All literal constants are read-only

* libguile/array-handle.c (initialize_vector_handle): Add mutable_p
  argument.  Unless the vector handle is mutable, null out its
  writable_elements member.
  (scm_array_get_handle): Adapt to determine mutability of the various
  arrays.
  (scm_array_handle_elements, scm_array_handle_writable_elements):
  Reverse the sense: instead of implementing read-only in terms of
  read-write, go the other way around, adding an assertion in the
  read-write case that the array handle is mutable.
* libguile/array-map.c (racp): Assert that the destination is mutable.
* libguile/bitvectors.c (SCM_F_BITVECTOR_IMMUTABLE, IS_BITVECTOR):
  (IS_MUTABLE_BITVECTOR): Add a flag to indicate immutability.
  (scm_i_bitvector_bits): Fix indentation.
  (scm_i_is_mutable_bitvector): New helper.
  (scm_array_handle_bit_elements)
  ((scm_array_handle_bit_writable_elements): Build writable_elements in
  terms of elements.
  (scm_bitvector_elements, scm_bitvector_writable_elements): Likewise.
  (scm_c_bitvector_set_x): Require a mutable bitvector for the
  fast-path.
  (scm_bitvector_to_list, scm_bit_count): Use read-only elements()
  function.
* libguile/bitvectors.h (scm_i_is_mutable_bitvector): New decl.
* libguile/bytevectors.c (INTEGER_ACCESSOR_PROLOGUE):
  (INTEGER_GETTER_PROLOGUE, INTEGER_SETTER_PROLOGUE):
  (INTEGER_REF, INTEGER_NATIVE_REF, INTEGER_SET, INTEGER_NATIVE_SET):
  (GENERIC_INTEGER_ACCESSOR_PROLOGUE):
  (GENERIC_INTEGER_GETTER_PROLOGUE, GENERIC_INTEGER_SETTER_PROLOGUE):
  (LARGE_INTEGER_NATIVE_REF, LARGE_INTEGER_NATIVE_SET):
  (IEEE754_GETTER_PROLOGUE, IEEE754_SETTER_PROLOGUE):
  (IEEE754_REF, IEEE754_NATIVE_REF, IEEE754_SET, IEEE754_NATIVE_SET):
  Setters require a mutable bytevector.
  (SCM_BYTEVECTOR_SET_FLAG): New helper.
  (SCM_BYTEVECTOR_SET_CONTIGUOUS_P, SCM_BYTEVECTOR_SET_ELEMENT_TYPE):
  Remove helpers.
  (SCM_VALIDATE_MUTABLE_BYTEVECTOR): New helper.
  (make_bytevector, make_bytevector_from_buffer): Use
  SCM_SET_BYTEVECTOR_FLAGS.
  (scm_c_bytevector_set_x, scm_bytevector_fill_x)
  (scm_bytevector_copy_x): Require a mutable bytevector.
* libguile/bytevectors.h (SCM_F_BYTEVECTOR_CONTIGUOUS)
  (SCM_F_BYTEVECTOR_IMMUTABLE, SCM_MUTABLE_BYTEVECTOR_P): New
  definitions.
* libguile/bytevectors.h (SCM_BYTEVECTOR_CONTIGUOUS_P): Just access one
  bit.
* libguile/srfi-4.c (DEFINE_SRFI_4_C_FUNCS): Implement
  writable_elements() in terms of elements().
* libguile/strings.c (scm_i_string_is_mutable): New helper.
* libguile/uniform.c (scm_array_handle_uniform_elements):
  (scm_array_handle_uniform_writable_elements): Implement
  writable_elements in terms of elements.
* libguile/vectors.c (SCM_VALIDATE_MUTABLE_VECTOR): New helper.
  (scm_vector_elements, scm_vector_writable_elements): Implement
  writable_elements in terms of elements.
  (scm_c_vector_set_x): Require a mutable vector.
* libguile/vectors.h (SCM_F_VECTOR_IMMUTABLE, SCM_I_IS_MUTABLE_VECTOR):
  New definitions.
* libguile/vm-engine.c (VM_VALIDATE_MUTABLE_BYTEVECTOR):
  (VM_VALIDATE_MUTABLE_VECTOR, vector-set!, vector-set!/immediate)
  (BV_BOUNDED_SET, BV_SET): Require mutable bytevector/vector.
* libguile/vm.c (vm_error_not_a_mutable_bytevector):
  (vm_error_not_a_mutable_vector): New definitions.
* module/system/vm/assembler.scm (link-data): Mark residualized vectors,
  bytevectors, and bitvectors as being read-only.

