Fix strtold on 32-bit sparc (and probably others) (BZ #16965)
Commit Message
This patch fixes an issue observed running the tst-strtod-round test on
32 bit sparc. In some conditions, strtold calls round_and_return, which in
turn calls __mpn_rshift with cnt = 0, while stdlib/rshift.c explicitly says
that cnts should satisfy 0 < CNT < BITS_PER_MP_LIMB. In this case, the code
end up doing a logical shift right of the same amount than the register,
which is undefined in the C standard.
Due to this bug, 32-bit sparc does not correctly convert the value
"0x1p-16446", but it is likely that other architectures are also
affected for other input values.
---
ChangeLog | 6 ++++++
stdlib/strtod_l.c | 11 ++++++++---
2 files changed, 14 insertions(+), 3 deletions(-)
Comments
On Tue, 20 May 2014, Aurelien Jarno wrote:
> This patch fixes an issue observed running the tst-strtod-round test on
> 32 bit sparc. In some conditions, strtold calls round_and_return, which in
> turn calls __mpn_rshift with cnt = 0, while stdlib/rshift.c explicitly says
> that cnts should satisfy 0 < CNT < BITS_PER_MP_LIMB. In this case, the code
> end up doing a logical shift right of the same amount than the register,
> which is undefined in the C standard.
OK (presuming you at least ran the stdlib tests on 32-bit sparc, and that
you include the usual NEWS update and bug closing when committing).
On Tue, May 20, 2014 at 02:34:14PM +0000, Joseph S. Myers wrote:
> On Tue, 20 May 2014, Aurelien Jarno wrote:
>
> > This patch fixes an issue observed running the tst-strtod-round test on
> > 32 bit sparc. In some conditions, strtold calls round_and_return, which in
> > turn calls __mpn_rshift with cnt = 0, while stdlib/rshift.c explicitly says
> > that cnts should satisfy 0 < CNT < BITS_PER_MP_LIMB. In this case, the code
> > end up doing a logical shift right of the same amount than the register,
> > which is undefined in the C standard.
>
> OK (presuming you at least ran the stdlib tests on 32-bit sparc, and that
> you include the usual NEWS update and bug closing when committing).
Yes I ran the full tests on both 32-bit sparc and x86-64, I haven't
seen any regression, and a progression for tst-strtod-round on sparc.
Thanks for the review.
@@ -1,3 +1,9 @@
+2014-05-20 Aurelien Jarno <aurelien@aurel32.net>
+
+ [BZ #16965]
+ * stdlib/strtod_l.c (round_and_return): Add code to shift limbs
+ when the shift amount is modulo the limb size.
+
2014-05-20 Will Newton <will.newton@linaro.org>
* sysdeps/unix/sysv/linux/aarch64/nptl/sysdep-cancel.h (PSEUDO):
@@ -243,9 +243,14 @@ round_and_return (mp_limb_t *retval, intmax_t exponent, int negative,
more_bits |= ((round_limb & ((((mp_limb_t) 1) << round_bit) - 1))
!= 0);
- (void) __mpn_rshift (retval, &retval[shift / BITS_PER_MP_LIMB],
- RETURN_LIMB_SIZE - (shift / BITS_PER_MP_LIMB),
- shift % BITS_PER_MP_LIMB);
+ /* __mpn_rshift requires 0 < shift < BITS_PER_MP_LIMB. */
+ if ((shift % BITS_PER_MP_LIMB) != 0)
+ (void) __mpn_rshift (retval, &retval[shift / BITS_PER_MP_LIMB],
+ RETURN_LIMB_SIZE - (shift / BITS_PER_MP_LIMB),
+ shift % BITS_PER_MP_LIMB);
+ else
+ for (i = 0; i < RETURN_LIMB_SIZE - (shift / BITS_PER_MP_LIMB); i++)
+ retval[i] = retval[i + (shift / BITS_PER_MP_LIMB)];
MPN_ZERO (&retval[RETURN_LIMB_SIZE - (shift / BITS_PER_MP_LIMB)],
shift / BITS_PER_MP_LIMB);
}