Programmer's conundrums

On 4/5/2006 at 12:03 PM Don Y wrote: Some machine don't have a carry bit, so there's nothing to access. IMOHO, the biggest problem with HLL's versus assembly is when an architecture provides for a very useful instruction, but said instruction doesn't exactly fit into the language. For example, the MMX instrtuctions on the Pentium aren't terribly accessible from most Cs. So they're implemented as intrinsics at best--and at that point, you might as well be writing assembly. On the other hand, HLLs are very useful for automatically selecting instructions and optimizing them. I don't know what real-world gains are made by realing the gain from the fact that LOOP is slower on a 486 than a DEC CX, JNZ pair, but it's nice not to have to code for it. On the other hand, few C compilers can generate parallel versions of the same subroutine and say "If I'm running on machne that supports <fill in the blank feature>, I'll just use the other version. I've got lots of x86 assembly code that does just that when its determined that MMX is present or 386+ (32 bit registers) is available. The speed improvements in some cases can be substantial. One pleasure of a good optimizer is clever removal of invariants from the inside of a loop. It allows one to write what one means without worrying that the implementation is inefficient. Does anyone know of any optimizing assemblers ? That is, ones that will move instructions around for optimum scheduling? Cheers, Chuck