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 D322D22370F; Wed, 28 May 2025 21:59:21 +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=1748469561; cv=none; b=e/wAq8TzJE/xSsLTsJKhdy7SED0ABJJgILFEN3xpDHZuzUpsIh/wNHQrBtwkeom1L5/A6T3d8+PwPG3oCxbG0hPsGixotU9mislNhb6nXeWeE9juPNfSHllwtJ4JGlWJ73apPW3u4LKsdNA2PsxxF4ig56uRZMuamy3EV21ETOc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748469561; c=relaxed/simple; bh=rTSHTbWhD0fIYsIngS/eYKjGxuLf3+dx2sFLpQ/GdZ4=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=oWq8ytWUFKTG6sekfOWcCOf7jcjv56su0BsQTghb+UXP1rLxkBUKnlVWCT+aMoUvFv4gpOtd4M/UXsyrY5fXfSu6G40H9R5pr3DBaR5u42zulTzzuutMIbblXqtJaDwQeUtygv8Odlee5mqyXg+R4UsIBn0VRf+DsxXs+e065bM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Cbjd9J45; 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="Cbjd9J45" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 50B68C4CEE3; Wed, 28 May 2025 21:59:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1748469561; bh=rTSHTbWhD0fIYsIngS/eYKjGxuLf3+dx2sFLpQ/GdZ4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=Cbjd9J45sepFgn5GqkwPTv4ImwQvkxSKOkk5dGqw3h6OSW/Vb7pCPFRvFKb3GtJM0 gSGanIZX6tlE0026pDYsp3H7s03uSkytwXvkXyI7UIKT3QLXlTWdgQ2vJpsPz3z9Jj EPceE/0C4VYrxsGQTLKrMYlLQPMixw3DCCLV2mlGUZ4oSuVRZeXQCWO+OldrgI/PfC LGF185uk/BbMT2fSqcWiAl23fMlM/T3gFnk3eTnHmdrQTAbgF+J+3MA9uHXIVH1gBO OL8wFKl3BpusYiHl1B/5XniEY1ffF2XlY9IEb7umAkETvKAAsh44sp6yn04ifOVYxY QK3K9yH3GY1Wg== Date: Wed, 28 May 2025 14:59:19 -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 01:32:10PM -0700, Ian Rogers wrote: > On Wed, May 28, 2025 at 12:56 PM Namhyung Kim wrote: > > > > 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? > > Not sure I follow. The patch does remove the code but it replaces it > with __builtin_unreachable() to basically state that the code here and > below can never be reached. Do you mean remove the exit from > usage_with_options? Then we'd need to fix all the callers, which would > be a larger patch. Perhaps it should be usage_with_options_and_exit() > to make it clearer that the code doesn't return. I was after doing > what was minimal for -Wunreachable-code but while trying to keep the > code clear. No, I meant we may not need the __builtin_unreachable() at the callsites. Would it complain this code? if (some_bad_option_use) usage_with_options(...); /* normal code path */ Thanks, Namhyung