Re: [RFC][Update][Patch 12/16]Fix undefined ">> 32" in revoke code

H. Peter Anvin
2006-06-30T03:15:56Z

Mingming Cao wrote:
> "val >> 32" is undefined if val is a 32-bit value, so this code is
> broken if CONFIG_LBD is not set. Make it safe for that case.
>
> Signed-off-by: Stephen Tweedie <sct@redhat.com>
> Signed-off-by: Mingming Cao <cmm@us.ibm.com>
>
>
> ---
>
> linux-2.6.17-ming/fs/jbd/revoke.c | 2 +-
> 1 files changed, 1 insertion(+), 1 deletion(-)
>
> diff -puN fs/jbd/revoke.c~jbd-revoke-32bit-shift-fix fs/jbd/revoke.c
> --- linux-2.6.17/fs/jbd/revoke.c~jbd-revoke-32bit-shift-fix 2006-06-28 16:47:09.695458913 -0700
> +++ linux-2.6.17-ming/fs/jbd/revoke.c 2006-06-28 16:47:09.699458454 -0700
> @@ -110,7 +110,7 @@ static inline int hash(journal_t *journa
> {
> struct jbd_revoke_table_s *table = journal->j_revoke;
> int hash_shift = table->hash_shift;
> - int hash = (int)block ^ (int)(block >> 32);
> + int hash = (int)block ^ (int)((block >> 31) >> 1);
>

It might be better to code it as:

(int)((u64)block >> 32)

... which gcc can trivially recognize as 0 if block is 32 bits. Not
sure if it can do that with the code above.

-hpa