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forwarding: use top bit instead of next cell

--- a/src/flisp.h

+++ b/src/flisp.h

@@ -95,7 +95,6 @@

 #define NONNUMERIC (0xff)

 #define valid_numtype(v) ((v) <= T_DOUBLE)

 #define UNBOUND ((value_t)1) // an invalid value

-#define TAG_FWD UNBOUND

 #define tag(x) ((x) & 7)

 #define ptr(x) ((void*)((uintptr_t)(x) & (~(uintptr_t)7)))

 #define tagptr(p, t) ((value_t)(p) | (t))

@@ -120,13 +119,12 @@

 #define mark_cons(c) bitvector_set(FL(consflags), cons_index(c))

 #define unmark_cons(c) bitvector_reset(FL(consflags), cons_index(c))

-#define isforwarded(v) (((value_t*)ptr(v))[0] == TAG_FWD)

-#define forwardloc(v) (((value_t*)ptr(v))[1])

+#define isforwarded(v) (*(value_t*)ptr(v) & TOP_BIT)

+#define forwardloc(v) (*(value_t*)ptr(v) ^ TOP_BIT)

 #define forward(v, to) \

 	do{ \

-		(((value_t*)ptr(v))[0] = TAG_FWD); \

-		(((value_t*)ptr(v))[1] = to); \

-	}while (0)

+		*(value_t*)ptr(v) = (value_t)(to) | TOP_BIT; \

+	}while(0)

 #define vector_size(v) (((size_t*)ptr(v))[0]>>2)

 #define vector_setsize(v, n) (((size_t*)ptr(v))[0] = ((n)<<2))

@@ -135,8 +133,8 @@

 // functions ending in _ are unsafe, faster versions

 #define car_(v) (((cons_t*)ptr(v))->car)

 #define cdr_(v) (((cons_t*)ptr(v))->cdr)

-#define car(v) (tocons((v))->car)

-#define cdr(v) (tocons((v))->cdr)

+#define car(v) (tocons(v)->car)

+#define cdr(v) (tocons(v)->cdr)

 #define fn_bcode(f) (((function_t*)ptr(f))->bcode)

 #define fn_vals(f) (((function_t*)ptr(f))->vals)

 #define fn_env(f) (((function_t*)ptr(f))->env)