lz_sarray: Prefetch portion of suffix array to be searched

Seems to improve performance of LZX and LZMS compression on large chunks
by ~5%.  Results may vary.

diff --git a/include/wimlib/compiler.h b/include/wimlib/compiler.h
index d32d092b964dfda938aa82cd4343edb085e9e103..21fabd995f01fa75a09f58e085f383495a6a339a 100644 (file)
--- a/include/wimlib/compiler.h
+++ b/include/wimlib/compiler.h
@@ -22,6 +22,7 @@
 #      define likely(x) __builtin_expect(!!(x), 1)
 #      define unlikely(x) __builtin_expect(!!(x), 0)
 #      define inline inline __attribute__((always_inline))
+#      define prefetch(x) __builtin_prefetch(x)
 #else
 #      define WIMLIBAPI
 #      define _always_inline_attribute inline
@@ -33,6 +34,7 @@
 #      define _aligned_attribute(size)
 #      define likely(x) (x)
 #      define unlikely(x) (x)
+#      define prefetch(x)
 #endif /* __GNUC__ */
 
 #endif /* _WIMLIB_COMPILER_H */

diff --git a/include/wimlib/lz_sarray.h b/include/wimlib/lz_sarray.h
index 5b63ae7c74e9dcafbb075d7ba2ab4559ac36284d..30dc2b30a480ea64d943ab73e6baedf71bfa86f6 100644 (file)
--- a/include/wimlib/lz_sarray.h
+++ b/include/wimlib/lz_sarray.h
@@ -34,7 +34,8 @@
 #ifndef _WIMLIB_LZ_SARRAY_H
 #define _WIMLIB_LZ_SARRAY_H
 
-#include "wimlib/compiler.h" /* must define '_always_inline_attribute'  */
+#include "wimlib/compiler.h" /* must define '_always_inline_attribute',
+                               'likely()', and 'prefetch()'.  */
 #include "wimlib/lz.h"       /* must define 'struct raw_match' and LZ_ASSERT()  */
 #include "wimlib/types.h"    /* must define 'bool', 'u8', 'u16, and 'u32'  */
 
@@ -285,6 +286,19 @@ lz_sarray_get_matches(struct lz_sarray *mf,
        /* Prepare for searching the current position.  */
        lz_sarray_update_salink(r, link);
 
+#if 1
+       /* Prefetch next position in SA and link.
+        *
+        * This can improve performance on large windows since the locations in
+        * SA and link at which each successive search begins are in general
+        * randomly distributed.  */
+       if (likely(i + 1 < mf->window_size)) {
+               const lz_sarray_pos_t next_r = ISA[i + 1];
+               prefetch(&SA[next_r]);
+               prefetch(&link[next_r]);
+       }
+#endif
+
        /* L = rank of next suffix to the left;
         * R = rank of next suffix to the right;
         * lenL = length of match between current position and the suffix with rank L;