X-Git-Url: https://git.hcoop.net/bpt/guile.git/blobdiff_plain/ba578eb04460833c5acb1d8f4c08d2facde15e80..eb3d623da57e6d31a58d95f932345fb761f9b701:/libguile/numbers.h

diff --git a/libguile/numbers.h b/libguile/numbers.h
index 6e382ea35..bba336bd4 100644
--- a/libguile/numbers.h
+++ b/libguile/numbers.h
@@ -4,7 +4,7 @@
 #define SCM_NUMBERS_H
 
 /* Copyright (C) 1995,1996,1998,2000,2001,2002,2003,2004,2005, 2006,
- *   2008, 2009, 2010, 2011, 2013 Free Software Foundation, Inc.
+ *   2008, 2009, 2010, 2011, 2013, 2014 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -49,19 +49,43 @@ typedef scm_t_int32 scm_t_wchar;
 #define SCM_MOST_POSITIVE_FIXNUM ((SCM_T_SIGNED_BITS_MAX-3)/4)
 #define SCM_MOST_NEGATIVE_FIXNUM (-SCM_MOST_POSITIVE_FIXNUM-1)
 
-/* SCM_SRS is signed right shift */
-#if (-1 == (((-1) << 2) + 2) >> 2)
-# define SCM_SRS(x, y) ((x) >> (y))
+/* SCM_SRS (X, Y) is signed right shift, defined as floor (X / 2^Y),
+   where Y must be non-negative and less than the width in bits of X.
+   It's common for >> to do this, but the C standards do not specify
+   what happens when X is negative.
+
+   NOTE: X must not perform side effects.  */
+#if (-1 >> 2 == -1) && (-4 >> 2 == -1) && (-5 >> 2 == -2) && (-8 >> 2 == -2)
+# define SCM_SRS(x, y)  ((x) >> (y))
 #else
-# define SCM_SRS(x, y) ((x) < 0 ? ~((~(x)) >> (y)) : ((x) >> (y)))
-#endif /* (-1 == (((-1) << 2) + 2) >> 2) */
+# define SCM_SRS(x, y)                                   \
+  ((x) < 0                                               \
+   ? -1 - (scm_t_signed_bits) (~(scm_t_bits)(x) >> (y))  \
+   : ((x) >> (y)))
+#endif
+
 
+/* The first implementation of SCM_I_INUM below depends on behavior that
+   is specified by GNU C but not by C standards, namely that when
+   casting to a signed integer of width N, the value is reduced modulo
+   2^N to be within range of the type.  The second implementation below
+   should be portable to all conforming C implementations, but may be
+   less efficient if the compiler is not sufficiently clever.
+
+   NOTE: X must not perform side effects.  */
+#ifdef __GNUC__
+# define SCM_I_INUM(x)  (SCM_SRS ((scm_t_signed_bits) SCM_UNPACK (x), 2))
+#else
+# define SCM_I_INUM(x)                                \
+  (SCM_UNPACK (x) > LONG_MAX                          \
+   ? -1 - (scm_t_signed_bits) (~SCM_UNPACK (x) >> 2)  \
+   : (scm_t_signed_bits) (SCM_UNPACK (x) >> 2))
+#endif
 
 #define SCM_I_INUMP(x)	(2 & SCM_UNPACK (x))
 #define SCM_I_NINUMP(x) (!SCM_I_INUMP (x))
 #define SCM_I_MAKINUM(x) \
-  (SCM_PACK ((((scm_t_signed_bits) (x)) << 2) + scm_tc2_int))
-#define SCM_I_INUM(x)   (SCM_SRS ((scm_t_signed_bits) SCM_UNPACK (x), 2))
+  (SCM_PACK ((((scm_t_bits) (x)) << 2) + scm_tc2_int))
 
 /* SCM_FIXABLE is true if its long argument can be encoded in an SCM_INUM. */
 #define SCM_POSFIXABLE(n) ((n) <= SCM_MOST_POSITIVE_FIXNUM)