Re: [Qemu-devel] [PATCHv4 2/9] cutils: add a function to find non-zero content in a buffer

[Peter Lieven <pl@kamp.de>]

Am 25.03.2013 um 09:53 schrieb Orit Wasserman <owasserm@redhat.com>:

> On 03/22/2013 02:46 PM, Peter Lieven wrote:
>> this adds buffer_find_nonzero_offset() which is a SSE2/Altivec
>> optimized function that searches for non-zero content in a
>> buffer.
>> 
>> 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 <pl@kamp.de>
>> ---
>> include/qemu-common.h |   13 +++++++++++++
>> util/cutils.c         |   45 +++++++++++++++++++++++++++++++++++++++++++++
>> 2 files changed, 58 insertions(+)
>> 
>> diff --git a/include/qemu-common.h b/include/qemu-common.h
>> index e76ade3..078e535 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..41c627e 100644
>> --- a/util/cutils.c
>> +++ b/util/cutils.c
>> @@ -143,6 +143,51 @@ 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 BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * sizeof(VECTYPE).
>> + * 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;
>> +    size_t i;
>> +
>> +    assert(len % (BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR
>> +        * sizeof(VECTYPE)) == 0);
>> +    assert(((uintptr_t) buf) % sizeof(VECTYPE) == 0);
>> +
>> +    if (*((const long *) buf)) {
>> +        return 0;
>> +    }
>> +
>> +    for (i = 0; i < len / sizeof(VECTYPE);
> Why not put len/sizeof(VECTYPE) in a variable?

are you afraid that there is a division at each iteration?

sizeof(VECTYPE) is a power of 2 so i think the compiler will optimize it
to a >> at compile time.

I would also be ok with writing len /= sizeof(VECTYPE) before the loop.

Peter

> Orit
>> +            i += BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) {
>> +        VECTYPE tmp0 = p[i + 0] | p[i + 1];
>> +        VECTYPE tmp1 = p[i + 2] | p[i + 3];
>> +        VECTYPE tmp2 = p[i + 4] | p[i + 5];
>> +        VECTYPE tmp3 = p[i + 6] | p[i + 7];
>> +        VECTYPE tmp01 = tmp0 | tmp1;
>> +        VECTYPE tmp23 = tmp2 | tmp3;
>> +        if (!ALL_EQ(tmp01 | tmp23, zero)) {
>> +            break;
>> +        }
>> +    }
>> +    return i * sizeof(VECTYPE);
>> +}
>> +
>> +/*
>>  * Checks if a buffer is all zeroes
>>  *
>>  * Attention! The len must be a multiple of 4 * sizeof(long) due to
>> 
>