On 8/24/15 2:06 PM, rumbu wrote:
BTW, 1.2 and 12.0 are directly representable as double
In C++:
printf("%.20f\r\n", 1.2);
printf("%.20f\r\n", 12.0);
will output:
1.20000000000000000000
12.00000000000000000000
Either upcasting to real is the wrong decision here, either the writeln
string conversion is wrong.
I don't think they are directly representable as floating point, because
they are have factors other than 2 in the decimal portion. From my
understanding, anything that only has to do with powers of 2 are
representable in floating point, just like you cannot represent 1/3 in
decimal exactly.
But there is definitely something weird going on with the casting.
I wrote this program:
testfp.d:
extern(C) void foo(double x);
void main() {
double x = 1.2;
foo(x);
}
testfp2.d:
extern(C) void foo(double x)
{
import std.stdio;
writeln(cast(ulong)(x * 10.0));
}
testfp2.c:
#include <stdio.h>
void foo(double x)
{
printf("%lld\n", (unsigned long long)(x * 10));
}
If I link testfp.d against testfp2.c, then it outputs 12. If I link
against testfp2.d, it outputs 11.
I have faith that printf and writeln properly output ulongs. Something
different happens with the cast. There can be no constant folding
operations or optimizations going on here, as this is done via separate
compilation. I'll re-open the bug report.
-Steve
