Am 09.12.2013 21:08, schrieb Jonathan Nieder:
> Karsten Blees wrote:
>
>> GCC supports __packed__ as of 2.3 (1992), so any other compilers
>> that copied the __attribute__ feature probably won't complain.
>
> Alas, it looks like HP C doesn't support __packed__ (not that I
> care much about HP C):
>
>
> http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/Online_Help/pragmas.htm#Attributes
>
Thanks for the link
> Maybe a macro expanding to __attribute__((aligned(1))) on the fields
> would work? The same macro could expand to __declspec(align(1)) in
> the MSVC build.
>
But alignment is not the same as packing. We still want the structure to be
8-byte aligned (i.e. variables of the type should start at 8-byte boundaries).
We just don't want the size of the structure to be padded to a multiple of 8,
so that we can extend it without penalty. (Besides, __attribute__((aligned)) /
__declspec(align) can only _increase_ the alignment, so aligned(1) would have
no effect).
Googling some more, I believe the most protable way to achieve this via
'compiler settings' is
#pragma pack(push)
#pragma pack(4)
struct hashmap_entry {
struct hashmap_entry *next;
unsigned int hash;
};
#pragma pack(pop)
This is supported by at least GCC, MSVC and HP (see your link). The downside is
that we cannot use macros (in git-compat-util.h) to emit #pragmas. But we
wouldn't have to, as compilers aren't supposed to barf on unknown #pragmas.
However, considering the portability issues, the macro solution (injecting just
the two fields instead of a struct) becomes more and more attractive in my
mind...
Karsten
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