From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 128298 invoked by alias); 8 Feb 2018 15:05:33 -0000 Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-owner@gcc.gnu.org Received: (qmail 128155 invoked by uid 89); 8 Feb 2018 15:05:32 -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,FREEMAIL_FROM,RCVD_IN_DNSWL_NONE,SPF_PASS,TIME_LIMIT_EXCEEDED autolearn=unavailable version=3.3.2 spammy= X-HELO: mail-lf0-f51.google.com Received: from mail-lf0-f51.google.com (HELO mail-lf0-f51.google.com) (209.85.215.51) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Thu, 08 Feb 2018 15:05:22 +0000 Received: by mail-lf0-f51.google.com with SMTP id g72so6856193lfg.5 for ; Thu, 08 Feb 2018 07:05:21 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:in-reply-to:references:from:date :message-id:subject:to:cc; bh=5YkGcUu+V7Eme/oeRzJb/NG5iq0fnuokf6BEwv5dFsA=; b=kOWQpXq5e3d08aFOBXe/Yz6BUPC+hd0kLiTo1bt9KEWmWCdC8N8SFzXafH2qyTR2UB lCbwHZXmmbdvwYuUHiYKlGksO16CK0XU12LhrGUmTcKSVWnTTcyI7/GMliqbVVA0TiM1 JKDEyPpt3tksFOpiGVX0RJ0+qwZU+cPpJDGo2ALVA05XCCeQzMYf3rC7ieH3D/tpcb5w 9JV8uI7YmW/a2QZRXLZbxhPKgdPrC/02KJ8n7opzl9TqHICNnLVNkjToS3jH1pSf3Q5t PdyRCBdmgRFAI/T/AVxujP/pC8SnIX5lq8IxaO5chyCO0UAfCmgIQ4gVb7BveuVOPVET dlmg== X-Gm-Message-State: APf1xPACw2lUdBwDVyIW9cGt0tX13H2iQYehTEuo0oN90mPmN+gfoJa5 q3gogA/xdw+mGJXXQ4w8R5PuLPpT7ddtjs/dgk0= X-Google-Smtp-Source: AH8x226JQl2lyLKwbFZOSGHN7bgFvr+JG6XpbJwSXwaWDyPk2NmousriVfvMVJuzs7LRYTBli4ZHJlr66ePS3k0XOHs= X-Received: by 10.25.163.68 with SMTP id m65mr743114lfe.83.1518102319723; Thu, 08 Feb 2018 07:05:19 -0800 (PST) MIME-Version: 1.0 Received: by 10.46.51.21 with HTTP; Thu, 8 Feb 2018 07:05:19 -0800 (PST) In-Reply-To: <132fbd97-4f0d-020f-1c0f-1d4097800233@polymtl.ca> References: <1517667601.3405.123.camel@gnu.org> <1b58e2df-5425-4f22-510c-d2e9f51040ba@polymtl.ca> <39845077-6bdf-f60d-9bfc-a491e7fa4fc7@gmail.com> <132fbd97-4f0d-020f-1c0f-1d4097800233@polymtl.ca> From: Richard Biener Date: Thu, 08 Feb 2018 15:05:00 -0000 Message-ID: Subject: Re: gdb 8.x - g++ 7.x compatibility To: Simon Marchi Cc: Martin Sebor , Manfred , gdb@sourceware.org, GCC Development Content-Type: text/plain; charset="UTF-8" X-IsSubscribed: yes X-SW-Source: 2018-02/txt/msg00102.txt.bz2 On Mon, Feb 5, 2018 at 6:06 AM, Simon Marchi wrote: > 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. For other reasons I've always wanted sth like DW_OP_addr; DW_OP_name: n; DW_OP_stack_value thus put symbolical expressions in locations and have the consumer look them up (in context obviously). That way gdb can also choose to print foo instead of foo<1> or foo<>. Of course that needs DWARF extensions. Richard. > Simon > > [1] https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/simark/template-suffix