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SCM_SRS: Improve fallback implemention to avoid unspecified behavior.

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* libguile/numbers.h (SCM_SRS): Rewrite preprocessor test to avoid
  left-shifting negative integers, and to test more comprehensively for
  the behavior we need.  Rewrite fallback implementation to avoid
  unspecified behavior.
This commit is contained in:
Mark H Weaver 2014-03-11 21:27:21 -04:00
parent 7f8ad91b99
commit e293c94c65
1 changed files with 14 additions and 5 deletions
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17
libguile/numbers.h
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@ -49,12 +49,21 @@ 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)
/* 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
#define SCM_I_INUMP(x) (2 & SCM_UNPACK (x))