Re: Asm and CTFE

[Alex Rønne Petersen]

On 12-11-2011 13:33, bearophile wrote:
This code doesn't compile with the latest DMD 2.057head:
import std.math: poly;
enum p = poly(2.0, [-2, 3, 5]);
void main() {}


The error:
...\dmd2\src\phobos\std\math.d(3752): Error: asm statements cannot be 
interpreted at compile time
test.d(2):        called from here: poly(2L,[-2L,3L,5L])


The code of poly is something like this:


real poly(real x, const real[] A) pure {
     version (D_InlineAsm_X86) {
         version (Windows) {
             asm { // assembler by W. Bright
                 mov     ECX,A[EBP];
                 dec     ECX;
                 lea     EDX,[ECX][ECX*8];
                 add     EDX,ECX;
                 add     EDX,A+4[EBP];
                 fld     real ptr [EDX];
                 jecxz   return_ST;
                 fld     x[EBP];
                 fxch    ST(1);
                 align   4;
         L2:     fmul    ST,ST(1);
                 fld     real ptr -10[EDX];
                 sub     EDX,10;
                 faddp   ST(1),ST;
                 dec     ECX;
                 jne     L2;
                 fxch    ST(1);
                 fstp    ST(0);
                 align   4;
         return_ST:;
             }
         } else
             static assert(0);
     } else {
         sizediff_t i = A.length - 1;
         real r = A[i];
         while (--i>= 0) {
             r *= x;
             r += A[i];
         }
         return r;
     }
}


So there is code able to run at compile time, but D_InlineAsm_X86 is set at 
compile time too, despite CTFE is not able to run assembly code. In Phobos 
there are other similar cases where not-assembly fallback code is available, 
but it can't be used by CTFE.

Do you know how to improve this situation, avoiding messy code too?

Bye,
bearophile

The problem is that DMD defines *all* version identifiers regardless of 
whether CTFE is active. I guess it's just a matter of removing the 
inline assembly identifiers when doing CTFE.

- Alex