module/system/vm/assembler.scm

@@ -1392,17 +1392,27 @@ should be .data or .rodata), and return the resulting linker object.
     (+ address
        (modulo (- alignment (modulo address alignment)) alignment)))
 
-  (define tc7-vector 13)
+  (define tc7-vector #x0d)
+  (define vector-immutable-flag #x80)
+
+  (define tc7-string #x15)
+  (define string-read-only-flag #x200)
+
+  (define tc7-stringbuf #x27)
   (define stringbuf-wide-flag #x400)
-  (define tc7-stringbuf 39)
-  (define tc7-narrow-stringbuf tc7-stringbuf)
-  (define tc7-wide-stringbuf (+ tc7-stringbuf stringbuf-wide-flag))
-  (define tc7-ro-string (+ 21 #x200))
+
   (define tc7-syntax #x3d)
-  (define tc7-program 69)
-  (define tc7-bytevector 77)
-  (define tc7-bitvector 95)
-  (define tc7-array 93)
+
+  (define tc7-program #x45)
+
+  (define tc7-bytevector #x4d)
+  ;; This flag is intended to be left-shifted by 7 bits.
+  (define bytevector-immutable-flag #x200)
+
+  (define tc7-array #x5d)
+
+  (define tc7-bitvector #x5f)
+  (define bitvector-immutable-flag #x80)
 
   (let ((word-size (asm-word-size asm))
         (endianness (asm-endianness asm)))
@@ -1447,9 +1457,10 @@ should be .data or .rodata), and return the resulting linker object.
        ((stringbuf? obj)
         (let* ((x (stringbuf-string obj))
                (len (string-length x))
-               (tag (if (= (string-bytes-per-char x) 1)
-                        tc7-narrow-stringbuf
-                        tc7-wide-stringbuf)))
+               (tag (logior tc7-stringbuf
+                            (if (= (string-bytes-per-char x) 1)
+                                0
+                                stringbuf-wide-flag))))
           (case word-size
             ((4)
              (bytevector-u32-set! buf pos tag endianness)
@@ -1491,15 +1502,15 @@ should be .data or .rodata), and return the resulting linker object.
         (write-placeholder asm buf pos))
 
        ((string? obj)
-        (let ((tag (logior tc7-ro-string (ash (string-length obj) 8)))) ; FIXME: unused?
+        (let ((tag (logior tc7-string string-read-only-flag)))
           (case word-size
             ((4)
-             (bytevector-u32-set! buf pos tc7-ro-string endianness)
+             (bytevector-u32-set! buf pos tag endianness)
              (write-placeholder asm buf (+ pos 4)) ; stringbuf
              (bytevector-u32-set! buf (+ pos 8) 0 endianness)
              (bytevector-u32-set! buf (+ pos 12) (string-length obj) endianness))
             ((8)
-             (bytevector-u64-set! buf pos tc7-ro-string endianness)
+             (bytevector-u64-set! buf pos tag endianness)
              (write-placeholder asm buf (+ pos 8)) ; stringbuf
              (bytevector-u64-set! buf (+ pos 16) 0 endianness)
              (bytevector-u64-set! buf (+ pos 24) (string-length obj) endianness))
@@ -1511,7 +1522,7 @@ should be .data or .rodata), and return the resulting linker object.
 
        ((simple-vector? obj)
         (let* ((len (vector-length obj))
-               (tag (logior tc7-vector (ash len 8))))
+               (tag (logior tc7-vector vector-immutable-flag (ash len 8))))
           (case word-size
             ((4) (bytevector-u32-set! buf pos tag endianness))
             ((8) (bytevector-u64-set! buf pos tag endianness))
@@ -1546,9 +1557,14 @@ should be .data or .rodata), and return the resulting linker object.
 
        ((simple-uniform-vector? obj)
         (let ((tag (if (bitvector? obj)
-                       tc7-bitvector
-                       (let ((type-code (array-type-code obj)))
-                         (logior tc7-bytevector (ash type-code 7))))))
+                       (logior tc7-bitvector
+                               bitvector-immutable-flag)
+                       (logior tc7-bytevector
+                               ;; Bytevector immutable flag also shifted
+                               ;; left.
+                               (ash (logior bytevector-immutable-flag
+                                            (array-type-code obj))
+                                    7)))))
           (case word-size
             ((4)
              (bytevector-u32-set! buf pos tag endianness)