newCTFE Status November 2018

[Nathan S.]

On Saturday, 8 December 2018 at 15:56:46 UTC, kinke wrote:
> On Saturday, 8 December 2018 at 07:08:54 UTC, Stefan Koch wrote:
>> On Saturday, 8 December 2018 at 04:06:11 UTC, Joakim wrote:
>>> Btw, do you know some way to get the current compile-time 
>>> real's precision in D code?
>>
>> this _may_ work:
>> static immutable ctRealPerc = () { return real.mant_dig; } ();
>
> Nope, these FP type properties represent the target type's. 
> There's just no way of knowing the compile-time precision at 
> the moment, at least for LDC (DMD is x86 only and currently 
> always uses a x87 `real_t`). For LDC, it's the host `real` 
> precision, except for MSVC hosts, where it uses x87 (Rainer's 
> custom implementation in dmd.root.longdouble).

This infallibly determines the precision used for calculations 
involving `real` in CTFE:

----
enum ctfeRealExponentBits =
() {
     real f = 2;
     foreach (i; 1 .. 2000)
     {
         real g = f * f;
         if (g == f)
             return i;
         f = g;
     }
     assert(0, "algorithm failure");
}();

enum ctfeMantissaBits =
() {
     real a = 1;
     real b = 0.5;
     int bits = 0;
     while (a + b != a)
     {
         bits = bits + 1;
         b = b / 2;
     }
     // Result matches what would be reported by mant_dig
     // (1 greater than the actual number of mantissa bits
     // except for 80-bit extended precision).
     return bits + 1;
}();
----