[Qemu-devel] [RFC][PATCH 2/9] add a function to find non-zero content in a buffer

[Qemu-devel] [RFC][PATCH 2/9] add a function to find non-zero content in a buffer

[Peter Lieven]

Signed-off-by: Peter Lieven <pl@kamp.de>
---
  include/qemu-common.h |    1 +
  util/cutils.c         |   30 ++++++++++++++++++++++++++++++
  2 files changed, 31 insertions(+)

diff --git a/include/qemu-common.h b/include/qemu-common.h
index a3555d0..c22c41e 100644
--- a/include/qemu-common.h
+++ b/include/qemu-common.h
@@ -362,6 +362,7 @@ size_t qemu_iovec_from_buf(QEMUIOVector *qiov, size_t offset,
  size_t qemu_iovec_memset(QEMUIOVector *qiov, size_t offset,
                           int fillc, size_t bytes);

+size_t buffer_find_nonzero_offset(const void *buf, size_t len);
  bool buffer_is_zero(const void *buf, size_t len);

  void qemu_progress_init(int enabled, float min_skip);
diff --git a/util/cutils.c b/util/cutils.c
index 1439da4..a09d8e8 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -143,6 +143,36 @@ int qemu_fdatasync(int fd)
  }

  /*
+ * Searches for an area with non-zero content in a buffer
+ *
+ * Attention! The len must be a multiple of 8 * sizeof(VECTYPE)
+ * and addr must bedue to restriction of optimizations in this function.
+ *
+ * The return value is the offset of the non-zero area rounded
+ * down to 8 * 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;
+    for (i = 0; i < len / sizeof(VECTYPE); i += 8) {
+		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
-- 
1.7.9.5

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

[Eric Blake]

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On 03/12/2013 09:48 AM, Peter Lieven wrote:
> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  include/qemu-common.h |    1 +
>  util/cutils.c         |   30 ++++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+)
> 

> 
>  /*
> + * Searches for an area with non-zero content in a buffer
> + *
> + * Attention! The len must be a multiple of 8 * sizeof(VECTYPE)
> + * and addr must bedue to restriction of optimizations in this function.

s/bedue/be XXX due/

don't know what XXX is supposed to be.  Instead of stating this in a
comment...

> + *
> + * The return value is the offset of the non-zero area rounded
> + * down to 8 * 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)
> +{

...why not instead assert() it as part of the contract?

> +    VECTYPE *p = (VECTYPE *)buf;
> +    VECTYPE zero = ZERO_SPLAT;
> +    size_t i;
> +    for (i = 0; i < len / sizeof(VECTYPE); i += 8) {
> +        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

oh, probably because of copy and paste.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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