Message ID | 20210511174106.703235-2-pbonzini@redhat.com (mailing list archive) |
---|---|
State | New, archived |
Headers | show |
Series | fix long division routines for ARM eabi | expand |
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@arm.com> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> > --- > 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)
On 12/05/21 12:39, Alexandru Elisei wrote: >> + >> + /* 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? Of course... Paolo
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; + 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)
Avoid possible uninitialized variables on machines where division by zero does not trap. Add __divmoddi4, and do not use 64-bit math unnecessarily in __moddi3 and __divdi3. Reported-by: Alexandru Elisei <alexandru.elisei@arm.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- lib/ldiv32.c | 28 +++++++++++++++++++++++++--- 1 file changed, 25 insertions(+), 3 deletions(-)