-fwrapv gives implementation defined behaviour for wrapping variables but not constants.

#include <limits.h>
#include <stdio.h>

int
main()
{
    printf(" LONG_MIN %ld\n", LONG_MIN);
    printf(" LONG_MAX  %ld\n", LONG_MAX);
    printf("-LONG_MAX %ld\n", -LONG_MIN); // line 9

    long lval = LONG_MIN;
    lval = lval * -1; // this is okay with -fwrapv
    printf("wrap lval  %ld\n", lval);
}

[download]
$ gcc -Wall -Wextra -Woverflow -fwrapv -fsanitize=undefined t.c -o t &
+& ./t
t.c: In function ‘main’:
t.c:9:31: warning: integer overflow in expression ‘-922337203685477580
+8’ of type ‘long int’ results in ‘-9223372036854775808’ [-Woverflow]
    9 |     printf("-LONG_MAX %ld\n", -LONG_MIN); // line 9
      |                               ^
 LONG_MIN -9223372036854775808
 LONG_MAX  9223372036854775807
-LONG_MAX -9223372036854775808
wrap lval  9223372036854775807

[download]

I see that the patch fixes it by changing to unsigned int so it seems the wrapping is irrelevant there.

In reply to Re^3: Why does $Config{ccflags} include "-fwrapv" on many gcc builds of perl ? by Anonymous Monk
in thread Why does $Config{ccflags} include "-fwrapv" on many gcc builds of perl ? by syphilis

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