Attempt to support MTE on Android arm64

Makefile

@@ -72,6 +72,10 @@ ifdef KRK_NO_STRESS_GC
   CFLAGS += -DKRK_NO_STRESS_GC=1
 endif
 
+ifdef KRK_HEAP_TAG_BYTE
+  CFLAGS += -DKRK_HEAP_TAG_BYTE=${KRK_HEAP_TAG_BYTE}
+endif
+
 .PHONY: help
 
 help:

src/kuroko/value.h

@@ -188,12 +188,37 @@ typedef union {
 	double   dbl;
 } KrkValueDbl;
 
+#ifdef KRK_SANITIZE_OBJECT_POINTERS
+/**
+ * Debugging tool for verifying we aren't trying to box NULL, which is not a valid object.
+ * Enable this and run the test suite and whatever else you can find.
+ */
+#include <assert.h>
+static inline uintptr_t _krk_sanitize(uintptr_t input) {
+	assert(input != 0);
+	return input;
+}
+#else
+#define _krk_sanitize(ptr) (ptr)
+#endif
+
+/**
+ * On platforms where heap pointers are tagged, we can try to force the tag bytes
+ * back into our truncated object pointers. On arm64 Android, you can try setting
+ * this to 0xb4 to fix issues with MTE.
+ */
+#ifdef KRK_HEAP_TAG_BYTE
+#define KRK_HEAP_TAG ((uintptr_t)KRK_HEAP_TAG_BYTE << 56)
+#else
+#define KRK_HEAP_TAG 0
+#endif
+
 #define NONE_VAL(value)     ((KrkValue)(KRK_VAL_MASK_LOW | KRK_VAL_MASK_NONE))
 #define NOTIMPL_VAL(value)  ((KrkValue)(KRK_VAL_MASK_LOW | KRK_VAL_MASK_NOTIMPL))
 #define BOOLEAN_VAL(value)  ((KrkValue)(((uint64_t)(value) & KRK_VAL_MASK_LOW) | KRK_VAL_MASK_BOOLEAN))
 #define INTEGER_VAL(value)  ((KrkValue)(((uint64_t)(value) & KRK_VAL_MASK_LOW) | KRK_VAL_MASK_INTEGER))
 #define KWARGS_VAL(value)   ((KrkValue)((uint32_t)(value) | KRK_VAL_MASK_KWARGS))
-#define OBJECT_VAL(value)   ((KrkValue)(((uintptr_t)(value) & KRK_VAL_MASK_LOW) | KRK_VAL_MASK_OBJECT))
+#define OBJECT_VAL(value)   ((KrkValue)((_krk_sanitize((uintptr_t)(value)) & KRK_VAL_MASK_LOW) | KRK_VAL_MASK_OBJECT))
 #define HANDLER_VAL(ty,ta)  ((KrkValue)((uint32_t)((((uint16_t)ty) << 16) | ((uint16_t)ta)) | KRK_VAL_MASK_HANDLER))
 #define FLOATING_VAL(value) (((KrkValueDbl){.dbl = (value)}).val)
 
@@ -206,7 +231,7 @@ typedef union {
 
 #define AS_NOTIMPL(value)   ((krk_integer_type)((value) & KRK_VAL_MASK_LOW))
 #define AS_HANDLER(value)   ((uint32_t)((value) & KRK_VAL_MASK_LOW))
-#define AS_OBJECT(value)    ((KrkObj*)(uintptr_t)((value) & KRK_VAL_MASK_LOW))
+#define AS_OBJECT(value)    ((KrkObj*)(uintptr_t)(((value) & KRK_VAL_MASK_LOW) | KRK_HEAP_TAG))
 #define AS_FLOATING(value)  (((KrkValueDbl){.val = (value)}).dbl)
 
 /* This is a silly optimization: because of the arrangement of the identifying