Re: The relationship between SCM and scm_t_bits.

[Dirk Herrmann <dirk@dirk-herrmanns-seiten.de>]

Dirk Herrmann wrote:

>  I have not yet given it a try, but I found the suggestion to use a
>  union quite appaling:
 > [some syntactically incorrect examples deleted :-)]

However, I have now given it a try, and it seems to work quite nicely.
I enclose a patch, which I would be glad to apply, if you, Marius, agree.
I did not do it yet, though, in order to give everybody the chance to
check whether it has any negative influence on performance.

Best regards,
Dirk


Diff agains the latest CVS:

Index: NEWS
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/NEWS,v
retrieving revision 1.427
diff -u -b -r1.427 NEWS
--- NEWS    6 May 2004 17:04:04 -0000    1.427
+++ NEWS    15 May 2004 16:02:26 -0000
@@ -584,16 +584,6 @@
 
 * Changes to the C interface
 
-** SCM_CELL_WORD_LOC has been deprecated.
-
-Use the new macro SCM_CELL_OBJECT_LOC instead, which return a pointer
-to a SCM, as opposed to a pointer to a scm_t_bits.
-
-This was done to allow the correct use of pointers into the Scheme
-heap.  Previously, the heap words were of type scm_t_bits and local
-variables and function arguments were of type SCM, making it
-non-standards-conformant to have a pointer that can point to both.
-
 ** New macros SCM_SMOB_DATA_2, SM_SMOB_DATA_3, etc.
 
 These macros should be used instead of SCM_CELL_WORD_2/3 to access the
Index: libguile/ChangeLog
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/ChangeLog,v
retrieving revision 1.2070
diff -u -b -r1.2070 ChangeLog
--- libguile/ChangeLog    15 May 2004 15:50:24 -0000    1.2070
+++ libguile/ChangeLog    15 May 2004 16:02:26 -0000
@@ -1,5 +1,23 @@
 2004-05-15  Dirk Herrmann  <dirk@dirk-herrmanns-seiten.de>
 
+    * deprecated.h (SCM_CELL_WORD_LOC): Un-deprecated.
+
+    * gc.h (scm_t_cell): Redefined to hold a union, since each cell
+    element either holds a scm_t_bits value or a SCM value.
+
+    (SCM_GC_CARD_BVEC, SCM_GC_SET_CARD_BVEC, SCM_GC_GET_CARD_FLAGS,
+    SCM_GC_SET_CARD_FLAGS, SCM_GC_CELL_OBJECT, SCM_GC_CELL_WORD,
+    SCM_GC_SET_CELL_OBJECT, SCM_GC_SET_CELL_WORD): Modified to work
+    with the new scm_t_cell.
+
+    (SCM_CELL_WORD_LOC): Un-deprecated.
+
+    * numbers.h (SCM_I_BIG_MPZ): Use SCM_CELL_WORD_LOC instead of
+    SCM_CELL_OBJECT_LOC, since we are not dealing with scheme objects
+    here.
+
+2004-05-15  Dirk Herrmann  <dirk@dirk-herrmanns-seiten.de>
+
     * numbers.h (SCM_SLOPPY_FRACTIONP): Removed.  It was not used
     throughout guile, has not been part of an official release yet,
     and the concept of sloppy predicates has never been a good idea.
Index: libguile/deprecated.h
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/deprecated.h,v
retrieving revision 1.19
diff -u -b -r1.19 deprecated.h
--- libguile/deprecated.h    6 May 2004 16:42:50 -0000    1.19
+++ libguile/deprecated.h    15 May 2004 16:02:26 -0000
@@ -300,11 +300,6 @@
 
 #define SCM_VALIDATE_OPDIR(pos, port) SCM_MAKE_VALIDATE (pos, port, OPDIRP)
 
-/* Deprecated because we can not safely cast a SCM* to a scm_t_bits*
- */
-
-#define SCM_CELL_WORD_LOC(x, n)   
((scm_t_bits*)SCM_CELL_OBJECT_LOC((x),(n)))
-
 void scm_i_init_deprecated (void);
 
 #endif
Index: libguile/gc.h
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/gc.h,v
retrieving revision 1.116
diff -u -b -r1.116 gc.h
--- libguile/gc.h    6 May 2004 16:41:08 -0000    1.116
+++ libguile/gc.h    15 May 2004 16:02:26 -0000
@@ -36,8 +36,10 @@
 
 typedef struct scm_t_cell
 {
-  SCM word_0;
-  SCM word_1;
+  union {
+    scm_t_bits word;
+    SCM object;
+  } elements[2];
 } scm_t_cell;
 
