Interesting GCC extensions

[Lutger]

Lutger wrote:

> bearophile wrote:
> 
>> Lutger:
>> 
>>> We don't need an extension for this! Look:
>>> 
>>> template Eval(string exp)
>>> {
>>>     enum Eval = mixin(exp);
>>> }
>>> 
>>> template IsConstant(string exp)
>>> {
>>>     enum IsConstant = __traits(compiles, Eval!exp);
>>> }
>> 
>> 
>> From what I see I think your code is useless for my purposes:
>> 
>> // D2 code
>> import std.stdio: writeln;
>> 
>> template Eval(string exp) {
>>     enum Eval = mixin(exp);
>> }
>> 
>> template IsConstant(string exp) {
>>     enum IsConstant = __traits(compiles, Eval!exp);
>> }
>> 
>> template Tuple(T...) { alias T Tuple; }
>> 
>> template Range(int stop) {
>>     static if (stop <= 0)
>>         alias Tuple!() Range;
>>     else
>>         alias Tuple!(Range!(stop-1), stop-1) Range;
>> }
>> 
>> long ipow(long x, int n) {
>>     long result = 1;
>> 
>>     static if (IsConstant!("n")) {
>>         pragma(msg, "constant");
>>         static if (n < 5) {
>>             foreach (i; Range!(n))
>>                 result *= x;
>>         } else {
>>             for (int i; i < n; i++)
>>                 result *= x;
>>         }
>>     } else {
>>         pragma(msg, "not constant");
>>         for (int i; i < n; i++)
>>             result *= x;
>>     }
>> 
>>     return result;
>> }
>> 
>> enum int w = 5;
>> void main() {
>>     int x = 2;
>>     const int y = 3;
>>     enum int z = 4;
>>     writeln(IsConstant!("x"), " ", IsConstant!("y"),
>>             " ", IsConstant!("z"), " ", IsConstant!("w"));
>>     // prints: false false false true
>> 
>>     ipow(10, x);
>>     ipow(10, y);
>>     ipow(10, z);
>> }
>> 
>> Even "z" is seen as not constant?
>> 
>> Bye,
>> bearophile
> 
> They are not compile time constants. Have you tried it with GCC? It gives
> the exact same results.

Actually not for z, I don't know why this doesn't work with dmd. It does say 
it is a constant.