[PATCH 7/9] hw/9pfs/9p-synth: avoid n-square issue in synth_readdir()

[Christian Schoenebeck]

This patch is just a temporary benchmark hack, not intended
to be merged!

synth driver's readdir() implementation has a severe n-square
performance problem. This patch is a quick and dirty hack to
prevent that performance problem from tainting the readdir()
benchmark results.

Signed-off-by: Christian Schoenebeck <address@hidden>
---
 hw/9pfs/9p-synth.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/hw/9pfs/9p-synth.c b/hw/9pfs/9p-synth.c
index 7eb210ffa8..1743f5409f 100644
--- a/hw/9pfs/9p-synth.c
+++ b/hw/9pfs/9p-synth.c
@@ -225,7 +225,7 @@ static void synth_direntry(V9fsSynthNode *node,
 }
 
 static struct dirent *synth_get_dentry(V9fsSynthNode *dir,
-                                            struct dirent *entry, off_t off)
+                                            struct dirent *entry, off_t off, 
V9fsSynthNode **hack)
 {
     int i = 0;
     V9fsSynthNode *node;
@@ -243,16 +243,37 @@ static struct dirent *synth_get_dentry(V9fsSynthNode *dir,
         /* end of directory */
         return NULL;
     }
+    *hack = node;
     synth_direntry(node, entry, off);
     return entry;
 }
 
 static struct dirent *synth_readdir(FsContext *ctx, V9fsFidOpenState *fs)
 {
-    struct dirent *entry;
+    struct dirent *entry = NULL;
     V9fsSynthOpenState *synth_open = fs->private;
     V9fsSynthNode *node = synth_open->node;
-    entry = synth_get_dentry(node, &synth_open->dent, synth_open->offset);
+
+    /*
+     * HACK: This is just intended for benchmark, to avoid severe n-square
+     * performance problem of synth driver's readdir implementation here which
+     * would otherwise unncessarily taint the benchmark results. By simply
+     * caching (globally) the previous node (of the previous synth_readdir()
+     * call) we can simply proceed to next node in chained list efficiently.
+     *
+     * not a good idea for any production code ;-)
+     */
+    static struct V9fsSynthNode *cachedNode = NULL;
+
+    if (!cachedNode) {
+        entry = synth_get_dentry(node, &synth_open->dent, synth_open->offset, 
&cachedNode);
+    } else {
+        cachedNode = cachedNode->sibling.le_next;
+        if (cachedNode) {
+            entry = &synth_open->dent;
+            synth_direntry(cachedNode, entry, synth_open->offset+1);
+        }
+    }
     if (entry) {
         synth_open->offset++;
     }
-- 
2.20.1