Hi Paolo,
Thanks for sending this so quickly!
On 5/11/21 6:41 PM, Paolo Bonzini wrote:
> Avoid possible uninitialized variables on machines where
> division by zero does not trap. Add __divmoddi4, and
According to the ARM Architecture Reference Manual for ARMv7-A (ARM DDI 0406C.d),
hardware floating point support is optional (page A2-54), so initializing the
remainder to zero in the case of zero division makes sense.
> do not use 64-bit math unnecessarily in __moddi3 and __divdi3.
>
> Reported-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> lib/ldiv32.c | 28 +++++++++++++++++++++++++---
> 1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/lib/ldiv32.c b/lib/ldiv32.c
> index 96f4b35..c39fccd 100644
> --- a/lib/ldiv32.c
> +++ b/lib/ldiv32.c
> @@ -1,6 +1,7 @@
> #include <stdint.h>
>
> extern uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem);
> +extern int64_t __divmoddi4(int64_t num, int64_t den, int64_t *p_rem);
> extern int64_t __moddi3(int64_t num, int64_t den);
> extern int64_t __divdi3(int64_t num, int64_t den);
> extern uint64_t __udivdi3(uint64_t num, uint64_t den);
> @@ -11,8 +12,11 @@ uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem)
> uint64_t quot = 0;
>
> /* Trigger a division by zero at run time (trick taken from iPXE). */
> - if (den == 0)
> + if (den == 0) {
> + if (p_rem)
> + *p_rem = 0;
> return 1/((unsigned)den);
> + }
>
> if (num >= den) {
> /* Align den to num to avoid wasting time on leftmost zero bits. */
> @@ -35,9 +39,27 @@ uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem)
> return quot;
> }
>
> +int64_t __divmoddi4(int64_t num, int64_t den, int64_t *p_rem)
> +{
> + int32_t nmask = num < 0 ? -1 : 0;
> + int32_t qmask = (num ^ den) < 0 ? -1 : 0;
> + uint64_t quot;
> +
> + /* Compute absolute values and do an unsigned division. */
> + num = (num + nmask) ^ nmask;
> + if (den < 0)
> + den = -den;
> +
> + /* Copy sign of num^den into quotient, sign of num into remainder. */
> + quot = (__divmoddi4(num, den, p_rem) + qmask) ^ qmask;
I see no early return statement in the function, it looks to me like the function
will recurse forever. Maybe you wanted to call here __*u*divmoddi4() (emphasis
added) instead?
Other than that, the function looks correct.
Thanks,
Alex
> + if (p_rem)
> + *p_rem = (*p_rem + nmask) ^ nmask;
> + return quot;
> +}
> +
> int64_t __moddi3(int64_t num, int64_t den)
> {
> - uint64_t mask = num < 0 ? -1 : 0;
> + int32_t mask = num < 0 ? -1 : 0;
>
> /* Compute absolute values and do an unsigned division. */
> num = (num + mask) ^ mask;
> @@ -50,7 +72,7 @@ int64_t __moddi3(int64_t num, int64_t den)
>
> int64_t __divdi3(int64_t num, int64_t den)
> {
> - uint64_t mask = (num ^ den) < 0 ? -1 : 0;
> + int32_t mask = (num ^ den) < 0 ? -1 : 0;
>
> /* Compute absolute values and do an unsigned division. */
> if (num < 0)
