[Issue 3749] cannot evaluate yl2x (log) and exp functions at compile time

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=3749



--- Comment #12 from Martin Nowak <code at dawg.eu> 2014-01-07 19:47:17 PST ---
(In reply to comment #11)
> (In reply to comment #10)
> > What's the state of this?
> > Having log/exp functions at compile time would be very useful, e.g. to
> > pregenerate scientific tables.
> 
> std.math now has pure generic implementations.
> 
> One of the main problems holding CTFE support back is that there is no
> straightforward way to do math operations such as isNaN, isInfinity, signbit,
> frexp which require access (and for some, manipulation) of the bits in float
> types.

We had the same issue with hashOf in druntime and now there is a huge machinery
to compute exponent and mantissa.
Could we allow to read specific floating point values through intrinsics at
compile time?
Something like exponent(float), signbit(float), mantissa(float)?

> Saying that, DMD has a problem that GDC doesn't - it uses the IASM
> implementations of std.math functions that can't be evaluated at compile time.

How do you treat std.math.log at runtime as intrinsic or does it run the same
code? Is there a performance penalty?

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