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?
#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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