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Allocate data for structures on an eight-byte boundary, as

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required by the tagging system.
* struct.c (alloc_struct): New function.
(scm_make_struct, scm_make_vtable_vtable): Call it.
* struct.h (scm_struct_n_extra_words): Bump to 3.
(scm_struct_i_ptr): New "field".
* gc.c (scm_gc_sweep): When we need to free the data, use the
information stored by alloc_struct to find the beginning of the
block allocated to the structure, so we can free it.
This commit is contained in:
Jim Blandy 1996-10-15 00:10:59 +00:00
parent 2dfc85c018
commit 14d1400fa7
3 changed files with 74 additions and 29 deletions
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86
libguile/struct.c
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@ -276,6 +276,65 @@ scm_struct_vtable_p (x)
: SCM_BOOL_F);
}
/* All struct data must be allocated at an address whose bottom three
bits are zero. This is because the tag for a struct lives in the
bottom three bits of the struct's car, and the upper bits point to
the data of its vtable, which is a struct itself. Thus, if the
address of that data doesn't end in three zeros, tagging it will
destroy the pointer.
This function allocates a block of memory, and returns a pointer at
least scm_struct_n_extra_words words into the block. Furthermore,
it guarantees that that pointer's least three significant bits are
all zero.
The argument n_words should be the number of words that should
appear after the returned address. (That is, it shouldn't include
scm_struct_n_extra_words.)
This function initializes the following fields of the struct:
scm_struct_i_ptr --- the actual stort of the block of memory; the
address you should pass to 'free' to dispose of the block.
This field allows us to both guarantee that the returned
address is divisible by eight, and allow the GC to free the
block.
scm_struct_i_n_words --- the number of words allocated to the
block, including the extra fields. This is used by the GC.
scm_struct_i_tag --- a unique tag assigned to this struct,
allocated according to struct_num.
Ugh. */
static SCM *alloc_struct SCM_P ((int n_words, char *who));
static SCM *
alloc_struct (n_words, who)
int n_words;
char *who;
{
int size = sizeof (SCM) * (n_words + scm_struct_n_extra_words) + 7;
SCM *block = (SCM *) scm_must_malloc (size, who);
/* Adjust the pointer to hide the extra words. */
SCM *p = block + scm_struct_n_extra_words;
/* Adjust it even further so it's aligned on an eight-byte boundary. */
p = (SCM *) (((SCM) p + 7) & ~7);
/* Initialize a few fields as described above. */
p[scm_struct_i_ptr] = (SCM) block;
p[scm_struct_i_n_words] = (SCM) (scm_struct_n_extra_words + n_words);
p[scm_struct_i_tag] = struct_num++;
return p;
}
SCM_PROC (s_make_struct, "make-struct", 2, 0, 1, scm_make_struct);
SCM
@ -292,22 +351,15 @@ scm_make_struct (vtable, tail_array_size, init)
SCM_ASSERT ((SCM_BOOL_F != scm_struct_vtable_p (vtable)),
vtable, SCM_ARG1, s_make_struct);
SCM_ASSERT (SCM_INUMP (tail_array_size), tail_array_size, SCM_ARG2, s_make_struct);
SCM_ASSERT (SCM_INUMP (tail_array_size), tail_array_size, SCM_ARG2,
s_make_struct);
layout = SCM_STRUCT_DATA (vtable)[scm_struct_i_layout];
basic_size = SCM_LENGTH (layout) / 2;
tail_elts = SCM_INUM (tail_array_size);
SCM_NEWCELL (handle);
SCM_DEFER_INTS;
data = (SCM*)scm_must_malloc (sizeof (SCM) * (scm_struct_n_extra_words
+ basic_size
+ tail_elts),
"structure");
data += scm_struct_n_extra_words;
data[scm_struct_i_n_words] = (SCM) (scm_struct_n_extra_words
+ basic_size
+ tail_elts);
data[scm_struct_i_tag] = struct_num++;
data = alloc_struct (basic_size + tail_elts, "make-struct");
SCM_SETCDR (handle, data);
SCM_SETCAR (handle, ((SCM)SCM_STRUCT_DATA (vtable)) + scm_tc3_cons_gloc);
init_struct (handle, tail_elts, init);
@ -334,8 +386,8 @@ scm_make_vtable_vtable (extra_fields, tail_array_size, init)
SCM_ASSERT (SCM_NIMP (extra_fields) && SCM_ROSTRINGP (extra_fields),
extra_fields, SCM_ARG1, s_make_vtable_vtable);
SCM_ASSERT (SCM_INUMP (tail_array_size), tail_array_size, SCM_ARG3, s_make_vtable_vtable);
SCM_ASSERT (SCM_INUMP (tail_array_size), tail_array_size, SCM_ARG2,
s_make_vtable_vtable);
fields = scm_string_append (scm_listify (required_vtable_fields,
extra_fields,
@ -345,15 +397,7 @@ scm_make_vtable_vtable (extra_fields, tail_array_size, init)
tail_elts = SCM_INUM (tail_array_size);
SCM_NEWCELL (handle);
SCM_DEFER_INTS;
data = (SCM *) scm_must_malloc (sizeof (SCM) * (scm_struct_n_extra_words
+ basic_size
+ tail_elts),
"structure");
data += scm_struct_n_extra_words;
data[scm_struct_i_n_words] = (SCM) (scm_struct_n_extra_words
+ basic_size
+ tail_elts);
data[scm_struct_i_tag] = struct_num++;
data = alloc_struct (basic_size + tail_elts, "make-vtable-vtable");
SCM_SETCDR (handle, data);
SCM_SETCAR (handle, ((SCM)data) + scm_tc3_cons_gloc);
SCM_STRUCT_LAYOUT (handle) = layout;