On 5/15/2017 9:55 PM, Ben Peart wrote:
On 5/15/2017 8:34 PM, Jeff King wrote:
On Tue, May 16, 2017 at 12:22:14AM +0000, brian m. carlson wrote:
On Mon, May 15, 2017 at 03:13:44PM -0400, Ben Peart wrote:
+ istate->last_update = (time_t)ntohll(*(uint64_t *)index);
+ index += sizeof(uint64_t);
+
+ ewah_size = ntohl(*(uint32_t *)index);
+ index += sizeof(uint32_t);
To answer the question you asked in your cover letter, you cannot write
this unless you can guarantee (((uintptr_t)index & 7) == 0) is true.
Otherwise, this will produce a SIGBUS on SPARC, Alpha, MIPS, and some
ARM systems, and it will perform poorly on PowerPC and other ARM
systems[0].
If you got that pointer from malloc and have only indexed multiples of 8
on it, you're good. But if you're not sure, you probably want to use
memcpy. If the compiler can determine that it's not necessary, it will
omit the copy and perform a direct load.
I think get_be32() does exactly what we want for the ewah_size read. For
the last_update one, we don't have a get_be64() yet, but it should be
easy to make based on the 16/32 versions.
Thanks for the pointers. I'll update this to use the existing get_be32
and have created a get_be64 and will use that for the last_update.
OK, now I'm confused as to the best path for adding a get_be64. This
one is trivial:
#define get_be64(p) ntohll(*(uint64_t *)(p))
but should the unaligned version be:
#define get_be64(p) ( \
(*((unsigned char *)(p) + 0) << 56) | \
(*((unsigned char *)(p) + 1) << 48) | \
(*((unsigned char *)(p) + 2) << 40) | \
(*((unsigned char *)(p) + 3) << 32) | \
(*((unsigned char *)(p) + 4) << 24) | \
(*((unsigned char *)(p) + 5) << 16) | \
(*((unsigned char *)(p) + 6) << 8) | \
(*((unsigned char *)(p) + 7) << 0) )
or would it be better to do it like this:
#define get_be64(p) ( \
((uint64_t)get_be32((unsigned char *)(p) + 0) << 32) | \
((uint64_t)get_be32((unsigned char *)(p) + 4) << 0)
or with a static inline function like git_bswap64:
or something else entirely?
I'm not sure why the different styles in this one file and which I
should be emulating.
(I note also that time_t is not necessarily 64-bits in the first place,
but David said something about this not really being a time_t).
The in memory representation is a time_t as that is the return value of
time(NULL) but it is converted to/from a 64 bit value when written/read
to the index extension so that the index format is the same no matter
the native size of time_t.
-Peff