Size difference in base class between GCC releases
I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. paul
Re: Size difference in base class between GCC releases
On 27 August 2012 19:48, Paul_Koningwrote: I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. This depends on whether the base class is a POD or not. According to a note in the Itanium C++ ABI the C++ standard requires that compilers not overlay the tail padding in a POD (I don't know off the top of my head where that is stated in the standard.) Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. GCC 3.4 and later conform to the Itanium C++ ABI, which specifies the behaviour you're seeing as required by the C++ standard, so 4.5 is correct.
Re: Size difference in base class between GCC releases
On Aug 27, 2012, at 3:33 PM, Jonathan Wakely wrote: On 27 August 2012 19:48, Paul_Koningwrote: I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. This depends on whether the base class is a POD or not. According to a note in the Itanium C++ ABI the C++ standard requires that compilers not overlay the tail padding in a POD (I don't know off the top of my head where that is stated in the standard.) Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. GCC 3.4 and later conform to the Itanium C++ ABI, which specifies the behaviour you're seeing as required by the C++ standard, so 4.5 is correct. Interesting. What if the base class is not a POD? It doesn't seem to be, if I remember the definition of POD correctly. paul
Re: Size difference in base class between GCC releases
On Mon, Aug 27, 2012 at 1:48 PM, paul_kon...@dell.com wrote: I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. paul Is this message http://gcc.gnu.org/ml/gcc/2002-08/msg00874.html relevant to your case? -- Gaby
Re: Size difference in base class between GCC releases
On Aug 27, 2012, at 4:05 PM, Gabriel Dos Reis wrote: On Mon, Aug 27, 2012 at 1:48 PM, paul_kon...@dell.com wrote: I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. paul Is this message http://gcc.gnu.org/ml/gcc/2002-08/msg00874.html relevant to your case? -- Gaby Yes, that looks like the exact case. And the mail thread seems to say that the 3.3.3 behavior I'm seeing is what G++ was doing at that time, as was HP -- but not Intel. So now we have it done differently in later compilers. I know this is changing data structure layouts in our code; I don't know yet if that is a problem (i.e., if it applies to layouts used in persistent data or in protocol messages). I assume there isn't some compiler switch I can use to control this behavior? paul
Re: Size difference in base class between GCC releases
On Mon, Aug 27, 2012 at 3:16 PM, paul_kon...@dell.com wrote: On Aug 27, 2012, at 4:05 PM, Gabriel Dos Reis wrote: On Mon, Aug 27, 2012 at 1:48 PM, paul_kon...@dell.com wrote: I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. paul Is this message http://gcc.gnu.org/ml/gcc/2002-08/msg00874.html relevant to your case? -- Gaby Yes, that looks like the exact case. And the mail thread seems to say that the 3.3.3 behavior I'm seeing is what G++ was doing at that time, as was HP -- but not Intel. So now we have it done differently in later compilers. Yes. I know this is changing data structure layouts in our code; I don't know yet if that is a problem (i.e., if it applies to layouts used in persistent data or in protocol messages). I assume there isn't some compiler switch I can use to control this behavior? Normally, any changes like this is controlled by -fabi-version; you can also get warnings with -Wabi. See the discussion at http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Dialect-Options.html#index-Wabi-168 if it is of any help. -- Gaby
Re: Size difference in base class between GCC releases
On 27 August 2012 20:49 Paul Koning wrote: On Aug 27, 2012, at 3:33 PM, Jonathan Wakely wrote: On 27 August 2012 19:48, Paul_Koningwrote: I'm doing some checking of data structure layouts in different releases of our code -- which were produced by different releases of GCC (3.3.3 vs. 4.5.4). One difference I'm seeing that is puzzling is in the handling of base classes. Specifically, the case where a base class has padding at the end to fill it out to a multiple of the alignment. In GCC 3.3.3, when such a class is used as a base class, that padding is omitted, and the first derived class data member starts right after the last base class real (not pad) data member. In GCC 4.5.4, the base class is used padding and all, the first derived class data member starts after the padding of the base class. This depends on whether the base class is a POD or not. According to a note in the Itanium C++ ABI the C++ standard requires that compilers not overlay the tail padding in a POD (I don't know off the top of my head where that is stated in the standard.) Which is correct? Or are both correct? This sort of thing is a potential cause of trouble if such a class is used as a container for persistent data. GCC 3.4 and later conform to the Itanium C++ ABI, which specifies the behaviour you're seeing as required by the C++ standard, so 4.5 is correct. Interesting. What if the base class is not a POD? It doesn't seem to be, if I remember the definition of POD correctly. G++ 3.4 and later can, and will, reuse the tail padding in a non-POD.
Re: Size difference in base class between GCC releases
On 27 August 2012 21:16, Paul Koning wrote:
On Aug 27, 2012, at 4:05 PM, Gabriel Dos Reis wrote:
Is this message
http://gcc.gnu.org/ml/gcc/2002-08/msg00874.html
relevant to your case?
-- Gaby
Yes, that looks like the exact case. And the mail thread seems to say that
the 3.3.3 behavior I'm seeing is what G++ was doing at that time, as was HP
-- but not Intel. So now we have it done differently in later compilers.
That mail is talking about reusing tail padding in non-PODs, and G++
still does that, i.e. this code compiles:
struct S1 {
virtual void f();
int i;
char c1;
};
struct S2 : public S1 {
char c2;
};
const S2 s2{};
static_assert( (s2.c2 - s2.c1) == 1, Reused tail padding );
Please check whether the code you're looking at involves a POD base
class, because that would explain why G++ 4.5 doesn't reuse the tail
padding (I have no idea if 3.3 does or doesn't, but using
-fabi-version=1 to request the G++ 3.2 ABI doesn't seem to cause tail
padding in PODs to be reused.)
