c - Type casting: double to char: multiple questions

Question

Consider this code:

#include <stdio.h>

int main(void) 
{
    /* TEST 1 */
    double d = 128;
    char ch = (char)d;
    printf("%d
", ch);

    /* TEST 2 */
    printf("%d
", (char)128.0);

    /* TEST 3 */
    char ch1 = (char)128.0;
    printf("%d
", ch1);
    return 0;
}

Results:

        gcc*  clang*  cl*
TEST 1  -128  -128    -128
TEST 2  127   0       -128
TEST 3  127   -2      -128

* latest version

Questions:

1. Why the results differ between tests (excluding cl)?
2. Why the results differ between compilers (excluding TEST 1)?
3. In case of UB/IB, where is the UB/IB exactly? What the standard says?
4. [Extra question] Why clang shows so different behavior? Where these 0 and -2 come from?

Answer 1

When CHAR_MAX == 127, (char)128.0 is undefined behavior (UB).

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined. C17dr § 6.3.1.4 1

This is not UB due to integer overflow. It is UB due to conversion rules.