Compile-time evaluation of real expressions?

Alex Rønne Petersen xtzgzorex at
Sat Jan 7 01:40:20 PST 2012

On 07-01-2012 01:31, H. S. Teoh wrote:
> 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?
> T

Executing asm at compile time is also a security risk.

- Alex

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