From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 122759 invoked by alias); 5 Feb 2018 05:07:14 -0000 Mailing-List: contact gdb-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-owner@sourceware.org Received: (qmail 122531 invoked by uid 89); 5 Feb 2018 05:07:13 -0000 Authentication-Results: sourceware.org; auth=none X-Virus-Found: No X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,SPF_HELO_PASS,SPF_PASS,T_RP_MATCHES_RCVD autolearn=unavailable version=3.3.2 spammy= X-HELO: smtp.polymtl.ca Received: from smtp.polymtl.ca (HELO smtp.polymtl.ca) (132.207.4.11) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Mon, 05 Feb 2018 05:07:11 +0000 Received: from simark.ca (simark.ca [158.69.221.121]) (authenticated bits=0) by smtp.polymtl.ca (8.14.7/8.14.7) with ESMTP id w1556w0L004708 (version=TLSv1/SSLv3 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 5 Feb 2018 00:07:03 -0500 Received: from [10.0.0.11] (192-222-251-162.qc.cable.ebox.net [192.222.251.162]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by simark.ca (Postfix) with ESMTPSA id DD55F1E4F4; Mon, 5 Feb 2018 00:06:57 -0500 (EST) Subject: Re: gdb 8.x - g++ 7.x compatibility To: Martin Sebor , Manfred Cc: gdb@sourceware.org, gcc@gcc.gnu.org References: <1517667601.3405.123.camel@gnu.org> <1b58e2df-5425-4f22-510c-d2e9f51040ba@polymtl.ca> <39845077-6bdf-f60d-9bfc-a491e7fa4fc7@gmail.com> From: Simon Marchi Message-ID: <132fbd97-4f0d-020f-1c0f-1d4097800233@polymtl.ca> Date: Mon, 05 Feb 2018 05:07:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: <39845077-6bdf-f60d-9bfc-a491e7fa4fc7@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-Poly-FromMTA: (simark.ca [158.69.221.121]) at Mon, 5 Feb 2018 05:06:58 +0000 X-IsSubscribed: yes X-SW-Source: 2018-02/txt/msg00021.txt.bz2 Hi Martin, Thanks for the reply. On 2018-02-04 02:17 PM, Martin Sebor wrote: > Printing the suffix is unhelpful because it leads to unnecessary > differences in diagnostics (even in non-template contexts). For > templates with non-type template parameters there is no difference > between, say A<1>, A<1U>, A<(unsigned) 1>, or even A when > Green is an enumerator that evaluates to 1, so including the suffix > serves no useful purpose. This is the part I don't understand. In Roman's example, spelling foo<10> and foo<10u> resulted in two different instantiations of the template, with different code. So that means it can make a difference, can't it? > In the GCC test suite, it would tend to > cause failures due to differences between the underlying type of > common typedefs like size_t and ptrdiff_t. Avoiding these > unnecessary differences was the main motivation for the change. > Not necessarily just in the GCC test suite but in all setups that > process GCC messages. Ok, I understand. > I didn't consider the use of auto as a template parameter but > I don't think it changes anything. There, just like in other > contexts, what's important is the deduced types and the values > of constants, not the minute details of how they are spelled. Well, it seems like using decltype on a template constant value is a way to make the type of constants important, in addition to their value. I know the standard seems to say otherwise (what Manfred quoted), but the reality seems different. I'm not a language expert so I can't tell if this is a deficiency in the language or not. > That said, it wasn't my intention to make things difficult for > the debugger. I hope so :). > But changing GCC back to include the suffix, > even just in the debug info, isn't a solution. There are other > compilers besides GCC that don't emit the suffixes, and there > even are some that prepend a cast to the number, so if GDB is > to be usable with all these kinds of producers it needs to be > able to handle all of these forms. As I said earlier, there are probably ways to make GDB cope with it. The only solution I saw (I'd like to hear about other ones) was to make GDB ignore the template part in DW_AT_name and re-build it from the DW_TAG_template_* DIEs in the format it expects. It can already do that somewhat, because, as you said, some compilers don't emit the template part in DW_AT_name. Doing so would cause major slowdowns in symbol reading, I've tried it for the sake of experimentation/discussion. I have a patch available on the "users/simark/template-suffix" branch in the binutils-gdb repo [1]. It works for Roman's example, but running the GDB testsuite shows that, of course, the devil is in the details. Consider something like this: template struct foo { virtual ~foo() {} }; int n; int main () { foo<&n> f; } The demangled name that GDB will be looking up is "foo<&n>". The debug info about the template parameter only contains the resulting address of n (the value of &n): <2>: Abbrev Number: 11 (DW_TAG_template_value_param) DW_AT_name : P DW_AT_type : <0x1ac> DW_AT_location : 10 byte block: 3 34 10 60 0 0 0 0 0 9f (DW_OP_addr: 601034; DW_OP_stack_value) I don't see how GDB could reconstruct the "&n" in the template, so that's where my idea falls short. Simon [1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/simark/template-suffix