Here's the assembly code for my alpha-blending routine:
ubyte[4] src = *cast(ubyte[4]*)(palette.ptr + 4 * *c);
ubyte[4] *p = cast(ubyte[4]*)(workpad + (offsetX + x)*4 + offsetY);
asm{    //moving the values to their destinations
movd    MM0, p;
movd    MM1, src;
movq    MM5, alpha;
movq    MM7, alphaMMXmul_const1;
movq    MM6, alphaMMXmul_const2;
                                                                        
punpcklbw       MM2, MM0;
punpcklbw       MM3, MM1;

paddw   MM6, MM5;       //1 + alpha
psubw   MM7, MM5;       //256 - alpha

pmulhuw MM2, MM6;       //src * (1 + alpha)
pmulhuw MM3, MM7;       //dest * (256 - alpha)
paddw   MM3, MM2;       //(src * (1 + alpha)) + (dest * (256 - alpha))
psrlw MM3, 8; //(src * (1 + alpha)) + (dest * (256 - alpha)) / 256
                                                                        
//moving the result to its place;
                                                                        
packuswb        MM4, MM3;
movd    p, MM4;
emms;
}

The two constants being referred here:
static immutable ushort[4] alphaMMXmul_const1 = [256,256,256,256];
static immutable ushort[4] alphaMMXmul_const2 = [1,1,1,1];

alpha is a ushort[4] containing the alpha value four times.

After some debugging, I found out that the p pointer becomes null at the end instead of pointing to a value. I have no experience with using in-line assemblers (although I made a few Hello World programs for MS-Dos with a stand-alone assembler), so I don't know when and how the compiler will interpret the types from D.

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