From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 111186 invoked by alias); 19 Aug 2018 13:35:26 -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 111176 invoked by uid 89); 19 Aug 2018 13:35:26 -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,SPF_PASS autolearn=ham version=3.3.2 spammy=H*F:D*ca, gap, H*Ad:D*ca, stepping X-HELO: simark.ca Received: from simark.ca (HELO simark.ca) (158.69.221.121) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with ESMTP; Sun, 19 Aug 2018 13:35:24 +0000 Received: from [10.0.0.11] (unknown [192.222.164.54]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by simark.ca (Postfix) with ESMTPSA id C59CC1E008; Sun, 19 Aug 2018 09:35:21 -0400 (EDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=simark.ca; s=mail; t=1534685722; bh=vRDvEEJAftcdqChq3KHrhwIX51gdOi+Fd5nh02ZamiQ=; h=Subject:To:References:From:Date:In-Reply-To:From; b=AK7VjCgsgdR6VgDvjkv+20t5/fQySTmTcmrkGZc4XRhcLa2jQlHYVjLVCm8nzujT/ g798JP4UOGQQaXn23wdYSZ5BFrMzjVTB1b2hK2t2kJOHoaxG8umiU/GP0a467M6puv I+DAq384pEd5JObUMXux4uxXr1d8DG0O9SNz8T6U= Subject: Re: [PATCH v2 0/8] Non-contiguous address range support To: Kevin Buettner , gdb-patches@sourceware.org References: <20180813165002.29ddb8dd@pinnacle.lan> From: Simon Marchi Message-ID: <59e1be90-1328-2b01-1d18-8d69e1d4eee0@simark.ca> Date: Sun, 19 Aug 2018 13:35:00 -0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <20180813165002.29ddb8dd@pinnacle.lan> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-SW-Source: 2018-08/txt/msg00443.txt.bz2 On 2018-08-13 7:50 p.m., Kevin Buettner wrote: > This is version 2 of an eight part patch series which adds further > support for non-contiguous address ranges to GDB. > > In this v2 series, I've addressed the concerns from Simon Marchi's > review of the earlier patch set. I've also changed my mind about how > return values *ADDRESS and *ENDADDR ought to be set for > find_pc_partial_function. I'll discuss this matter in the remarks > preceding the relevant patches. > > Everything below this point was copy/pasted from the introductory > message for the v1 patch set... > > 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... > Hi Kevin, The patchset seems quite outdated (based on an old commit). Could you send a rebased version? Thanks, Simon