3 ms·

I enjoyed this article, I always find the argument that "these days you can't beat the compiler" fishy. I remember more than 25 years ago I wrote an 8080 machin

I enjoyed this article, I always find the argument that "these days you can't beat the compiler" fishy. I remember more than 25 years ago I wrote an 8080 machine code interpreter in 8088/86 assembly. My innermost interpreter comprised 3 lines of asm; lodsb ;al = *si++ mov ah,al ;eg if al=3eh (=mvi a,dd), then ax=0x3e3e jmp ax ;eg goto 0x3e3e. Preloader puts the emulation ; code for mvi a,dd followed by another copy ; of the inner interpreter at cs:0x3e3e. This approach uses 64K to sparsely store the emulation code for each of the 256 opcodes at 0x0000,0x0101,0x0202 ... etc. which seemed a pretty outrageous waste of memory at the time. But it was worth it because none of the 3 inner interpreter instructions modified the flags, which meant I could use the native 8086 flags to directly emulate the 8080 flags, a huge speed win. Remember that the 8086 had LAHF and SAHF instructions to help solve this very problem, yet my approach avoiding the whole flags issue was still twice as quick as commercial equivalents. Okay admittedly this is all irrelevant dinosaur nostalgia from the perspective of modern technology, but it reinforces the point that there's no substitute for human creativity when it comes to real hardcore optimization tricks.

froydnj 15y ago
> I enjoyed this article, I always find the argument that "these days you can't beat the compiler" fishy. I enjoyed your example; very clever. I think the "these days you can't beat the compiler" should really be understood to carry a qualifier like "for compiling things that we understand well enough and that occur often enough that it's worth teaching the compiler how to optimize them". Knuth's maxim "Science is what we understand well enough to explain to a computer. Art is everything else we do." seems particularly apt here. Problems where humans can apply very domain-specific knowledge to come up with clever solutions are not the sort of problems that a general-purpose compiler is going to generate good code for. Knowing when you're dealing with one of those problems (e.g. writing inner loops for multi-precision arithmetic or vectorizing multimedia codecs) is part of being a good programmer. But to a first or even second approximation, you are not dealing with one of those problems, so you might as well just sit back and let the compiler do its job.
- billforsternz 15y ago
  I was with you until "but to a first or even second approximation, you are not dealing with one of those problems, so ...". Perhaps there is a typo there and it should have been something like "but when to a first or even...."

froydnj 15y ago

> I enjoyed this article, I always find the argument that "these days you can't beat the compiler" fishy. I enjoyed your example; very clever. I think the "these days you can't beat the compiler" should really be understood to carry a qualifier like "for compiling things that we understand well enough and that occur often enough that it's worth teaching the compiler how to optimize them". Knuth's maxim "Science is what we understand well enough to explain to a computer. Art is everything else we do." seems particularly apt here. Problems where humans can apply very domain-specific knowledge to come up with clever solutions are not the sort of problems that a general-purpose compiler is going to generate good code for. Knowing when you're dealing with one of those problems (e.g. writing inner loops for multi-precision arithmetic or vectorizing multimedia codecs) is part of being a good programmer. But to a first or even second approximation, you are not dealing with one of those problems, so you might as well just sit back and let the compiler do its job.

billforsternz 15y ago
I was with you until "but to a first or even second approximation, you are not dealing with one of those problems, so ...". Perhaps there is a typo there and it should have been something like "but when to a first or even...."