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June 10, 2012
Is it possible to force CTFE?
Three related questions:

1) Is there a way to force a function to be always executed at 
compile time (when it's possible to do so) no matter what context 
it's called in?

2) Is it possible to specialize a function based on whether or 
not the parameter that was passed in is a compile time constant?

3) Does any D compiler currently optimize out a conditional 
branch which _can_ be evaluated at compile time (but which isn't 
forced into CTFE)? Like:

int getValue(bool b)
{
    return b ? 123 : 456;
}

//...
   auto value = getValue(true);
June 10, 2012
Re: Is it possible to force CTFE?
> 1) Is there a way to force a function to be always executed at 
> compile time (when it's possible to do so) no matter what 
> context it's called in?

No, but you could wrap it in a template to force it to always
execute at compile time. Of course it could then only be called
at compile time.

> 2) Is it possible to specialize a function based on whether or 
> not the parameter that was passed in is a compile time constant?

No.

> 3) Does any D compiler currently optimize out a conditional 
> branch which _can_ be evaluated at compile time (but which 
> isn't forced into CTFE)? Like:
>
> int getValue(bool b)
> {
>     return b ? 123 : 456;
> }
>
> //...
>    auto value = getValue(true);

DMD, LDC and GDC all do this when compiling with optimizations
turned on. They all compile these functions to exactly the
same code:

auto foo()
{
    return getValue(true);
}

auto bar()
{
    return 123;
}
June 10, 2012
Re: Is it possible to force CTFE?
On 06/10/2012 09:04 AM, Tommi wrote:
> Three related questions:
>
> 1) Is there a way to force a function to be always executed at compile
> time (when it's possible to do so) no matter what context it's called in?
>

No there is not. You could use a template that calls a private function 
at compile time instead. What is your use case?

> 2) Is it possible to specialize a function based on whether or not the
> parameter that was passed in is a compile time constant?
>

This has been discussed before, but there is not.


1-2) could be introduced later when D gets AST macros.

> 3) Does any D compiler currently optimize out a conditional branch which
> _can_ be evaluated at compile time (but which isn't forced into CTFE)?
> Like:
>
> int getValue(bool b)
> {
>      return b ? 123 : 456;
> }
>
> //...
>     auto value = getValue(true);

Yes, DMD/GDC/LDC should be able to do this to different extents.
June 10, 2012
Re: Is it possible to force CTFE?
On Sunday, 10 June 2012 at 10:23:09 UTC, Timon Gehr wrote:
> No there is not. You could use a template that calls a private 
> function at compile time instead. What is your use case?

I was just thinking about a situation where a property 
accessor/mutator methods are not as simple as read/assign value, 
such as in this silly example:

struct Flipping123
{
    private int m_number = 123;

    @property bool isPositive()
    {
        return m_number >= 0;
    }

    @property void isPositive(bool b)
    {
        m_number = b ? 123 : -123;
    }
}

//...
    Flipping123 fl;
    fl.isPositive = false; // I'd rather not have cond. branching 
in release mode
September 27, 2012
Re: Is it possible to force CTFE?
On Sunday, 10 June 2012 at 10:16:23 UTC, jerro wrote:
>
> No, but you could wrap it in a template to force it to always
> execute at compile time.

So, I just realized, I could have just this one convenience 
template that I can use whenever I want to force an expression to 
be evaluated at compile-time. Like so:

template ct(alias expr)
{
     enum ct = expr;
}

int fun(int a, int b)
{
     return a + b;
}

//... and use it like:

ct!(fun(1, 2))

That's not *too* inconvenient. Although, best would be a function 
attribute that would force the compiler to apply ctfe 
aggressively whenever it can with calls to that function.
September 27, 2012
Re: Is it possible to force CTFE?
Tommi:

> 2) Is it possible to specialize a function based on whether or 
> not the parameter that was passed in is a compile time constant?

I am interested in this since some years. I think it's useful, 
but I don't know if it can be implemented. I don't remember 
people discussing about this much.

Bye,
bearophile
September 27, 2012
Re: Is it possible to force CTFE?
On 2012-09-27 15:01, bearophile wrote:
> Tommi:
>
>> 2) Is it possible to specialize a function based on whether or not the
>> parameter that was passed in is a compile time constant?
>
> I am interested in this since some years. I think it's useful, but I
> don't know if it can be implemented. I don't remember people discussing
> about this much.

There's the if (__ctfe) hack. Also using only template parameters will 
force the function to be CTFE.

-- 
/Jacob Carlborg
September 27, 2012
Re: Is it possible to force CTFE?
Jacob Carlborg:

>> I am interested in this since some years. I think it's useful, 
>> but I don't know if it can be implemented. I don't remember
>> people discussing about this much.
>
> There's the if (__ctfe) hack. Also using only template 
> parameters will force the function to be CTFE.

This is quite far from what I was discussing about here...

Bye,
bearophile
September 28, 2012
Re: Is it possible to force CTFE?
One use case I can think of for specializing functions based on 
whether or not its arguments are compile-time evaluable:

// Big container that can't be accessed in constant time:
immutable cachedResults = init();

double getResult(<args>)
    if (areCompileTimeConstants!(<args>) == false)
{
    return cachedResults.at(<args>);
}

double getResult(<args>)
    if (areCompileTimeConstants!(<args>) == true)
{
    // Computing the result takes long time
    ...
    return computedResult;
}

Point being that A) cachedResults takes so much memory we don't 
want to evaluate it at compile-time and bloat the executable, and 
B) accessing cachedResults takes some non-trivial time, so we 
don't want to do that at runtime if it can be done at 
compile-time. Don't know how common this kind of thing would be 
though.


But, that made me think...
In a perfect world, I think, the compiler would always evaluate 
all possible functions at compile-time, given that doing so would 
produce a smaller (or equal size) executable than what 
not-evaluating-at-compile-time would produce. For example 
(assuming the following initialization functions are compile-time 
evaluable):

// The following wouldn't be evaluated at compile time,
// because that function call (probably) wouldn't take
// as much space in the executable as million ints:

int[1_000_000] bigArray = initBigArray();

// The following would be always evaluated at compile time,
// because a single int value would take less space in the
// executable than the function call:

int myValue = initMyValue();

Although, to speed up test compilations, we'd need a compiler 
flag to disable this "aggressive" CTFE behaviour.
September 28, 2012
Re: Is it possible to force CTFE?
On Friday, 28 September 2012 at 17:52:55 UTC, Tommi wrote:
> In a perfect world, I think, the compiler would always evaluate 
> all possible functions at compile-time, given that doing so 
> would produce a smaller (or equal size) executable than what 
> not-evaluating-at-compile-time would produce.


Or, a simpler rule (for both the compiler and the coder):
Have a compiler flag where you set a value (in bytes), and if a 
function returns a type that's size is not larger than the set 
value, the compiler would execute all calls to that function at 
compile-time (if possible).
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