Subject: Re: struct hashmap_entry packing

From: Jeff King <[email-masked]>
Date: Fri, 1 Aug 2014 18:37:39 -0400

On Tue, Jul 29, 2014 at 10:40:12PM +0200, Karsten Blees wrote:

> > The sizeof() has to be the same regardless of whether the hashmap_entry
> > is standalone or in another struct, and therefore must be padded up to
> > 16 bytes. If we stored "x" in that padding in the combined struct, it
> > would be overwritten by our memset.
> > 
> 
> The struct-packing patch was ultimately dropped because there was no way
> to reliably make it work on all platforms. See [1] for discussion, [2] for
> the final, 'most compatible' version.

Thanks for the pointers; I should have guessed there was more to it and
searched the archive myself.

> Hmmm. Now that we have "__attribute__((packed))" in pack-bitmap.h, perhaps
> we should do the same for stuct hashmap_entry? (Which was the original
> proposal anyway...). Only works for GCC, but that should cover most builds
> / platforms.

I don't see any reason to avoid the packed attribute, if it helps us. As
you noted, anything using __attribute__ probably supports it, and if
not, we can conditionally #define PACKED_STRUCT or something, like we do
for NORETURN. Since it's purely an optimization, if another compiler
doesn't use it, no big deal.

That being said, I don't know if those padding bytes are actually
causing a measurable slowdown. It may not even be worth the trouble.

> Btw.: Using struct-packing on 'struct bitmap_disk_entry' means that the
> binary format of .bitmap files is incompatible between GCC and other
> builds, correct?

The on-disk format is defined by JGit; if there are differences between
the builds, that's a bug (and I would not be too surprised if there is
one, as bitmaps have gotten very extensive testing on 32- and 64-bit
gcc, but probably not much elsewhere).

We do use structs to represent disk structures in other bits of the
packfile code (e.g., struct pack_idx_header), but the struct is vanilla
enough that we assume every compiler gives us two tightly-packed 32-bit
integers without having to bother with the "packed" attribute (and it
seems to have worked in practice).

We should probably be more careful with that bitmap code. It looks like
it wouldn't be too bad to drop it. I'll see if I can come up with a
patch.

-Peff