Re: [PATCH 1/2] lib: test_scanf: Fix incorrect use of type_min() with unsigned types

On 24/05/2021 17.59, Richard Fitzgerald wrote:

sparse was producing warnings of the form:

 sparse: cast truncates bits from constant value (ffff0001 becomes 1)

The problem was that value_representable_in_type() compared unsigned types
against type_min(). But type_min() is only valid for signed types because
it is calculating the value -type_max() - 1. 

... and casts that to (T), so it does produce 0 as it should. E.g. for
T==unsigned char, we get

#define type_min(T) ((T)((T)-type_max(T)-(T)1))
(T)((T)-255 - (T)1)
(T)(-256)

which is 0 of type unsigned char.

The minimum value of an

unsigned is obviously 0, so only type_max() need be tested.

That part is true.

But type_min and type_max have been carefully created to produce values
of the appropriate type that actually represent the minimum/maximum
representable in that type, without invoking UB. If this program doesn't
produce the expected results for you, I'd be very interested in knowing
your compiler version:

#include <stdio.h>

#define is_signed_type(type)       (((type)(-1)) < (type)1)
#define __type_half_max(type) ((type)1 << (8*sizeof(type) - 1 -
is_signed_type(type)))
#define type_max(T) ((T)((__type_half_max(T) - 1) + __type_half_max(T)))
#define type_min(T) ((T)((T)-type_max(T)-(T)1))

int main(int argc, char *argv[])
{
#define p(T, PT, fmt) do {					\
		PT vmin = type_min(T);				\
		PT vmax = type_max(T);				\
		printf("min(%s) = "fmt", max(%s) = "fmt"\n",#T, vmin, #T, vmax); \
	} while (0)

	p(_Bool, int, "%d");
	p(unsigned char, int, "%d");
	p(signed char, int, "%d");
	p(unsigned int, unsigned int, "%u");
	p(unsigned long long, unsigned long long, "%llu");
	p(signed long long, signed long long, "%lld");
	
	return 0;
}

With or without this, these tests are tautological when T is "long long"
or "unsigned long long". I don't know if that is intended. But it won't,
say, exclude ~0ULL if that is in the numbers[] array from being treated
as fitting in a "long long".

Rasmus