyWHEPu/f9QEy3rDPtNSxLipFXSmy7Yrp0b/RpTbfGGe9SCAqXqKgS1ijBfkX8EuSBDpEVKvDReqo9sjy049/MoE2kjcY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=bq7WOE57; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="bq7WOE57" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5C08AC4CEE3; Wed, 28 May 2025 19:56:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1748462195; bh=pnfecmKSpQYkvxOkvpTwWbsi+mXim4QALt0Lzr4hQOI=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=bq7WOE57EBGZ7ojGxYtDeoRGXQOvOAzkr0vDVuW5dz+pNOk+MY8RxNLpMdMcJ//T3 28oBNmdhcZWJMktvsSY9a/z8XCaWiOW3m/TRpylW1qLphUXMS1RE/AuCrwC0+b12jC gUBrziJ7VWcpsvGZJfP/xY09H2Dww9tDDoxM8T/JKR7npqUrslsg5rrBfxM7UEFSgO PlljObfXP0oRWfnp5OpUvkjN5dkN7HFk9R5TlEkQ2kxhuDqKnNngmi3gcHMPUANvLV BSRFC6dYpFV8y1KdoKXN0R2C1jjS+9Znert72fLGlZHNZigk86iRRA1n1XLnvJxWai 0vofM2t+KXmbg== Date: Wed, 28 May 2025 12:56:33 -0700 From: Namhyung Kim To: Ian Rogers Cc: Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Mark Rutland , Alexander Shishkin , Adrian Hunter , Kan Liang , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org Subject: Re: [RFC PATCH v1] perf build: Fix build for clang's -Wunreachable-code Message-ID: References: <20250410202647.1899125-1-irogers@google.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, May 28, 2025 at 11:35:00AM -0700, Ian Rogers wrote: > On Wed, May 28, 2025 at 11:24 AM Namhyung Kim wrote: > > > > On Tue, May 27, 2025 at 01:53:37PM -0700, Ian Rogers wrote: > > > On Fri, Apr 11, 2025 at 3:14 PM Ian Rogers wrote: > > > > > > > > On Fri, Apr 11, 2025 at 2:34 PM Namhyung Kim wrote: > > > > > > > > > > Hi Ian, > > > > > > > > > > On Thu, Apr 10, 2025 at 01:26:47PM -0700, Ian Rogers wrote: > > > > > > Clang's unreachable code warning is able to catch bugs like the famous > > > > > > "goto fail" which gcc's unreachable code warning fails to warn about > > > > > > (it will complain about misleading indent). The changes here are > > > > > > sufficient to get perf building with clang with -Wunreachable code, > > > > > > but they don't really fix any bugs. Posting as an RFC to see if anyone > > > > > > things this is worth pursuing. > > > > > > > > > > I'm not sure if it's useful and don't see what kind of bugs it can > > > > > address. The proposed changes don't look like an improvement. > > > > > > > > The goto fail case was in OpenSSL the code from a bad merge: > > > > ``` > > > > if (...) > > > > goto fail; > > > > goto fail; > > > > ``` > > > > Meaning the fail path was always taken and checking on the non-fail > > > > code never executed. Newer GCCs will warn of this because of the > > > > "misleading indent" but clang won't. It is easy to imagine similar > > > > mistakes creeping in, so using compiler warnings to avoid the bug > > > > could be useful. > > > > It doesn't look very convincing to me but it might be valuable in some > > rare cases. But the proposed changes - basically replace exit() to > > __builtin_unreachable() - seem weird. Why is calling it a problem? I > > guess it already has some kind of annotation like "noreturn"? > > Yep. The exit is incorrect (depending on your notion of correct, I'd > go with clang's notion as they've had to consider this for a while) as > it can never be executed. I've added the __builtin_unreachable() to > document that you can never get to that statement, as otherwise it can > make the code readability harder with the code looking like it will > fall through after calling something like usage_with_options (which is > noreturn). In unoptimized builds __builtin_unreachable() will fail if > executed, so it is a bit more active than just a comment. Oh I see, usage_with_options() calls exit() inside so any code after that won't be executed. Hmm.. isn't it better to remove those codes then? Thanks, Namhyung