util: Avoid strict aliasing bugs in xxhash.

XXH32 is doing access through u32 *, and with strict aliasing the compiler
gets to assume that those are independent of the u16 writes we did in
fd6_texture_key setup, and based on various tweaks to the code, would
result in bad hashes computed after inlining.  The failure was:

../src/util/hash_table.c:326:_mesa_hash_table_search_pre_hashed: Assertion
`ht->key_hash_function == ((void *)0) || hash == ht->key_hash_function(key)'
failed.)

By setting these two flags, we always take the unaligned,
memcpy-the-32-bit-data path.  I believe this should be same perf on x86
(which will happily unaligned load 32 bits in the end), while it will be
slower on arm (where you have to a special unaligned load operation iirc).
This should still be far faster than our old hash.

Fixes: edd62619a1c4 ("freedreno: replace fnv1a hash function with xxhash")
Acked-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5271>

diff --git a/src/util/xxhash.h b/src/util/xxhash.h
index c0c8f44b600b1ae08ac15d4feca50fea5b72f10d..f7a4b40578ea8161c8cf59788ce3124dee06b427 100644 (file)
--- a/src/util/xxhash.h
+++ b/src/util/xxhash.h
@@ -69,6 +69,15 @@ XXH64       13.8 GB/s            1.9 GB/s
 XXH32        6.8 GB/s            6.0 GB/s
 */
 
+/* Mesa leaves strict aliasing on in the compiler, and this code likes to
+ * dereference the passed in data as u32*, which means that the compiler is
+ * free to move the u32 read before the write of the struct members being
+ * hashed, and in practice it did in freedreno.  Forcing these two things
+ * prevents it.
+ */
+#define XXH_FORCE_ALIGN_CHECK 0
+#define XXH_FORCE_MEMORY_ACCESS 0
+
 #if defined (__cplusplus)
 extern "C" {
 #endif