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In 'ash' and 'round-ash', handle right shift count of LONG_MIN.

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Fixes <https://bugs.gnu.org/21901>.
Reported by Zefram <zefram@fysh.org>.

* libguile/numbers.c: Add another top-level 'verify' to ensure that
LONG_MIN is not a fixnum.
(scm_ash, scm_round_ash): Ensure that when the shift count is LONG_MIN,
it is not handled via the normal code path, to avoid signed overflow
when the shift count is negated.
* test-suite/tests/numbers.test: Add tests.
This commit is contained in:
Mark H Weaver 2018-10-14 05:29:52 -04:00 • committed by Andy Wingo
parent e6100f64bb
commit 00973cbd2e
2 changed files with 35 additions and 2 deletions
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13
libguile/numbers.c
View file

@ -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)))

24
test-suite/tests/numbers.test
View file

@ -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