Store the BM table in mg_ptr instead of after SvCUR().

Previously the 256 byte Boyer-Moore table was stored in the buffer of SvPVX()
after the raw string by extending the buffer.

Given that the scalar is alway upgraded to add PERL_MAGIC_bm magic, to clear
the table and other flags, there's no extra memory cost in using mg_ptr in the
MAGIC struct to point directly to the table.

I believe that this removes the last place in the core that stores data beyond
SvCUR().

1 files changed, 28 insertions, 11 deletions

diff --git a/util.c b/util.c
index 165e61a918..8c836c2faf 100644
--- a/util.c
+++ b/util.c
@@ -549,6 +549,7 @@ Perl_fbm_compile(pTHX_ SV *sv, U32 flags)
     STRLEN len;
     STRLEN rarest = 0;
     U32 frequency = 256;
+    MAGIC *mg;
 
     PERL_ARGS_ASSERT_FBM_COMPILE;
 
@@ -575,29 +576,45 @@ Perl_fbm_compile(pTHX_ SV *sv, U32 flags)
     SvIOK_off(sv);
     SvNOK_off(sv);
     SvVALID_on(sv);
+
+    /* "deep magic", the comment used to add. The use of MAGIC itself isn't
+       really. MAGIC was originally added in 79072805bf63abe5 (perl 5.0 alpha 2)
+       to call SvVALID_off() if the scalar was assigned to.
+
+       The comment itself (and "deeper magic" below) date back to
+       378cc40b38293ffc (perl 2.0). "deep magic" was an annotation on
+       str->str_pok |= 2;
+       where the magic (presumably) was that the scalar had a BM table hidden
+       inside itself.
+
+       As MAGIC is always present on BMs [in Perl 5 :-)], we can use it to store
+       the table instead of the previous (somewhat hacky) approach of co-opting
+       the string buffer and storing it after the string.  */
+
+    assert(!mg_find(sv, PERL_MAGIC_bm));
+    mg = sv_magicext(sv, NULL, PERL_MAGIC_bm, &PL_vtbl_bm, NULL, 0);
+    assert(mg);
+
     if (len > 2) {
 	/* Shorter strings are special-cased in Perl_fbm_instr(), and don't use
 	   the BM table.  */
-	const unsigned char *sb;
 	const U8 mlen = (len>255) ? 255 : (U8)len;
+	const unsigned char *const sb = s + len - mlen; /* first char (maybe) */
 	register U8 *table;
 
-	Sv_Grow(sv, len + 256 + PERL_FBM_TABLE_OFFSET);
-	table
-	    = (unsigned char*)(SvPVX_mutable(sv) + len + PERL_FBM_TABLE_OFFSET);
-	s = table - 1 - PERL_FBM_TABLE_OFFSET;	/* last char */
+	Newx(table, 256, U8);
 	memset((void*)table, mlen, 256);
+	mg->mg_ptr = (char *)table;
+	mg->mg_len = 256;
+
+	s += len - 1; /* last char */
 	i = 0;
-	sb = s - mlen + 1;			/* first char (maybe) */
 	while (s >= sb) {
 	    if (table[*s] == mlen)
 		table[*s] = (U8)i;
 	    s--, i++;
 	}
-    } else {
-	Sv_Grow(sv, len + PERL_FBM_TABLE_OFFSET);
     }
-    sv_magic(sv, NULL, PERL_MAGIC_bm, NULL, 0);	/* deep magic */
 
     s = (const unsigned char*)(SvPVX_const(sv));	/* deeper magic */
     for (i = 0; i < len; i++) {
@@ -772,8 +789,8 @@ Perl_fbm_instr(pTHX_ unsigned char *big, register unsigned char *bigend, SV *lit
 	return NULL;
 
     {
-	register const unsigned char * const table
-	    = little + littlelen + PERL_FBM_TABLE_OFFSET;
+	const MAGIC *const mg = mg_find(littlestr, PERL_MAGIC_bm);
+	const unsigned char * const table = (const unsigned char *) mg->mg_ptr;
 	register const unsigned char *oldlittle;
 
 	--littlelen;			/* Last char found by table lookup */