Compile-time evaluation of real expressions?

H. S. Teoh hsteoh at
Fri Jan 6 16:31:14 PST 2012

On Sat, Jan 07, 2012 at 12:49:46AM +0100, Alex Rønne Petersen wrote:
> On 07-01-2012 00:37, Jonathan M Davis wrote:
> >On Saturday, January 07, 2012 00:03:39 Alex Rønne Petersen wrote:
> >>Most likely those functions are just implemented using inline
> >>assembly, therefore not usable in CTFE.
> >
> >Yeah, several functions in std.math use inline assembly. So, for them
> >to be able to be used at compile time, either the compiler must be
> >expanded to be able to run asm statements at compile time (which may
> >or may not be planned and may or may not be reasonable), or those
> >functions need another branch (using __cfte in an if condition) which
> >doesn't use assembly. Or I suppose that if the extra check for __ctfe
> >isn't considered particularly acceptable (after all, they're already
> >using assembly), then separate functions meant specifically for CTFE
> >would be necessary.

>From my limited experience, I'd say that having two versions of the
function is probably the least painful way to go.

> Allowing asm in CTFE would probably be way more work than it's worth.
> You'd basically need full-blown analysis of x86 assembly plus an
> interpreter. Even then, x86 is not typed, so it's going to be a major
> pain...

I admit I've no idea how the D compiler implements compile-time
evaluation, but is it possible for the compiler to actually emit code
for compile-time functions containing asm blocks and, say, execute it in
a sandbox, and read the values out from the machine registers? Or does
this create more problems than it solves?


Verbing weirds language. -- Calvin (& Hobbes)

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