I think GNU is the standard. I know that you can do _asm with GNU.

Also, are you doing command line compilation? I think you have to add a whole ton of crap to the make to do inline assembly.

Also, maybe it's different for your compiler, but I always use references for parameters in inline assembly. I don't remember if you need to do that with GNU, I know one of them required you to do that.
Code:
mov      edi, [DestAddr]
       mov      ecx, [MixLen]
       mov      esi, [SrcAddr]