From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 103416 invoked by alias); 11 Jul 2018 15:27:45 -0000 Mailing-List: contact gdb-patches-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Archive: List-Post: List-Help: , Sender: gdb-patches-owner@sourceware.org Received: (qmail 103398 invoked by uid 89); 11 Jul 2018 15:27:44 -0000 Authentication-Results: sourceware.org; auth=none X-Spam-SWARE-Status: No, score=-1.9 required=5.0 tests=BAYES_00,KAM_SHORT,SPF_HELO_PASS autolearn=ham version=3.3.2 spammy=Kevin, H*M:lan, weighs, blockvector X-HELO: mx1.redhat.com Received: from mx1.redhat.com (HELO mx1.redhat.com) (209.132.183.28) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Wed, 11 Jul 2018 15:27:43 +0000 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id D975ABB9C1 for ; Wed, 11 Jul 2018 15:27:41 +0000 (UTC) Received: from pinnacle.lan (ovpn-116-191.phx2.redhat.com [10.3.116.191]) by smtp.corp.redhat.com (Postfix) with ESMTPS id AB4B35C582 for ; Wed, 11 Jul 2018 15:27:41 +0000 (UTC) Date: Wed, 11 Jul 2018 15:27:00 -0000 From: Kevin Buettner To: gdb-patches@sourceware.org Subject: Re: [PATCH 0/8] Non-contiguous address range support Message-ID: <20180711082740.05e94750@pinnacle.lan> In-Reply-To: <20180625233239.49dc52ea@pinnacle.lan> References: <20180625233239.49dc52ea@pinnacle.lan> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-IsSubscribed: yes X-SW-Source: 2018-07/txt/msg00310.txt.bz2 Ping. On Mon, 25 Jun 2018 23:32:39 -0700 Kevin Buettner wrote: > This eight part patch sequence adds (further) support for > non-contiguous address ranges to GDB. > > This sequence of patches was motivated by GCC bug 84550: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84550 > > There is a test case posted to that bug along with some analysis of > the underlying problem. > > There is also a GDB bug for the same issue; it's 23021, but at the > moment, there is little there aside from a link to the GCC bug > mentioned above. But here's a link anyway: > > https://sourceware.org/bugzilla/show_bug.cgi?id=23021 > > A quick synopsis of the problem is as follows... > > Recent versions of gcc can generate code in which a function is split > into at least two non-contiguous address ranges. As I understand it, > the idea here is to separate code which gcc does not expect to execute > in normal operation from the rest of the code. Doing this may result > in better cache locality for the normal case. The generated code for > the example in GCC bug 84550 separated a call to abort() from the rest > of the code comprising the function. > > In the course of my investigation, I identified at least four > problems: > > 1) Stepping into a function from a function which occupies non-contiguous > address ranges does not always work. It was not uncommon to see the > program run to completion when attempting to do a step. > > 2) Setting a breakpoint on a function with non-contiguous address ranges > causes a breakpoint to be placed on more than one location. When a > breakpoint is set on the "cold" address range, this is almost certainly > incorrect. The breakpoint should instead be set only on code near the > entry point(s). > > 3) The disassemble command did not work correctly. E.g. here is what I > found during my analysis of 84550: > > (gdb) x/i 'main.cold.0' > 0x4010e0 : mov %rax,%rdi > (gdb) x/i main > 0x4011a0
: push %r12 > (gdb) disassemble main > Dump of assembler code for function main(): > 0x00000000004010e0 <+0>: mov %rax,%rdi > ... > [No addresses starting at 0x4011a0 are shown] > > 4) Display of addresses associated with the non-contiguous function are > confusing. E.g. in the above example, note that GDB thinks that > the address associated with main.cold.0 is , but that there's > also a minsym called main which is displayed as
. > > There are probably several other problems which are related those > identified above. > > I discovered that the stepping problem could be "fixed" by disabling > the find_pc_partial_function cache. This cache keeps track of the > most recent result (of calling find_pc_partial_function). If > find_pc_partial_function is called with an address which falls within > the cache range, then that's considered to be a cache hit and the most > recent result is returned. Obviously, this won't work correctly for > functions which occupy non-contiguous (disjoint) address ranges where > other functions might be placed in the gap. > > So one of the problems that needed to be solved was to make the > caching code work correctly. It is interesting to note that stepping > _did_ work when the cache was disabled. This is/was due to GDB > already having some (albeit incomplete) support for non-contiguous > addresses in the form of blockvector address maps. Code responsible > for mapping addresses to blocks (which form the lower levels of > find_pc_partial_function) handle this case correctly. > > To solve the problem of incorrect disassembly, we need to be able > to iterate over all of the ranges associated with a block. > > Finally, we need to distinguish between the entry pc and the lowest > address in a block. I discovered that this lack of distinction was > the cause of the remainder of the bugs including some which seemed to > be introduced by fixing the problems noted above. Once this > distinction is made, it will be straightforward to add full support for > DW_AT_entry_pc. I considered adding this support as part of this > patch series, but decided to wait until the community weighs in on my > work thus far...