On Wed, Jun 27, 2018 at 12:10:12PM -0600, Jason Gunthorpe wrote:
> On Wed, Jun 27, 2018 at 11:36:03AM +0200, Rasmus Villemoes wrote:
> > OK. The requirement of everything having the same type for the
> > check_*_overflow when gccs builtins are not available was mostly a
> > consequence of my inability to implement completely type-generic
> > versions (but also to enforce some sanity, so people don't do
> > check_add_overflow( s8, size_t, int*)). There's no gcc builtin for
> > shift, but if it's relatively simple to one allowing a and *d to have
> > different types, then why not. It's of course particularly convenient
> > to allow a bare "1" (i.e. int) as a while having *d have some random
> > type.
>
> Yes
>
> > Wouldn't check_shift_overflow(-1, 4, &someint) just put -16 in someint
> > and report no overflow? That's what I'd expect, if negative values are
> > to be supported at all.
>
> I would say that is not a desired outcome, bitshift is defined on
> bits, if the caller wanted something defined as signed multiply they
> should use multiply.
>
> IMHO, nobody writes 'a << b' expecting sign preservation..
>
> > Well, the types you can check at compile-time, the values not, so you
> > still have to define the result, i.e. contents of *d, for negative
> > values (even if we decide that "overflow" should always be signalled in
> > that case).
>
> Why do a need to define a 'result' beyond whatever the not-undefined
> behavior shift expression produces?
>
> > What about more like this?
> > check_shift_overflow(a, s, d) ({
> > // Shift is always performed on the machine's largest
> > unsigned
> > u64 _a = a;
> > typeof(s) _s = s;
> > typeof(d) _d = d;
> > // Make s safe against UB
> > unsigned int _to_shift = _s >= 0 && _s < 8*sizeof(*d) : _s ? 0;
> > *_d = (_a << _to_shift);
> > // s is malformed
> > (_to_shift != _s ||
> > // d is a signed type and became negative
> > *_d < 0 ||
> > // a is a signed type and was negative
> > _a < 0 ||
> > // Not invertable means a was truncated during
> > shifting
> > (*_d >> _to_shift) != a))
> > })
> > I'm not seeing a UB with this?
> >
> > Something like that might work, but you're not there yet. In
> > particular, your test for whether a is negative is thwarted by using
> > u64 for _a and testing _a < 0...
>
> Oops, yes that was intended to be 'a', and of course we need to
> capture it..
>
> Leon? Seems like agreement, Can you work with this version?
Yes, sure, I waited for an agreement.
>
> #include <stdint.h>
> #include <stdbool.h>
> #include <assert.h>
>
> #define u64 uint64_t
>
> /*
> * Compute *d = (a << s)
> *
> * Returns true if '*d' cannot hold the result or 'a << s' doesn't make sense.
> * - 'a << s' causes bits to be lost when stored in d
> * - 's' is garbage (eg negative) or so large that a << s is guarenteed to be 0
> * - 'a' is negative
> * - 'a << s' sets the sign bit, if any, in '*d'
> * *d is not defined if false is returned.
> */
> #define check_shift_overflow(a, s, d) \
> ({ \
> typeof(a) _a = a; \
> typeof(s) _s = s; \
> typeof(d) _d = d; \
> u64 _a_full = _a; \
> unsigned int _to_shift = \
> _s >= 0 && _s < 8 * sizeof(*d) ? _s : 0; \
> \
> *_d = (_a_full << _to_shift); \
> \
> (_to_shift != _s || *_d < 0 || _a < 0 || \
> (*_d >> _to_shift) != a); \
> })
>
> int main(int argc, const char *argv[])
> {
> int32_t s32;
> uint32_t u32;
>
> assert(check_shift_overflow(1, 0, &s32) == false && s32 == (1 << 0));
> assert(check_shift_overflow(1, 1, &s32) == false && s32 == (1 << 1));
> assert(check_shift_overflow(1, 30, &s32) == false && s32 == (1 << 30));
> assert(check_shift_overflow(1, 31, &s32) == true);
> assert(check_shift_overflow(1, 32, &s32) == true);
> assert(check_shift_overflow(-1, 1, &s32) == true);
> assert(check_shift_overflow(-1, 0, &s32) == true);
>
> assert(check_shift_overflow(1, 0, &u32) == false && u32 == (1 << 0));
> assert(check_shift_overflow(1, 1, &u32) == false && u32 == (1 << 1));
> assert(check_shift_overflow(1, 30, &u32) == false && u32 == (1 << 30));
> assert(check_shift_overflow(1, 31, &u32) == false && u32 == (1UL << 31));
> assert(check_shift_overflow(1, 32, &u32) == true);
> assert(check_shift_overflow(-1, 1, &u32) == true);
> assert(check_shift_overflow(-1, 0, &u32) == true);
>
> assert(check_shift_overflow(0xFFFFFFFF, 0, &u32) == false && u32 == (0xFFFFFFFFUL << 0));
> assert(check_shift_overflow(0xFFFFFFFF, 1, &u32) == true);
> assert(check_shift_overflow(0xFFFFFFFF, 0, &s32) == true);
> assert(check_shift_overflow(0xFFFFFFFF, 1, &s32) == true);
> }
>
> Thanks,
> Jason
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