 /*
@@ -73,12 +75,14 @@
 #define SCM_GC_CARD_N_CELLS        256
 #define SCM_GC_SIZEOF_CARD         SCM_GC_CARD_N_CELLS * sizeof 
(scm_t_cell)
 
-#define SCM_GC_CARD_BVEC(card)  ((scm_t_c_bvec_long *) ((card)->word_0))
+#define SCM_GC_CARD_BVEC(card) \
+  ((scm_t_c_bvec_long *) ((card)->elements[0].word))
 #define SCM_GC_SET_CARD_BVEC(card, bvec) \
-    ((card)->word_0 = (SCM) (bvec))
-#define SCM_GC_GET_CARD_FLAGS(card) ((long) ((card)->word_1))
+  ((card)->elements[0].word = (scm_t_bits) (bvec))
+#define SCM_GC_GET_CARD_FLAGS(card) \
+  ((long) ((card)->elements[1].word))
 #define SCM_GC_SET_CARD_FLAGS(card, flags) \
-    ((card)->word_1 = (SCM) (flags))
+  ((card)->elements[1].word = (flags))
 
 #define SCM_GC_GET_CARD_FLAG(card, shift) \
  (SCM_GC_GET_CARD_FLAGS (card) & (1L << (shift)))
@@ -141,12 +145,13 @@
  * in debug mode.  In particular these macros will even work for free 
cells,
  * which should never be encountered by user code.  */
 
-#define SCM_GC_CELL_OBJECT(x, n) (((SCM *)SCM2PTR (x)) [n])
-#define SCM_GC_CELL_WORD(x, n)   (SCM_UNPACK (SCM_GC_CELL_OBJECT ((x), 
(n))))
+#define SCM_GC_CELL_WORD(x, n) (((SCM2PTR (x))->elements[n]).word)
+#define SCM_GC_CELL_OBJECT(x, n) (((SCM2PTR (x))->elements[n]).object)
 
-#define SCM_GC_SET_CELL_OBJECT(x, n, v) ((((SCM *)SCM2PTR (x)) [n]) = (v))
+#define SCM_GC_SET_CELL_OBJECT(x, n, v) \
+  (((SCM2PTR (x))->elements[n]).object = (v))
 #define SCM_GC_SET_CELL_WORD(x, n, v)  \
-  (SCM_GC_SET_CELL_OBJECT ((x), (n), SCM_PACK (v)))
+  (((SCM2PTR (x))->elements[n]).word = (v))
 
 #define SCM_GC_CELL_TYPE(x) (SCM_GC_CELL_OBJECT ((x), 0))
 
@@ -214,6 +219,7 @@
   (SCM_GC_SET_CELL_OBJECT ((x), 1, (v)))
 
 
+#define SCM_CELL_WORD_LOC(x, n)   (&SCM_CELL_WORD ((x), (n)))
 #define SCM_CELL_OBJECT_LOC(x, n) (&SCM_CELL_OBJECT ((x), (n)))
 #define SCM_CARLOC(x)             (SCM_CELL_OBJECT_LOC ((x), 0))
 #define SCM_CDRLOC(x)             (SCM_CELL_OBJECT_LOC ((x), 1))
Index: libguile/numbers.h
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/numbers.h,v
retrieving revision 1.82
diff -u -b -r1.82 numbers.h
--- libguile/numbers.h    15 May 2004 15:50:24 -0000    1.82
+++ libguile/numbers.h    15 May 2004 16:02:26 -0000
@@ -144,7 +144,7 @@
 #define SCM_COMPLEX_IMAG(x) (SCM_COMPLEX_MEM (x)->imag)
 
 /* Each bignum is just an mpz_t stored in a double cell starting at 
word 1. */
-#define SCM_I_BIG_MPZ(x) (*((mpz_t *) (SCM_CELL_OBJECT_LOC((x),1))))
+#define SCM_I_BIG_MPZ(x) (*((mpz_t *) (SCM_CELL_WORD_LOC ((x), 1))))
 #define SCM_BIGP(x) (!SCM_IMP (x) && SCM_TYP16 (x) == scm_tc16_big)
 
 #define SCM_NUMBERP(x) (SCM_INUMP(x) || SCM_NUMP(x))




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