From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by sourceware.org (Postfix) with ESMTPS id E1DB73858D33 for ; Mon, 9 Jan 2023 07:11:06 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org E1DB73858D33 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=suse.de Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=suse.de Received: from relay2.suse.de (relay2.suse.de [149.44.160.134]) by smtp-out2.suse.de (Postfix) with ESMTP id 010DA76BAA; Mon, 9 Jan 2023 07:11:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1673248262; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Ehcq5wPYgQ1PTUgIcfGOF58Eo7/2JIiHr2PoahnXSmc=; b=Jm+ERqkAE3q5S8XCtWu5yAIRjXR9MWJJKtUdeSCeY0CIASulx00ETOuZdKVGvRNcY0da0H pCWsiWc3STEU1CTY2In6bJ1c0zo58OYu4Yk4YEV+igX4aFBtkIjM5llcc+Nf8PET5bW03I egqgSS8KcVWHZEKdGUK8Qc22HJsxKso= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1673248262; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Ehcq5wPYgQ1PTUgIcfGOF58Eo7/2JIiHr2PoahnXSmc=; b=KjvRrhhV40HtOT1LP0cqC/RqRESlvQjHcGNjIUf8cSJYcgsLSM7g0AM5NsEli4SWucR7nv 2VEBSAb7PMOAGmAw== Received: from wotan.suse.de (wotan.suse.de [10.160.0.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by relay2.suse.de (Postfix) with ESMTPS id D5DE12C141; Mon, 9 Jan 2023 07:11:01 +0000 (UTC) Date: Mon, 9 Jan 2023 07:11:01 +0000 (UTC) From: Richard Biener To: Qing Zhao cc: gcc Patches Subject: Re: gcc-13/changes.html: Mention -fstrict-flex-arrays and its impact In-Reply-To: <47CF80FD-1B6B-4024-B493-79D45859E65A@oracle.com> Message-ID: References: <7EE40B3B-7A01-48A1-B4BE-B0E3103C31A3@oracle.com> <47CF80FD-1B6B-4024-B493-79D45859E65A@oracle.com> User-Agent: Alpine 2.22 (LSU 394 2020-01-19) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Spam-Status: No, score=-11.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Thu, 22 Dec 2022, Qing Zhao wrote: > > > > On Dec 22, 2022, at 2:09 AM, Richard Biener wrote: > > > > On Wed, 21 Dec 2022, Qing Zhao wrote: > > > >> Hi, Richard, > >> > >> Thanks a lot for your comments. > >> > >>> On Dec 21, 2022, at 2:12 AM, Richard Biener wrote: > >>> > >>> On Tue, 20 Dec 2022, Qing Zhao wrote: > >>> > >>>> Hi, > >>>> > >>>> This is the patch for mentioning -fstrict-flex-arrays and -Warray-bounds=2 changes in gcc-13/changes.html. > >>>> > >>>> Let me know if you have any comment or suggestions. > >>> > >>> Some copy editing below > >>> > >>>> Thanks. > >>>> > >>>> Qing. > >>>> > >>>> ======================================= > >>>> From c022076169b4f1990b91f7daf4cc52c6c5535228 Mon Sep 17 00:00:00 2001 > >>>> From: Qing Zhao > >>>> Date: Tue, 20 Dec 2022 16:13:04 +0000 > >>>> Subject: [PATCH] gcc-13/changes: Mention -fstrict-flex-arrays and its impact. > >>>> > >>>> --- > >>>> htdocs/gcc-13/changes.html | 15 +++++++++++++++ > >>>> 1 file changed, 15 insertions(+) > >>>> > >>>> diff --git a/htdocs/gcc-13/changes.html b/htdocs/gcc-13/changes.html > >>>> index 689178f9..47b3d40f 100644 > >>>> --- a/htdocs/gcc-13/changes.html > >>>> +++ b/htdocs/gcc-13/changes.html > >>>> @@ -39,6 +39,10 @@ a work-in-progress.

