Inline assembly and CTFE

[Walter Bright]

On 10/3/2013 3:25 PM, Joseph Cassman wrote:
> As of DMD 2.063.2 it looks to me like inline assembly language blocks are not
> usable in compile-time code. For example, the following code
>
>    void main() {
>      enum a = abc();
>    }
>    ulong abc() {
>      asm { mov RAX,1; }
>      return 1;
>    }
>
> will produce the error "Error: asm statements cannot be interpreted at compile
> time".
>
> Are there plans to eventually support the use of asm blocks in CTFE?

No. Simulating a CPU is way, way beyond its charter.


> If it were possible then I could use such a function in a static if test. What I
> am looking to do is use a function that calls CPUID (etc.) in a static if
> statement to determine which code to generate, somewhat like the following.
>
>    bool isAVXPossible() { asm { ... use CPUID, etc. ... } }
>    void foo() {
>      static if(isAVXPossible()) {
>        asm { ... generate AVX version ... }
>      else {
>        asm { ... generate non AVX version ... }
>      }
>    }
>
> I cannot use version(D_SIMD) because I need some more specificity in the
> versioning. Similarly, version(X86) and version(X86_64) are convenient but I
> would like to be more exact. Also, I tried the functionality in core.cpuid but
> it also seems only able to execute at run-time. When trying to use it in a
> static if statement I get the error "Error: static variable xfeatures cannot be
> read at compile time".
>
> Any ideas on how to implement isAVXPossible() and its related functionality
> without using a runtime test are appreciated.


But you don't want a compile time test for that! You want a runtime test, as it 
should depend on what machine the program is running under, which may not at all 
be the same one it compiled on.