From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by sourceware.org (Postfix) with ESMTPS id 8E86A3858CDA for ; Tue, 10 Jan 2023 08:06:19 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 8E86A3858CDA 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-out1.suse.de (Postfix) with ESMTP id C897F4E0CD; Tue, 10 Jan 2023 08:06:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_rsa; t=1673337977; 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=6t/SsoyOLkUepr9lr3ZZrarY5fKRM3EBgFSK6UWZBRg=; b=qkRI0m3tyFi+cNip8/Q8YZwxgCm9Zx9ArP4Cez5Zs6MikbCWkBpE2YFNI28WsKEl1RSlcl G2zIzfoNk0RENmFB05BJRfPLpb6MaoGP4U9aMlFq+RfMjmUWy55EYztDocr0iTs2dh/yrq LrY2BeqhCvN3bZq+QgCOFt2VwuxThOs= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.de; s=susede2_ed25519; t=1673337977; 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=6t/SsoyOLkUepr9lr3ZZrarY5fKRM3EBgFSK6UWZBRg=; b=yPR5YoeWsz24p7eviEOFR7YkMxZ5OhpsvAVvnNJXhF3DCOAnz6CdWKoWyL+uDqDAZya7O5 mKd7Jj5NIh6B21CQ== 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 A73CB2C141; Tue, 10 Jan 2023 08:06:17 +0000 (UTC) Date: Tue, 10 Jan 2023 08:06:17 +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: <16412745-F470-44A1-8886-4BE669FDA7A4@oracle.com> Message-ID: References: <7EE40B3B-7A01-48A1-B4BE-B0E3103C31A3@oracle.com> <47CF80FD-1B6B-4024-B493-79D45859E65A@oracle.com> <16412745-F470-44A1-8886-4BE669FDA7A4@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 Mon, 9 Jan 2023, Qing Zhao wrote: > > > > On Jan 9, 2023, at 2:11 AM, Richard Biener wrote: > > > > 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? > > The function ?strict_flex_array_level_of? is intended to query the LEVEL of strict_flex_array, only check DECL_NOT_FLEXARRAY is not enough. > > So, I think the major question here is: > > Do we need the LEVEL of strict_flex_array information in the Middle end? > > The current major use of LEVEL of strict_flex_array in the middle end is two places: > > 1. In the routine ?component_ref_size?: to determine the size of the trailing array based on the level of the strict_flex_array. > 2. In the routine ?array_bounds_checker::check_array_ref?: to issue different information for -Wstrict-flex-array based on different level. > > > Just double checked the above 1, and 2. Without LEVEL of strict_flex_array info, 1 should be fine > 2, as you mentioned previously, the major impact will be that the LEVEL information is lost in the issued message, but that might be not a big > issue. > > So, I will try to eliminate the reference to ?flag_strict_flex_array? in the middle end, replace it with ?DECL_NOT_FLEXARRAY?, and come up with > an updated patch for this change. > > How do you think? Yes, that sounds good. > > > >>> > >>> 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. > Yes. Agreed. > > > > >>> > >>>>> 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. > > Shall we issue warning for such mismatches? Where is the place I can add such warnings? I'm not sure - we'd have to restrict it to "used" types and in principle only when actual objects pass from one TU to another with different flex-array semantics. Otherwise we'll risk tons of diagnostics when people "forget" -fstrict-flex-arrays on some TUs but pull in common headers. The C++ ODR diagnostics reside in ipa-devirt.cc, I'm not sure diagnostics on flex arrays would fit there. I just wanted to bring this up, I do not have a good idea how or where to implement it. Richard. > thanks. > > Qing > > > > Richard. > > -- Richard Biener SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg, Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman; HRB 36809 (AG Nuernberg)