Re: [Qemu-devel] [PATCHv5 03/10] cutils: add a function to find non-zero content in a buffer

[Peter Lieven]

Am 26.03.2013 um 11:38 schrieb Juan Quintela <address@hidden>:

> Peter Lieven <address@hidden> wrote:
>> this adds buffer_find_nonzero_offset() which is a SSE2/Altivec
>> optimized function that searches for non-zero content in a
>> buffer.
>> 
>> the function starts full unrolling only after the first few chunks have
>> been checked one by one. analyzing real memory page data has revealed
>> that non-zero pages are non-zero within the first 256-512 bits in
>> most cases. as this function is also heavily used to check for zero memory
>> pages this tweak has been made to avoid the high setup costs of the fully
>> unrolled check for non-zero pages.
>> 
>> due to the optimizations used in the function there are restrictions
>> on buffer address and search length. the function
>> can_use_buffer_find_nonzero_content() can be used to check if
>> the function can be used safely.
>> 
>> Signed-off-by: Peter Lieven <address@hidden>
>> ---
>> include/qemu-common.h |   13 ++++++++++++
>> util/cutils.c         |   55 
>> +++++++++++++++++++++++++++++++++++++++++++++++++
>> 2 files changed, 68 insertions(+)
>> 
>> diff --git a/include/qemu-common.h b/include/qemu-common.h
>> index 9022646..7c7c244 100644
>> --- a/include/qemu-common.h
>> +++ b/include/qemu-common.h
>> @@ -472,4 +472,17 @@ void hexdump(const char *buf, FILE *fp, const char 
>> *prefix, size_t size);
>> #define ALL_EQ(v1, v2) ((v1) == (v2))
>> #endif
>> 
>> +#define BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR 8
>> +static inline bool
>> +can_use_buffer_find_nonzero_offset(const void *buf, size_t len)
>> +{
>> +    if (len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
>> +               * sizeof(VECTYPE)) == 0
>> +        && ((uintptr_t) buf) % sizeof(VECTYPE) == 0) {
>> +        return true;
>> +    }
>> +    return false;
>> +}
>> +size_t buffer_find_nonzero_offset(const void *buf, size_t len);
>> +
>> #endif
>> diff --git a/util/cutils.c b/util/cutils.c
>> index 1439da4..0314a18 100644
>> --- a/util/cutils.c
>> +++ b/util/cutils.c
>> @@ -143,6 +143,61 @@ int qemu_fdatasync(int fd)
>> }
>> 
>> /*
>> + * Searches for an area with non-zero content in a buffer
>> + *
>> + * Attention! The len must be a multiple of
>> + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE)
>> + * and addr must be a multiple of sizeof(VECTYPE) due to
>> + * restriction of optimizations in this function.
>> + *
>> + * can_use_buffer_find_nonzero_offset() can be used to check
>> + * these requirements.
>> + *
>> + * The return value is the offset of the non-zero area rounded
>> + * down to a multiple of sizeof(VECTYPE) for the first
>> + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR chunks and down to
>> + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE)
>> + * afterwards.
>> + *
>> + * If the buffer is all zero the return value is equal to len.
>> + */
>> +
>> +size_t buffer_find_nonzero_offset(const void *buf, size_t len)
>> +{
>> +    VECTYPE *p = (VECTYPE *)buf;
>> +    VECTYPE zero = ZERO_SPLAT;
> 
> If you have to resplit it anyways, what about changing this to:
> 
> -    VECTYPE *p = (VECTYPE *)buf;
> -    VECTYPE zero = ZERO_SPLAT;
> +    const VECTYPE *p = buf;
> +    const VECTYPE zero = ZERO_SPLAT;

i would change it to 

const VECTYPE zero = (VECTYPE) {0};

and drop patch 2 again.

Peter


>     size_t i;
> 
> From the "I hate casts" film?
> 
> Thanks, Juan.