Peter Lieven <p...@kamp.de> 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 <p...@kamp.de>
> ---
> 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;
size_t i;
>From the "I hate casts" film?
Thanks, Juan.
