Re: What's cooking in git.git (Dec 2013, #02; Fri, 6)

From: Jonathan Nieder <jrnieder@gmail.com>
Date: Dec 9, 2013, 23:45 UTC
Message-ID: <20131209234500.GY29959@google.com>
In-Reply-To: <52A64FE9.5010004@gmail.com>

Karsten Blees wrote:

> (Besides, __attribute__((aligned)) / __declspec(align) can only
> _increase_ the alignment, so aligned(1) would have no effect).

Good catch.

Show 15 quoted lines

> 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.

Technically this can be done using macros:

 #if (gcc)
 #	define BEGIN_PACKED _Pragma("pack(push,4)")
 #	define END_PACKED _Pragma("pack(pop)")
 #elif (msvc)
 #	define BEGIN_PACKED __pragma(pack(push,4))
 #	define END_PACKED __pragma(pack(pop))
 #else
 	/* Just tolerate a little padding. */
 #	define BEGIN_PACKED
 #	define END_PACKED
 #end

Then you can do:

 BEGIN_PACKED
 struct hashmap_entry {
 	...
 };
 END_PACKED

Whether that's nicer or uglier than the alternatives I leave to you. ;-)

Thanks, Jonathan