Create and use single_1bit_pos32()

This moves the code from regcomp.c to inline.h that calculates the
position of the lone set bit in a U32.  This is in preparation for use
by other call sites.

1 files changed, 19 insertions, 0 deletions

diff --git a/inline.h b/inline.h
index 55b26e0d78..9ec02265bd 100644
--- a/inline.h
+++ b/inline.h
@@ -664,6 +664,25 @@ Perl_is_utf8_invariant_string_loc(const U8* const s, STRLEN len, const U8 ** ep)
     return TRUE;
 }
 
+PERL_STATIC_INLINE unsigned
+Perl_single_1bit_pos32(U32 word)
+{
+    /* Given a 32-bit word known to contain all zero bits except one 1 bit,
+     * find and return the 1's position: 0..31 */
+
+#ifdef PERL_CORE    /* macro not exported */
+    ASSUME(isPOWER_OF_2(word));
+#else
+    ASSUME(word && (word & (word-1)) == 0);
+#endif
+
+    /* The position of the only set bit in a word can be quickly calculated
+     * using deBruijn sequences.  See for example
+     * https://en.wikipedia.org/wiki/De_Bruijn_sequence */
+    return PL_deBruijn_bitpos_tab32[(word * PERL_deBruijnMagic32_)
+                                                    >> PERL_deBruijnShift32_];
+}
+
 #ifndef EBCDIC
 
 PERL_STATIC_INLINE unsigned int