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Fri, 29 Nov 2019 14:43:18 +0000 From: Wilco Dijkstra To: =?Windows-1252?Q?Martin_Li=9Aka?= , GCC Patches Subject: Re: [PATCH] PR85678: Change default to -fno-common Date: Fri, 29 Nov 2019 14:46:00 -0000 Message-ID: References: ,<2600d107-dc05-95b0-8864-5a9c4d7bb85e@suse.cz> In-Reply-To: <2600d107-dc05-95b0-8864-5a9c4d7bb85e@suse.cz> Authentication-Results-Original: spf=none (sender IP is ) smtp.mailfrom=Wilco.Dijkstra@arm.com; x-checkrecipientrouted: true x-ms-oob-tlc-oobclassifiers: OLM:8882;OLM:8882; X-Forefront-Antispam-Report-Untrusted: SFV:NSPM;SFS:(10009020)(4636009)(396003)(366004)(376002)(136003)(39860400002)(346002)(189003)(199004)(33656002)(52536014)(110136005)(229853002)(3846002)(606006)(6306002)(6436002)(9686003)(54896002)(6116002)(26005)(7736002)(71200400001)(446003)(74316002)(11346002)(478600001)(66066001)(71190400001)(14454004)(81156014)(8676002)(81166006)(236005)(5660300002)(186003)(6506007)(25786009)(256004)(76116006)(86362001)(102836004)(8936002)(99286004)(6246003)(66946007)(7696005)(55016002)(316002)(66476007)(2906002)(66446008)(76176011)(64756008)(66556008);DIR:OUT;SFP:1101;SCL:1;SRVR:VI1PR0801MB2077;H:VI1PR0801MB2127.eurprd08.prod.outlook.com;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;MX:1;A:1; received-spf: None (protection.outlook.com: arm.com does not designate permitted sender hosts) X-MS-Exchange-SenderADCheck: 1 X-Microsoft-Antispam-Untrusted: BCL:0; X-Microsoft-Antispam-Message-Info-Original: qcxu0tH9R4f8kIMOG6KzNHrOk5qEreeP/WXULrjWHhwhp7eLQXnjMXH1nOzG8gydp/3qv2LMJnLi33mhW3C6cGX+v+izd4QtOnmmJk7MtdSaY0RH1kiL8uJBEF5fBP3R+UG9SzwSQ3WkfHZjOGfSrSNFa+qyEN9ayA/YmpBeVHu5mwnD1oUBpTM75fLFuDXQQpySMJ+oLeSfkXWFgn3eJQ81giDZ1m5C8z7Y3XUPZ8IWZ9cahHSv8GhWsEgeT87OcwHYdQbnaZVphfV4lE6+6KjK7O5piYYv2dLAer4OZuleWEU0wU0i+dTy9Z+M1p8Uyydu6vtDC+dBbiNQUetMUc9yEvc5005GAEQ0+LuimcpCjnsWgBhhBKOqmRtE0IpyuS8tS8TjpImIXGISm7ey9DxMGopOVzbZ3zWVceNySijQpxtZ3aCx9txz/lF57b8UXoceiMVb4Tb09FzwmU7zBgmJkyT6wM9PufrVPppdHJQ= x-ms-exchange-transport-forked: True Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Original-Authentication-Results: spf=none (sender IP is ) smtp.mailfrom=Wilco.Dijkstra@arm.com; Return-Path: Wilco.Dijkstra@arm.com X-MS-Exchange-Transport-CrossTenantHeadersStripped: AM5EUR03FT009.eop-EUR03.prod.protection.outlook.com X-MS-Office365-Filtering-Correlation-Id-Prvs: 84d4c5f6-002c-4b09-9990-08d774da795f X-SW-Source: 2019-11/txt/msg02663.txt.bz2 Hi Martin, > I've noticed quite significant package failures caused by the revision. How significant? Is it mostly the common mistake of forgetting extern? > Would you please consider documenting this change in porting_to.html > (and in changes.html) for GCC 10 release? Sure, I already had a patch for changes.html - I've added an initial portin= g_to as well: [wwwdocs] Document -fcommon default change Add an entry for the default change. Passes the W3 validator. -- diff --git a/htdocs/gcc-10/changes.html b/htdocs/gcc-10/changes.html index f0f0d312171a54afede176f06ce76f9c8abaebc4..980e4e591781d04aa888ba59889= 81006bd30dd1f 100644 --- a/htdocs/gcc-10/changes.html +++ b/htdocs/gcc-10/changes.html @@ -47,6 +47,13 @@ a work-in-progress.

=20

General Improvements

+

The following GCC command line options have been introduced or improved= .

+
    +
  • GCC now defaults to -fno-common. In C, global variable= s with + multiple tentative definitions will result in linker errors. + Global variable accesses are also more efficient on various targets. +
    • +
=20

The following built-in functions have been introduced.

    diff --git a/htdocs/gcc-10/porting_to.html b/htdocs/gcc-10/porting_to.html new file mode 100644 index 0000000000000000000000000000000000000000..2e652f6aa4bd3259a316af0c72a= b7eb96bab53b7 --- /dev/null +++ b/htdocs/gcc-10/porting_to.html @@ -0,0 +1,65 @@ + + + + + +Porting to GCC 10 + + + + +

    Porting to GCC 10

    + +

    +The GCC 10 release series differs from previous GCC releases in +a number of ways. Some of these are a result +of bug fixing, and some old behaviors have been intentionally changed +to support new standards, or relaxed in standards-conforming ways to +facilitate compilation or run-time performance. +

    + +

    +Some of these changes are user visible and can cause grief when +porting to GCC 10. This document is an effort to identify common issues +and provide solutions. Let us know if you have suggestions for improvement= s! +

    + + + + +

    C language issues

    + +

    Default to -fno-common

    + +

    + A common mistake in C is omitting extern when declaring a g= lobal + variable in a header file. If the header is included by several files it + results in multiple definitions of the same variable. In previous GCC + versions this error is ignored. GCC 10 defaults to -fno-common, + which means a linker error will now be reported. + To fix this, use extern in header files when declaring glob= al + variables, and ensure each global is defined in exactly one C file. + As a workaround, legacy C code can be compiled with -fcommon. +

    + 
    
+      int x;  // tentative definition - avoid in header files
+
+      extern int y;  // correct declaration in a header file
+
    + + + + +=20 + + + +