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-rw-r--r--libguile/numbers.c13
-rw-r--r--test-suite/tests/numbers.test24
2 files changed, 35 insertions, 2 deletions
diff --git a/libguile/numbers.c b/libguile/numbers.c
index 21dcdb62c..a11ed3a8c 100644
--- a/libguile/numbers.c
+++ b/libguile/numbers.c
@@ -5053,6 +5053,11 @@ round_right_shift_exact_integer (SCM n, long count)
assert (0);
}
+/* 'scm_ash' and 'scm_round_ash' assume that fixnums fit within a long,
+ and moreover that they can be negated without overflow. */
+verify (SCM_MOST_NEGATIVE_FIXNUM >= LONG_MIN + 1
+ && SCM_MOST_POSITIVE_FIXNUM <= LONG_MAX);
+
SCM_DEFINE (scm_ash, "ash", 2, 0, 0,
(SCM n, SCM count),
"Return @math{floor(@var{n} * 2^@var{count})}.\n"
@@ -5078,7 +5083,9 @@ SCM_DEFINE (scm_ash, "ash", 2, 0, 0,
if (SCM_I_INUMP (count)) /* fast path, not strictly needed */
bits_to_shift = SCM_I_INUM (count);
- else if (scm_is_signed_integer (count, LONG_MIN, LONG_MAX))
+ else if (scm_is_signed_integer (count, LONG_MIN + 1, LONG_MAX))
+ /* We exclude LONG_MIN to ensure that 'bits_to_shift' can be
+ negated without overflowing. */
bits_to_shift = scm_to_long (count);
else if (scm_is_false (scm_positive_p (scm_sum (scm_integer_length (n),
count))))
@@ -5130,7 +5137,9 @@ SCM_DEFINE (scm_round_ash, "round-ash", 2, 0, 0,
if (SCM_I_INUMP (count)) /* fast path, not strictly needed */
bits_to_shift = SCM_I_INUM (count);
- else if (scm_is_signed_integer (count, LONG_MIN, LONG_MAX))
+ else if (scm_is_signed_integer (count, LONG_MIN + 1, LONG_MAX))
+ /* We exclude LONG_MIN to ensure that 'bits_to_shift' can be
+ negated without overflowing. */
bits_to_shift = scm_to_long (count);
else if (scm_is_true (scm_negative_p (scm_sum (scm_integer_length (n),
count)))
diff --git a/test-suite/tests/numbers.test b/test-suite/tests/numbers.test
index 8cecb06ad..59e370ec9 100644
--- a/test-suite/tests/numbers.test
+++ b/test-suite/tests/numbers.test
@@ -5390,11 +5390,19 @@
(for-each (lambda (n)
(for-each (lambda (count) (test n count))
`(-1000
+ ,(* 2 (- fixnum-bit))
+ ,(- -3 fixnum-bit)
+ ,(- -2 fixnum-bit)
+ ,(- -1 fixnum-bit)
,(- fixnum-bit)
,(- (- fixnum-bit 1))
-3 -2 -1 0 1 2 3
,(- fixnum-bit 1)
,fixnum-bit
+ ,(+ fixnum-bit 1)
+ ,(+ fixnum-bit 2)
+ ,(+ fixnum-bit 3)
+ ,(* 2 fixnum-bit)
1000)))
(list 0 1 3 23 -1 -3 -23
fixnum-max
@@ -5423,6 +5431,22 @@
'(#b11001 #b11100 #b11101 #b10001 #b10100 #b10101)))
(list 0 64 -64 (* 64 fixnum-max) (* 64 fixnum-min)))
+ ;; Right shift by LONG_MIN, typically (ash -1 63) and (ash -1 31)
+ ;; depending on the word size, where negating the shift count
+ ;; overflows. See <https://bugs.gnu.org/21901>.
+ (pass-if-equal "Right shift of positive integer by (ash -1 63) bits"
+ 0
+ (ash-variant 123 (ash -1 63)))
+ (pass-if-equal "Right shift of negative integer by (ash -1 63) bits"
+ (if rounded? 0 -1)
+ (ash-variant -123 (ash -1 63)))
+ (pass-if-equal "Right shift of positive integer by (ash -1 31) bits"
+ 0
+ (ash-variant 123 (ash -1 31)))
+ (pass-if-equal "Right shift of negative integer by (ash -1 31) bits"
+ (if rounded? 0 -1)
+ (ash-variant -123 (ash -1 31)))
+
;; Huge shift counts
(pass-if-equal "Huge left shift of 0"
0