From: Yan Liu <yanliu@ca.ibm.com>
To: gcc@gnu.org
Subject: question on class layout with virtual base classes
Date: Tue, 31 Aug 2004 19:55:00 -0000 [thread overview]
Message-ID: <OF9D7C1C0F.E69DCD53-ON85256F01.0067264F-85256F01.006AE804@ca.ibm.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 2582 bytes --]
I found that g++ 3.3, 3.4 and 3.5 have different class layout for the
following testcase:
struct A {char a;};
struct B: virtual public A {};
struct C:public B{
int c;
virtual void f() {};
};
struct D {
char d;
};
struct E {int:3;};
struct F: public C, virtual E, public D {
char f1;
virtual void f(){}
};
int main()
{F f;}
g++ 3.3's information is:
Configured with: ../configure --enable-threads=posix --prefix=/usr
--with-local-prefix=/usr/local --infodir=/usr/share/info
--mandir=/usr/share/man --enable-languages=c,c++,f77,objc,java,ada
--disable-checking --libdir=/usr/lib --enable-libgcj
--with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib
--with-system-zlib --enable-shared --enable-__cxa_atexit
--host=powerpc-suse-linux --build=powerpc-suse-linux
--target=powerpc-suse-linux --enable-targets=powerpc64-suse-linux
--enable-biarch
Thread model: posix
gcc version 3.3.3 (SuSE Linux)
g++3.4's information is:
Configured with: ../gcc/configure
--prefix=/.../torolab.ibm.com/fs/projects/vabld/run/gcc/linux_ppc/3.4-dev
--enable-languages=c,c++,f77,objc --enable-altivec --enable-shared
powerpc-suse-linux
Thread model: posix
gcc version 3.4.0 20040212 (prerelease)
g++ 3.5's information is:
Configured with: /private/var/tmp/gcc/gcc-1765.obj~1/src/configure
--disable-checking --prefix=/usr --disable-libmudflap --mandir=/share/man
--enable-languages=c,objc,c++ --disable-libada
--program-transform-name=/^[cg][^+.-]*$/s/$/-3.5/
--with-gxx-include-dir=/include/gcc/darwin/3.5/c++
--build=powerpc-apple-darwin --host=powerpc-apple-darwin
--target=powerpc-apple-darwin
Thread model: posix
gcc version 3.5.0-tree-ssa 20040321 (merged 20040529) (Apple Computer, Inc.
build 1765)
Attached files are the class and vft layout results with three g++
compilers using -fdump-class-hierarchy option.
(See attached file: temp4.g++35.class)(See attached file:
temp4.g++33.class)(See attached file: temp4.C.g++34_32.class)
For the virtual base class A in the class F's layout, I thought tail
padding rule should only apply to non-virtual base classes. Before mapping
the virtual base class A, the current class size should be increased to the
mutiplication of alignment ( which is 4 at this moment), and A should be
put at offset 12 rather at offset 10.
I understant that C++ ABI does not explicitly specify the tail padding
rule. I would like to take this opportunity to understand the tail padding
rules and empty base class mapping strategies you are using.
Your kind help is highly appreciated. Thanks.
