c++/8054: Object layout: excess padding bytes

c++/8054: Object layout: excess padding bytes

[grigory]

>Number:         8054
>Category:       c++
>Synopsis:       Object layout: excess padding bytes
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Sep 26 03:26:07 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Grigory Zagorodnev
>Release:        3.2
>Organization:
>Environment:
IA-32 Red Hat Linux
IA-64 Red Hat Linux

Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --disable-checking --host=i386-redhat-linux --with-system-zlib --enable-__cxa_atexit
Thread model: posix
gcc version 3.2 (Red Hat Linux Rawhide 3.2-1)
>Description:
For the given class (most_derived) G++ compiler gives wrong data layout.

Test-case
---------
	struct base {
	    short m;
	    base() { m = 0x1111; };
	};

	struct derived: virtual base {
	};

	struct most_derived: derived {
	    short m;
	    most_derived() { m = 0x2222; };
	};

        #include <stdio.h>
	int main() // Run-time checking
	{
		most_derived obj;
		if( ((size_t) &(((base *)&obj)->m) -  (size_t) &obj) != sizeof(void *) + sizeof(obj.m) ){
			puts("Failed");
			return 1;
		}
		puts("Passed");
		return 0;
	}

Layout
------
According to the C++ ABI, the generalized data layout of class 'most_derived' should be:
	[0] Virtual table entry pointer
	[x] most_derived::m
	[y] base::m
Where offset 'x' is equal to size of pointer, and 'y' = x + sizeof(most_derived::m).

For ia32 architecture, the expected layout is:
	[0] Virtual table entry pointer
	[4] most_derived::m
	[6] base::m	

However, G++ compiler gives different layout:
	[0] Virtual table entry pointer
	[4] most_derived::m
	[8] base::m	
I.e. g++ emits extra padding bytes after most_derived::m, thus breaking the ABI rules.

There is a class layout obtained by "g++ -fdump-class-hierarchy ...":
	Class most_derived
	   size=12 align=4
	most_derived (0x402222c0) 0
	    vptridx=0 vptr=((&most_derived::_ZTV12most_derived) + 12)
	  derived (0x40222300) 0 nearly-empty
	      primary-for most_derived (0x402222c0)
	      subvttidx=4
	    base (0x40222340) 8 virtual canonical
	        vbaseoffset=-12
>How-To-Repeat:
You can run listed test-case for run-time validation.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: