[Qemu-devel] [PATCH 2/2] migration/xbzrle: make xbzrle_encode_buffer little easier to read

[Wei Yang]

The encoding process could be described below:

    for [content]
        get length of a run
        encode this run

By refactoring the code with this logic, it maybe a little easier to
understand.

Signed-off-by: Wei Yang <address@hidden>
---
 migration/xbzrle.c | 153 +++++++++++++++++++++------------------------
 1 file changed, 73 insertions(+), 80 deletions(-)

diff --git a/migration/xbzrle.c b/migration/xbzrle.c
index 1ba482ded9..25c69708ec 100644
--- a/migration/xbzrle.c
+++ b/migration/xbzrle.c
@@ -14,6 +14,59 @@
 #include "qemu/cutils.h"
 #include "xbzrle.h"
 
+static int next_run(uint8_t *old_buf, uint8_t *new_buf, int off, int slen,
+                    bool zrun)
+{
+    uint32_t len = 0;
+    long res;
+
+    res = (slen - off) % sizeof(long);
+
+    /* first unaligned bytes */
+    while (res) {
+        uint8_t xor = old_buf[off + len] ^ new_buf[off + len];
+
+        if (!(zrun ^ !!xor)) {
+            break;
+        }
+        len++;
+        res--;
+    }
+
+    if (res) {
+        return len;
+    }
+
+    /* word at a time for speed, use of 32-bit long okay */
+    while (off + len < slen) {
+        /* truncation to 32-bit long okay */
+        unsigned long mask = (unsigned long)0x0101010101010101ULL;
+        long xor = (*(long *)(old_buf + off + len)) ^
+                   (*(long *)(new_buf + off + len));
+
+        if (zrun && !(zrun ^ !!xor)) {
+            break;
+        } else if (!zrun && ((xor - mask) & ~xor & (mask << 7))) {
+            break;
+        }
+
+        len += sizeof(long);
+    }
+
+    /* go over the rest */
+    while (off + len < slen) {
+        uint8_t xor = old_buf[off + len] ^ new_buf[off + len];
+
+        if (!(zrun ^ !!xor)) {
+            break;
+        }
+
+        len++;
+    }
+
+    return len;
+}
+
 /*
   page = zrun nzrun
        | zrun nzrun page
@@ -27,103 +80,43 @@
 int xbzrle_encode_buffer(uint8_t *old_buf, uint8_t *new_buf, int slen,
                          uint8_t *dst, int dlen)
 {
-    uint32_t zrun_len = 0, nzrun_len = 0;
-    int d = 0, i = 0;
-    long res;
-    uint8_t *nzrun_start = NULL;
+    bool zrun = true;
+    int len, src_off = 0, dst_off = 0;
 
     g_assert(!(((uintptr_t)old_buf | (uintptr_t)new_buf | slen) %
                sizeof(long)));
 
-    while (i < slen) {
+    for (; src_off < slen; src_off += len, zrun = !zrun) {
         /* overflow */
-        if (d + 2 > dlen) {
+        if (dst_off + 2 > dlen) {
             return -1;
         }
 
-        /* not aligned to sizeof(long) */
-        res = (slen - i) % sizeof(long);
-        while (res && old_buf[i] == new_buf[i]) {
-            zrun_len++;
-            i++;
-            res--;
-        }
+        len = next_run(old_buf, new_buf, src_off, slen, zrun);
 
-        /* word at a time for speed */
-        if (!res) {
-            while (i < slen &&
-                   (*(long *)(old_buf + i)) == (*(long *)(new_buf + i))) {
-                i += sizeof(long);
-                zrun_len += sizeof(long);
+        if (zrun) {
+            /* buffer unchanged */
+            if (len == slen) {
+                return 0;
             }
-
-            /* go over the rest */
-            while (i < slen && old_buf[i] == new_buf[i]) {
-                zrun_len++;
-                i++;
+            /* skip last zero run */
+            if (src_off + len == slen) {
+                return dst_off;
             }
         }
 
-        /* buffer unchanged */
-        if (zrun_len == slen) {
-            return 0;
-        }
-
-        /* skip last zero run */
-        if (i == slen) {
-            return d;
-        }
-
-        d += uleb128_encode_small(dst + d, zrun_len);
-
-        zrun_len = 0;
-        nzrun_start = new_buf + i;
-
-        /* overflow */
-        if (d + 2 > dlen) {
-            return -1;
-        }
-        /* not aligned to sizeof(long) */
-        res = (slen - i) % sizeof(long);
-        while (res && old_buf[i] != new_buf[i]) {
-            i++;
-            nzrun_len++;
-            res--;
-        }
-
-        /* word at a time for speed, use of 32-bit long okay */
-        if (!res) {
-            /* truncation to 32-bit long okay */
-            unsigned long mask = (unsigned long)0x0101010101010101ULL;
-            while (i < slen) {
-                unsigned long xor;
-                xor = *(unsigned long *)(old_buf + i)
-                    ^ *(unsigned long *)(new_buf + i);
-                if ((xor - mask) & ~xor & (mask << 7)) {
-                    /* found the end of an nzrun within the current long */
-                    while (old_buf[i] != new_buf[i]) {
-                        nzrun_len++;
-                        i++;
-                    }
-                    break;
-                } else {
-                    i += sizeof(long);
-                    nzrun_len += sizeof(long);
-                }
+        dst_off += uleb128_encode_small(dst + dst_off, len);
+        if (!zrun) {
+            /* overflow */
+            if (dst_off + len > dlen) {
+                return -1;
             }
+            memcpy(dst + dst_off, new_buf + src_off, len);
+            dst_off += len;
         }
-
-        d += uleb128_encode_small(dst + d, nzrun_len);
-        /* overflow */
-        if (d + nzrun_len > dlen) {
-            return -1;
-        }
-        memcpy(dst + d, nzrun_start, nzrun_len);
-        d += nzrun_len;
-        nzrun_len = 0;
     }
 
-    return d;
+    return dst_off;
 }
 
 int xbzrle_decode_buffer(uint8_t *src, int slen, uint8_t *dst, int dlen)
-- 
2.19.1