Yan Liu
email: yanliu@ca.ibm.com
[-- Attachment #2: temp4.g++35.class --]
[-- Type: application/octet-stream, Size: 2200 bytes --]
Class A
size=1 align=1
base size=1 base align=1
A (0x41032040) 0
Vtable for B
B::_ZTV1B: 3u entries
0 4u
4 0u
8 (int (*)(...))(&_ZTI1B)
VTT for B
B::_ZTT1B: 1u entries
0 (const void*)((void*)((&B::_ZTV1B) + 12u))
Class B
size=8 align=4
base size=4 base align=4
B (0x41032180) 0 nearly-empty
vptridx=0u vptr=((&B::_ZTV1B) + 12u)
A (0x410321c0) 4 virtual
vbaseoffset=-0x0000000000000000c
Vtable for C
C::_ZTV1C: 4u entries
0 8u
4 0u
8 (int (*)(...))(&_ZTI1C)
12 C::f
Construction vtable for B (0x41032500 instance) in C
C::_ZTC1C0_1B: 3u entries
0 8u
4 0u
8 (int (*)(...))(&_ZTI1B)
VTT for C
C::_ZTT1C: 2u entries
0 (const void*)((void*)((&C::_ZTV1C) + 12u))
4 (const void*)((void*)((&C::_ZTC1C0_1B) + 12u))
Class C
size=12 align=4
base size=8 base align=4
C (0x410324c0) 0
vptridx=0u vptr=((&C::_ZTV1C) + 12u)
B (0x41032500) 0 nearly-empty
primary-for C (0x410324c0)
subvttidx=4u
A (0x41032540) 8 virtual
vbaseoffset=-0x0000000000000000c
Class D
size=1 align=1
base size=1 base align=1
D (0x41032880) 0
Class E
size=4 align=4
base size=4 base align=4
E (0x41032980) 0
Vtable for F
F::_ZTV1F: 5u entries
0 12u
4 10u
8 0u
12 (int (*)(...))(&_ZTI1F)
16 F::f
Construction vtable for C (0x41032a80 instance) in F
F::_ZTC1F0_1C: 4u entries
0 10u
4 0u
8 (int (*)(...))(&_ZTI1C)
12 C::f
Construction vtable for B (0x41032ac0 instance) in F
F::_ZTC1F0_1B: 3u entries
0 10u
4 0u
8 (int (*)(...))(&_ZTI1B)
VTT for F
F::_ZTT1F: 3u entries
0 (const void*)((void*)((&F::_ZTV1F) + 16u))
4 (const void*)((void*)((&F::_ZTC1F0_1C) + 12u))
8 (const void*)((void*)((&F::_ZTC1F0_1B) + 12u))
Class F
size=16 align=4
base size=10 base align=4
F (0x41032a40) 0
vptridx=0u vptr=((&F::_ZTV1F) + 16u)
C (0x41032a80) 0
primary-for F (0x41032a40)
subvttidx=4u
B (0x41032ac0) 0 nearly-empty
primary-for C (0x41032a80)
subvttidx=8u
A (0x41032b00) 10 virtual
vbaseoffset=-0x0000000000000000c
E (0x41032b40) 12 virtual
vbaseoffset=-0x00000000000000010
D (0x41032b80) 8
[-- Attachment #3: temp4.g++33.class --]
[-- Type: application/octet-stream, Size: 1744 bytes --]
Class A
size=1 align=1
A (0x40673cc0) 0
Vtable for B
B::_ZTV1B: 3 entries
0 4
4 0
8 &_ZTI1B
VTT for B
B::_ZTT1B: 1 entries
0 ((&B::_ZTV1B) + 12)
Class B
size=8 align=4
B (0x40673e40) 0 nearly-empty
vptridx=0 vptr=((&B::_ZTV1B) + 12)
A (0x40673e80) 4 virtual canonical
vbaseoffset=-12
Vtable for C
C::_ZTV1C: 4 entries
0 8
4 0
8 &_ZTI1C
12 C::f()
Construction vtable for B (0x40675380 instance) in C
C::_ZTC1C0_1B: 3 entries
0 8
4 0
8 &_ZTI1B
VTT for C
C::_ZTT1C: 2 entries
0 ((&C::_ZTV1C) + 12)
4 ((&C::_ZTC1C0_1B) + 12)
Class C
size=12 align=4
C (0x40675340) 0
vptridx=0 vptr=((&C::_ZTV1C) + 12)
B (0x40675380) 0 nearly-empty
primary-for C (0x40675340)
subvttidx=4
A (0x406753c0) 8 virtual canonical
vbaseoffset=-12
Class D
size=1 align=1
D (0x40675940) 0
Class E
size=4 align=4
E (0x40675a80) 0
