diff options
Diffstat (limited to 'src/lisp.h')
| -rw-r--r-- | src/lisp.h | 32 |
1 files changed, 26 insertions, 6 deletions
diff --git a/src/lisp.h b/src/lisp.h index cc82edb2f5f..8c274cddb56 100644 --- a/src/lisp.h +++ b/src/lisp.h @@ -168,6 +168,10 @@ extern int suppress_checking EXTERNALLY_VISIBLE; #define GCTYPEBITS 3 #endif +#ifndef VALBITS +#define VALBITS (BITS_PER_EMACS_INT - GCTYPEBITS) +#endif + #ifndef NO_DECL_ALIGN # ifndef DECL_ALIGN # if HAVE_ATTRIBUTE_ALIGNED @@ -191,7 +195,28 @@ extern int suppress_checking EXTERNALLY_VISIBLE; || defined DARWIN_OS || defined __sun) /* We also need to be able to specify mult-of-8 alignment on static vars. */ # if defined DECL_ALIGN -# define USE_LSB_TAG +/* On hosts where VALBITS is greater than the pointer width in bits, + USE_LSB_TAG is: + + a. unnecessary, because the top bits of an EMACS_INT are unused, + + b. slower, because it typically requires extra masking, and + + c. harmful, because it can create Lisp_Object values that are so scrambled + that mark_maybe_object cannot decipher them. mark_maybe_object assumes + that EMACS_INT values are contiguous, but a host where EMACS_INT is + wider than a pointer might allocate the top half of an EMACS_INT in + (say) a 32-bit word on the stack, putting the bottom half in a 32-bit + register that is saved elsewhere in a jmp_buf. When this happens, + since USE_LSB_TAG is not defined the bottom half alone is a valid + pointer that mark_maybe_pointer can follow; but if USE_LSB_TAG were + defined, the bottom half would not be a valid pointer and neither + mark_maybe_object nor mark_maybe_pointer would follow it. + + So, define USE_LSB_TAG only on hosts where it might be useful. */ +# if UINTPTR_MAX >> VALBITS != 0 +# define USE_LSB_TAG +# endif # endif #endif @@ -309,11 +334,6 @@ enum Lisp_Fwd_Type Lisp_Fwd_Kboard_Obj, /* Fwd to a Lisp_Object field of kboards. */ }; -/* These values are overridden by the m- file on some machines. */ -#ifndef VALBITS -#define VALBITS (BITS_PER_EMACS_INT - GCTYPEBITS) -#endif - #ifdef USE_LISP_UNION_TYPE #ifndef WORDS_BIGENDIAN |