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Date: Sun, 24 Apr 2016 17:58:43 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20160424215843.GA1122@flamenco>
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Subject: Re: [Qemu-devel] [PATCH v3 08/11] qht: QEMU's fast,
 resizable and scalable Hash Table
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To: Richard Henderson <rth@twiddle.net>
Cc: QEMU Developers <qemu-devel@nongnu.org>, MTTCG Devel <mttcg@greensocs.com>, Alex =?iso-8859-1?Q?Benn=E9e?= <alex.bennee@linaro.org>, Paolo Bonzini <pbonzini@redhat.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, Sergey Fedorov <serge.fdrv@gmail.com>

On Sun, Apr 24, 2016 at 13:01:31 -0700, Richard Henderson wrote:
> On 04/19/2016 04:07 PM, Emilio G. Cota wrote:
> >+static void qht_insert__locked(struct qht *ht, struct qht_map *map,
> >+                               struct qht_bucket *head, void *p, uint32_t hash)
> >+{
> >+    struct qht_bucket *b = head;
> >+    struct qht_bucket *prev = NULL;
> >+    struct qht_bucket *new = NULL;
> >+    int i;
> >+
> >+    for (;;) {
> >+        if (b == NULL) {
> >+            b = qemu_memalign(QHT_BUCKET_ALIGN, sizeof(*b));
> >+            memset(b, 0, sizeof(*b));
> >+            new = b;
> >+        }
> >+        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> >+            if (b->hashes[i]) {
> >+                continue;
> >+            }
> 
> Surely that's b->pointers[i] != NULL.  We've made no provision that the hash
> function must return non-zero.

Good catch! I initially banned 0 from being a valid hash value,
knowing that I'd have to eventually test how stupid this assumption
was, but I forgot to do so.

It turns out that banning any hash value is a stupid idea.
For example, looping over [0,0xffffffff] shows that
xxh32(0x7aac3812, seed=1) == 0.

Will fix this in v4 -- the only assumption will be that the passed
pointer should be !NULL.

> >+static inline bool qht_remove__locked(struct qht_map *map, struct qht_bucket *b,
> >+                                      const void *p, uint32_t hash)
> >+{
> >+    int i;
> >+
> >+    do {
> >+        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> >+            if (b->hashes[i] == hash && b->pointers[i] == p) {
> 
> Don't you only need to test p here?

Fair point. Will fix in v4.

Thanks,

		Emilio