> >>>>
  • Legacy debug info compression option -gz=zlib-gnu was removed > >>>> and the option is ignored right now.
    • > >>>>
  • New debug info compression option value -gz=zstd has been added.
    • > >>>> +
  • -Warray-bounds=2 will no longer issue warnings for out of bounds > >>>> + accesses to trailing struct members of one-element array type anymore. Please > >>>> + add -fstrict-flex-arrays=level to control how the compiler treat > >>>> + trailing arrays of structures as flexible array members.
    • > >>> > >>> "Instead it diagnoses accesses to trailing arrays according to > >>> -fstrict-flex-arrays." > >> > >> Okay. > >>> > >>>> > >>>> > >>>> > >>>> @@ -409,6 +413,17 @@ a work-in-progress.

    > >>>>

    Other significant improvements

    > >>>> > >>>> > >>>> +

    Treating trailing arrays as flexible array members

    > >>>> + > >>>> +
      > >>>> +
    • GCC can now control when to treat the trailing array of a structure as a > >>>> + flexible array member for the purpose of accessing the elements of such > >>>> + an array. By default, all trailing arrays of structures are treated as > >>> > >>> all trailing arrays in aggregates are treated > >> Okay. > >>> > >>>> + flexible array members. Use the new command-line option > >>>> + -fstrict-flex-array=level to control how GCC treats the trailing > >>>> + array of a structure as a flexible array member at different levels. > >>> > >>> -fstrict-flex-arrays to control which trailing array > >>> members are streated as flexible arrays. > >> > >> Okay. > >> > >>> > >>> I've also just now noticed that there's now a flag_strict_flex_arrays > >>> check in the middle-end (in array bound diagnostics) but this option > >>> isn't streamed or handled with LTO. I think you want to replace that > >>> with the appropriate DECL_NOT_FLEXARRAY check. > >> > >> We need to know the level value of the strict_flex_arrays on the struct > >> field to issue proper warnings at different levels. DECL_NOT_FLEXARRAY > >> does not include such info. So, what should I do? Streaming the > >> flag_strict_flex_arrays with LTO? > > > > But you do > > > > if (compref) > > { > > /* Try to determine special array member type for this > > COMPONENT_REF. */ > > sam = component_ref_sam_type (arg); > > /* Get the level of strict_flex_array for this array field. */ > > tree afield_decl = TREE_OPERAND (arg, 1); > > strict_flex_array_level = strict_flex_array_level_of (afield_decl); > > > > I see that function doesn't look at DECL_NOT_FLEXARRAY but just > > checks attributes (those are streamed in LTO). > > Yes, checked both flag_strict_flex_arrays and attributes. > > There are two places in middle end calling ?strict_flex_array_level_of? function, > one inside ?array_bounds_checker::check_array_ref?, another one inside ?component_ref_size?. > Shall we check DECL_NOT_FLEXARRAY field instead of calling ?strict_flex_array_level_of? in both places? I wonder if that function should check DECL_NOT_FLEXARRAY? > > > > OK, so I suppose the diagnostic itself would become just less precise > > as in "trailing array %qT should not be used as a flexible array member" > > without the "for level N and above" part of the diagnostic? > > Yes, that might be the major impact. > > If only check DECL_NOT_FLEXARRAY, we will lose such information. Does that matter? I think the main information is preserved in diagnosing the flex vs. non-flex array assumption. > > > >>> We might also want > >>> to see how inlining accesses from TUs with different -fstrict-flex-arrays > >>> setting behaves when accessing the same structure (and whether we might > >>> want to issue an ODR style diagnostic there). > > > > This mixing also means streaming -fstrict-flex-arrays won't be of much > > help in general. > > Then under such situation, i.e, different -fstrict-flex-arrays levels for the same structure from different TUs, how should we handle it? I think in similar situations we try to DWIM, but in some cases it will result in "garbage" behavior. I don't think there's anything we can do here besides trying to diagnose such mismatches with -flto at the WPA stage. Richard.