#275 pack-struct option is broken

[Anonymous]

$ i686-w64-mingw32-gcc --version
i686-w64-mingw32-gcc (GCC) 4.7.0 20120224 (Fedora MinGW 4.7.0-0.5.20120224.fc16_cross)

I am attaching test case that you can easily see the problem.

$ gcc -m32 test-ps.c
$ ./a.out
size: 12
$ gcc -fpack-struct -m32 test-ps.c
size: 6
$ i686-w64-mingw32-gcc test-ps.c
$ wine ./a-1.exe
size: 12
$ i686-w64-mingw32-gcc -fpack-struct test-ps.c
$ wine ./a.exe
size: 9

[Anonymous]

test case

[Kai Tietz]

The gcc 4.7 gcc for mingw has by default -mms-bitfields activated. Which means struct-layout has changed and alignment in struct.
You can force a specific layout by using attribute ms_struct or by attribute gcc_struct.

The difference between those two layouts is in bitfield packing and in alignment of those field members.

For example:

struct {
unsigned long long c : 1;
unsigned int a : 1;
unsigned int b : 1;
}

will be layout differently.

For gcc-layout all bits geting merged together within one bitfield. gcc struct makes no difference in actual type-sizes for bitfields.
For ms-layout c will end in a 8-byte field, and just a and b getting merged together as they have same type-size.

This is to be expected behavior. You might want to try here -mno-ms-bitfields.

[Kai Tietz]

Ah, I missed that default-alignment is treated now for ms_struct union/struct-s too.
You can solve the issue also by additing explicit #pragma pack(1) of __attribute__ ((pack(1))).

The cause why it gets aligned to 9 is that. Default alignment of fields is 4 on 32-bit.
So first char a[1]; is placed in 4-bytes, long b also, and the last element has 4 too.
By enabling struct-pack, All fields in structure getting merged.
So char a[1]; has one byte size, long b has 4 byte size, plus char c[1] has 1. But by structure-alignment of 4, structure gets expanded in the last field to 4.

I see that here alignment is one too big ... hmm, maybe an issue indeed.

Work-a-round use explicit #pragma pack(1), if you don't want structure alignment at end.

[Anonymous]

Adding -mno-ms-bitfields does fix the issue that I am having. I was expecting 6 and not 8 (or 9). I did not know about the defaults change. Should I report the 9 issue upstream (gcc)?

[Anonymous]

I was too quick to say it is fixed. There seem to be other byte alignment issues. I will continue investigating.

[Kai Tietz]

The general issue is that the __attribute__ ((__packed)) applies only to last field of struct. This is the cause for the size of 9. Trick is here '#pragma pack(1)'. By it you get expected sizes. The issue is that this field-alignment gets applied even for struct/union's marked to be packed (or via -fpack-struct option).

[Ozkan Sezer]

The remark of "__attribute__ ((__packed)) applies only to last field of struct" is curious: I never experienced that before in any gcc versions, and if that is a gcc-4.7 thing it is a devastatingly incompatible change. For the record, I just compiled Michael's testcase with gcc45 for win32 and it prints 6 when ran under wine. Same when compiled for x86-linux. Is your remark accurate Kai?

[Anonymous]

My last comment was a false alarm. Adding "-mno-ms-bitfields" solves all my issues. I use software that heavily relies on bitfields and packed structs (yes, it is very old).

I agree with sezero, even if I am an outside nobody. This is a drastic change in defaults.

Perhaps this "bug" needs to be changed to an "enhancement" to change the defaults back to pre-4.7 values to prevent further problems with other projects.

[Michael Zeilfelder]

I run into this as well and I don't think the change is correct. I get now _different_ results from compiling with Microsoft Compiler and packing when using -m-ms-bitfields while I get _identical_ results when compiling with -mno-ms-bitfields. And when __attribute__ ((__packed)) applies only to last field of struct then m[no]-ms-bitfields shouldn't really make a difference in my case:

#include <cstdio>

#if defined(_MSC_VER)
#pragma pack( push, packing )
#pragma pack( 1 )
#endif

struct Header
{
unsigned char IdLength;
unsigned char ColorMapType;
unsigned char ImageType;
unsigned char FirstEntryIndex[2];
unsigned short ColorMapLength;
unsigned char ColorMapEntrySize;
unsigned char XOrigin[2];
unsigned char YOrigin[2];
unsigned short ImageWidth;
}
#if defined(__GNUC__)
__attribute__((packed))
#endif
;

#if defined(_MSC_VER)
#pragma pack( pop, packing )
#endif

int main ()
{
printf("sizeof(Header): %d\n", sizeof(Header));
return 0;
}

But my results for size are:
With VS 2010 (32 bit as well as 64-bit): 14
Compiled with gcc 4.7 and -mno-ms-bitfields: 14
gcc 4.7 and new default (or with -mms-bitfields explicitly): 16

So gcc 4.7 is now incompatible with MS compiler as well as with gcc 4.6 as far as I can see.