https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99707
Bug ID: 99707
Summary: missing -Woverflow in floating-point to integer
conversion for known but non-constant value
Product: gcc
Version: 10.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: vincent-gcc at vinc17 dot net
Target Milestone: ---
Consider the following code (from PR93806 Comment 29):
#include <stdio.h>
int main (void)
{
volatile double d = -1.0;
double x = d;
unsigned int i = x;
printf ("%u\n", i);
if (x == -1.0)
printf ("%u\n", i);
return 0;
}
First note that if instead of "unsigned int i = x;", one has "unsigned int i =
-1.0;", then one gets a warning with GCC 10.2.1:
conv-warn.c: In function ‘main’:
conv-warn.c:7:20: warning: overflow in conversion from ‘double’ to ‘unsigned
in’ changes value from ‘-1.0e+0’ to ‘0’ [-Woverflow]
7 | unsigned int i = -1.0;
| ^
But the original code does not give any warning, even though with -O1, GCC uses
the fact that x == -1.0 to optimize and give a strange result (because the
variable i appears to have two different values):
4294967295
0
Thus GCC should know about the undefined behavior and warn.
