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defined FWD_BIT for forwarding during GC

Don't interfere with potentially already tagged pointers.
At least on supported hardware with arm64 cpu, the top byte
may be used for that - use 55th bit instead.

--- a/src/flisp.h

+++ b/src/flisp.h

@@ -41,7 +41,7 @@

 typedef uintptr_t value_t;

-#ifdef BITS64

+#if defined(BITS64)

 typedef int64_t fixnum_t;

 #define FIXNUM_BITS 62

 #define TOP_BIT (1ULL<<63)

@@ -55,6 +55,10 @@

 #define PRIdFIXNUM PRId32

 #endif

+#if !defined(FWD_BIT)

+#define FWD_BIT TOP_BIT

+#endif

+

 typedef struct {

 	value_t car;

 	value_t cdr;

@@ -119,11 +123,11 @@

 #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) & TOP_BIT)

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

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

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

 #define forward(v, to) \

 	do{ \

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

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

 	}while(0)

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

--- a/src/posix/platform.h

+++ b/src/posix/platform.h

@@ -52,10 +52,13 @@

 extern char *os_version;

 #define __os_version__ os_version

-#ifndef __SIZEOF_POINTER__

+#if !defined(__SIZEOF_POINTER__)

 #error pointer size unknown

 #elif __SIZEOF_POINTER__ == 8

 #define BITS64

+#if defined(__aarch64__)

+#define FWD_BIT (1ULL<<55)

+#endif

 #endif

 #define nil NULL

-- 

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