From patchwork Thu Oct 15 19:30:29 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jeff King X-Patchwork-Id: 11840091 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id E763BC433E7 for ; Thu, 15 Oct 2020 19:30:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8D6A9206DD for ; Thu, 15 Oct 2020 19:30:31 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2391559AbgJOTaa (ORCPT ); Thu, 15 Oct 2020 15:30:30 -0400 Received: from cloud.peff.net ([104.130.231.41]:33378 "EHLO cloud.peff.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2391541AbgJOTaa (ORCPT ); Thu, 15 Oct 2020 15:30:30 -0400 Received: (qmail 24178 invoked by uid 109); 15 Oct 2020 19:30:30 -0000 Received: from Unknown (HELO peff.net) (10.0.1.2) by cloud.peff.net (qpsmtpd/0.94) with ESMTP; Thu, 15 Oct 2020 19:30:30 +0000 Authentication-Results: cloud.peff.net; auth=none Received: (qmail 32202 invoked by uid 111); 15 Oct 2020 19:30:29 -0000 Received: from coredump.intra.peff.net (HELO sigill.intra.peff.net) (10.0.0.2) by peff.net (qpsmtpd/0.94) with (TLS_AES_256_GCM_SHA384 encrypted) ESMTPS; Thu, 15 Oct 2020 15:30:29 -0400 Authentication-Results: peff.net; auth=none Date: Thu, 15 Oct 2020 15:30:29 -0400 From: Jeff King To: Junio C Hamano Cc: git@vger.kernel.org, Denton Liu Subject: [PATCH v2 2/2] config.mak.dev: build with -fno-common Message-ID: <20201015193029.GB1129761@coredump.intra.peff.net> References: <20201015192845.GD1108210@coredump.intra.peff.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201015192845.GD1108210@coredump.intra.peff.net> Precedence: bulk List-ID: X-Mailing-List: git@vger.kernel.org It's an easy mistake to define a variable in a header with "int x;" when you really meant to only declare the variable as "extern int x;" instead. Clang and gcc will both allow this when building with "-fcommon"; they put these "tentative definitions" in a common block which the linker is able to resolve. This is the default in clang and was the default in gcc until gcc-10, since it helps some legacy code. However, we would prefer not to rely on this because: - using "extern" makes the intent more clear (so it's a style issue, but it's one the compiler can help us catch) - according to the gcc manpage, it may yield a speed and code size penalty So let's build explicitly with -fno-common when the DEVELOPER knob is set, which will let developers using clang and older versions of gcc notice these problems. I didn't bother making this conditional on a particular version of gcc. As far as I know, this option has been available forever in both gcc and clang, so old versions don't need to avoid it. And we already expect gcc and clang options throughout config.mak.dev, so it's unlikely anybody setting the DEVELOPER knob is using anything else. It's a noop on gcc-10, of course, but it's not worth trying to exclude it there. Note that there's nothing to fix in the code; we already don't have any issues here. But if you want to test the patch, you can add a bare "int x;" into cache.h, which will cause the link step to fail. Signed-off-by: Jeff King --- New in this version. It could be queued separately from the first. config.mak.dev | 1 + 1 file changed, 1 insertion(+) diff --git a/config.mak.dev b/config.mak.dev index 89b218d11a..89fefacd94 100644 --- a/config.mak.dev +++ b/config.mak.dev @@ -15,6 +15,7 @@ DEVELOPER_CFLAGS += -Wpointer-arith DEVELOPER_CFLAGS += -Wstrict-prototypes DEVELOPER_CFLAGS += -Wunused DEVELOPER_CFLAGS += -Wvla +DEVELOPER_CFLAGS += -fno-common ifndef COMPILER_FEATURES COMPILER_FEATURES := $(shell ./detect-compiler $(CC))