Vtable for F
F::_ZTV1F: 5 entries
0 16
4 12
8 0
12 &_ZTI1F
16 F::f()
Construction vtable for C (0x40675bc0 instance) in F
F::_ZTC1F0_1C: 4 entries
0 12
4 0
8 &_ZTI1C
12 C::f()
Construction vtable for B (0x40675c00 instance) in F
F::_ZTC1F0_1B: 3 entries
0 12
4 0
8 &_ZTI1B
VTT for F
F::_ZTT1F: 3 entries
0 ((&F::_ZTV1F) + 16)
4 ((&F::_ZTC1F0_1C) + 12)
8 ((&F::_ZTC1F0_1B) + 12)
Class F
size=20 align=4
F (0x40675b80) 0
vptridx=0 vptr=((&F::_ZTV1F) + 16)
C (0x40675bc0) 0
primary-for F (0x40675b80)
subvttidx=4
B (0x40675c00) 0 nearly-empty
primary-for C (0x40675bc0)
subvttidx=8
A (0x40675c40) 12 virtual canonical
vbaseoffset=-12
E (0x40675c80) 16 virtual canonical
vbaseoffset=-16
D (0x40675cc0) 8
[-- Attachment #4: temp4.C.g++34_32.class --]
[-- Type: application/octet-stream, Size: 2200 bytes --]
Class A
size=1 align=1
base size=1 base align=1
A (0x4013ff80) 0
Vtable for B
B::_ZTV1B: 3u entries
0 4u
4 0u
8 (int (*)(...))(&_ZTI1B)
VTT for B
B::_ZTT1B: 1u entries
0 (const void*)((void*)((&B::_ZTV1B) + 12u))
Class B
size=8 align=4
base size=4 base align=4
B (0x40140280) 0 nearly-empty
vptridx=0u vptr=((&B::_ZTV1B) + 12u)
A (0x401402c0) 4 virtual
vbaseoffset=-0x0000000000000000c
Vtable for C
C::_ZTV1C: 4u entries
0 8u
4 0u
8 (int (*)(...))(&_ZTI1C)
12 C::f
Construction vtable for B (0x401407c0 instance) in C
C::_ZTC1C0_1B: 3u entries
0 8u
4 0u
8 (int (*)(...))(&_ZTI1B)
VTT for C
C::_ZTT1C: 2u entries
0 (const void*)((void*)((&C::_ZTV1C) + 12u))
4 (const void*)((void*)((&C::_ZTC1C0_1B) + 12u))
Class C
size=12 align=4
base size=8 base align=4
C (0x40140780) 0
vptridx=0u vptr=((&C::_ZTV1C) + 12u)
B (0x401407c0) 0 nearly-empty
primary-for C (0x40140780)
subvttidx=4u
A (0x40140800) 8 virtual
vbaseoffset=-0x0000000000000000c
Class D
size=1 align=1
base size=1 base align=1
D (0x40140d40) 0
Class E
size=1 align=1
base size=1 base align=1
E (0x4014b000) 0
Vtable for F
F::_ZTV1F: 5u entries
0 11u
4 10u
8 0u
12 (int (*)(...))(&_ZTI1F)
16 F::f
Construction vtable for C (0x4014b2c0 instance) in F
F::_ZTC1F0_1C: 4u entries
0 10u
4 0u
8 (int (*)(...))(&_ZTI1C)
12 C::f
Construction vtable for B (0x4014b300 instance) in F
F::_ZTC1F0_1B: 3u entries
0 10u
4 0u
8 (int (*)(...))(&_ZTI1B)
VTT for F
F::_ZTT1F: 3u entries
0 (const void*)((void*)((&F::_ZTV1F) + 16u))
4 (const void*)((void*)((&F::_ZTC1F0_1C) + 12u))
8 (const void*)((void*)((&F::_ZTC1F0_1B) + 12u))
Class F
size=12 align=4
base size=10 base align=4
F (0x4014b280) 0
vptridx=0u vptr=((&F::_ZTV1F) + 16u)
C (0x4014b2c0) 0
primary-for F (0x4014b280)
subvttidx=4u
B (0x4014b300) 0 nearly-empty
primary-for C (0x4014b2c0)
subvttidx=8u
A (0x4014b340) 10 virtual
vbaseoffset=-0x0000000000000000c
E (0x4014b380) 11 virtual
vbaseoffset=-0x00000000000000010
D (0x4014b3c0) 8
next reply other threads:[~2004-08-31 19:28 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-08-31 19:55 Yan Liu [this message]
2004-09-01 15:27 ` Giovanni Bajo
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