Use 16bit to store KMP jump table. Set max needle to 8192

Also malloc the memory instead of putting it onto the stack if it is
going to be more than 4069 bytes. Not 100% sure this is needed, but
don't want to allocate too much to the stack since some platforms
may have smaller stack size than current versions of Linux, Windows
or MacOS do.

src/strings/ops.c

@@ -1,7 +1,7 @@
 #include "platform/memmem.h"
 #include "moar.h"
 #define MVM_DEBUG_STRANDS 0
-#define MVM_string_KMP_max_pattern_length 4096
+#define MVM_string_KMP_max_pattern_length 8192
 /* Max value possible for MVMuint32 MVMStringBody.num_graphs */
 #define MAX_GRAPHEMES     0xFFFFFFFFLL
 
@@ -1023,7 +1023,11 @@ static MVMint64 string_equal_at_ignore_case(MVMThreadContext *tc, MVMString *Hay
         return n_fc_graphs <= H_graphs + H_expansion - H_offset ? 1 : 0;
     return 0;
 }
-static void knuth_morris_pratt_process_pattern (MVMThreadContext *tc, MVMString *pat, MVMGrapheme32 *next, MVMStringIndex pat_graphs) {
+/* Processes the pattern. The pattern must be able to store negative and positive
+ * numbers. It must be able to store at least 1/2 the length of the needle,
+ * though possibly more (though I am not sure it's possible for it to be more than
+ * 1/2). */
+static void knuth_morris_pratt_process_pattern (MVMThreadContext *tc, MVMString *pat, MVMint16 *next, MVMStringIndex pat_graphs) {
     MVMint64 i = 0;
     MVMint64 j = next[0] = -1;
     while (i < pat_graphs) {
@@ -1045,13 +1049,21 @@ static MVMint64 knuth_morris_pratt_string_index (MVMThreadContext *tc, MVMString
     MVMint64 text_offset   = H_offset;
     MVMStringIndex Haystack_graphs = MVM_string_graphs_nocheck(tc, Haystack);
     MVMStringIndex needle_graphs   = MVM_string_graphs_nocheck(tc, needle);
-    MVMGrapheme32    *next = NULL;
+    MVMint16         *next = NULL;
     MVMString *flat_needle = NULL;
+    size_t next_size = (1 + needle_graphs) * sizeof(MVMint16);
+    int    next_is_malloced = 0;
     assert(needle_graphs <= MVM_string_KMP_max_pattern_length);
     /* Empty string is found at start of string */
     if (needle_graphs == 0)
         return 0;
-    next = alloca((1 + needle_graphs) * sizeof(MVMGrapheme32));
+    /* Allocate max 8K onto the stack, otherwise malloc */
+    if (next_size < 4096)
+        next = alloca(next_size);
+    else {
+        next = MVM_malloc(next_size);
+        next_is_malloced = 1;
+    }
     /* If the needle is a strand, flatten it, otherwise use the original string */
     flat_needle = needle->body.storage_type == MVM_STRING_STRAND
         ? collapse_strands(tc, needle)
@@ -1069,11 +1081,14 @@ static MVMint64 knuth_morris_pratt_string_index (MVMThreadContext *tc, MVMString
             if (needle_offset == -1 || MVM_string_get_grapheme_at_nocheck(tc, flat_needle, needle_offset)
                                     == MVM_string_gi_cached_get_grapheme(tc, &H_gic, text_offset)) {
                 text_offset++; needle_offset++;
-                if (needle_offset == needle_graphs)
+                if (needle_offset == needle_graphs) {
+                    if (next_is_malloced) MVM_free(next);
                     return text_offset - needle_offset;
+                }
             }
             else needle_offset = next[needle_offset];
         }
+        if (next_is_malloced) MVM_free(next);
         return -1;
     }
     while (text_offset < Haystack_graphs && needle_offset < needle_graphs) {