Bug 18861

Summary: 'reinterpret_cast' in gdb seems to be doing a 'static_cast'.
Product: gdb Reporter: cs120hcc
Component: c++Assignee: Not yet assigned to anyone <unassigned>
Status: RESOLVED FIXED    
Severity: normal CC: ssbssa, tromey
Priority: P2    
Version: HEAD   
Target Milestone: 15.1   
Host: Target:
Build: Last reconfirmed:
Attachments: Sample Code and Repro Transcript

Description cs120hcc 2015-08-21 21:27:27 UTC 
Created attachment 8544 [details]
Sample Code and Repro Transcript

Problem: 'reinterpret_cast' in gdb seems to be doing a 'static_cast'.
 
For example, assume we have two unrelated base classes, 'fred' and 'ginger', and another class 'hillary' that first extends 'fred' then 'ginger'.  Typically, a 'hillary' object would have the same address as the inherited 'fred' object.  The inherited 'ginger' object typically follows the 'fred' data.  (It depends on the compiler implementation, but don't let that distract you from the description of the gdb bug.)
 
'reinterpret_cast' typically does not change the value of a pointer that is casts.  Specifically, given 'ginger*' pointer 'g', 'reinterpret_cast<hillary*>(g)' should have the same value as 'static_cast<hillary*>(static_cast<void*>(g))' per 5.2.10.7 of the 2011 and 2014 versions of the C++ standard (N3242 and N4296).  Notice that it does work properly in code generated by g++.  However, in the various versions of gdb, it does not.
 
The bug is reproduced is several GDB versions and distributions.
> GNU gdb (GDB) 7.4.1-debian          installed using apt-get
> GNU gdb (GDB) 7.9.1                 downloaded from gnu.org/software/gdb/download/
> GNU gdb (GDB) 7.10.50.20150818-cvs  downloaded from gnu.org/software/gdb/download/
> GNU gdb (Gentoo 7.5.1 p2) 7.5.1     installed using portage
 
Attached is sample code with several transcripts showing the problem.
Comment 1 Sourceware Commits 2024-03-20 17:02:54 UTC 
The master branch has been updated by Hannes Domani <ssbssa@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=23cdd9431ad424b092c65419d47ef4601168a1c9

commit 23cdd9431ad424b092c65419d47ef4601168a1c9
Author: Hannes Domani <ssbssa@yahoo.de>
Date:   Wed Mar 20 18:02:06 2024 +0100

    Fix reinterpret_cast for classes with multiple inheritance
    
    Currently a reinterpret_cast may change the pointer value if
    multiple inheritance is involved:
    ```
    (gdb) p r
    $1 = (Right *) 0x22f75c
    (gdb) p reinterpret_cast<LeftRight*>(r)
    $2 = (LeftRight *) 0x22f758
    ```
    
    It's because value_cast is called in this case, which automatically
    does up- and downcasting.
    
    Fixed by simply using the target pointer type in a copy of the
    original value:
    ```
    (gdb) p r
    $1 = (Right *) 0x3bf87c
    (gdb) p reinterpret_cast<LeftRight*>(r)
    $2 = (LeftRight *) 0x3bf87c
    ```
    
    Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=18861
    Approved-By: Tom Tromey <tom@tromey.com>
Comment 2 Hannes Domani 2024-03-20 17:04:02 UTC 
Fixed.