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I write assembly most days (I write system libraries--largely floating-point and vector code--for a living). In general the assembly that I write is between 1.

I write assembly most days (I write system libraries--largely floating-point and vector code--for a living). In general the assembly that I write is between 1.5x and 3x faster than what the best compilers currently produce from equivalent C code (obviously for completely trivial tasks, you usually can’t beat the compiler, but those generally don’t need to be abstracted into a library either; there are also outliers where the code I write is orders of magnitude faster than what the compiler produces). For most programmers most of the time, that’s not enough of a payoff to bother. For library functions that are used by thousands of applications and millions of users, even small performance wins can have huge impact. Partially my advantage comes from the fact that I have more knowledge of microarchitecture than compilers do, but the real key is that I am hired to do a very different job than the compiler is: if your compiler took a day (or even an hour) to compile a small program, you simply wouldn’t use it, no matter how good the resulting code was. On the other hand, for a library function with significant impact on system performance, I can easily justify spending a week “compiling” it to be as efficient as possible.

revscat 13y ago
I haven't dealt with assembly in many years, and if you don't mind my asking, what tools do you use to do this these days?
- stephencanon 13y ago
  I don’t think the tools really change; architecture reference manuals, your favorite text editor, and an assembler. Processor simulators are sometimes useful for resolving the most perplexing quandaries.
  - codemac 13y ago
    With the new AVX 512 announcement from Intel, I was trying to use the sde to see what I could get away with in terms of reducing cycles for certain operations by loading larger values.. Have you used the Intel SDE? Do you know how helpful the -mix histogram is or isn't? Or just general docs on usage?
    - stephencanon 13y ago
      SDE has never been particularly interesting to me, but I can certainly see that it could be useful for some cases. In general, the simulator I want is one that completely models the CPU and generates detailed traces (like the late great SimG4).
      - nkurz 13y ago
        the simulator I want is one that completely models the CPU and generates detailed traces I'd like one also. Lacking that, have you found any tools close enough to this to be useful? Intel's IACA is better than pen and paper, but rarely replaces it. Is there an AMD equivalent? Are PTLsim or MARSSx86 useful? (sorry for presuming x86 if not what you work on)
mjn 13y ago
if your compiler took a day (or even an hour) to compile a small program, you simply wouldn’t use it, no matter how good the resulting code was Some research has been moving in that direction, since there does seem to be a demand for it. After all, if someone is willing to wait for you to spend a week hand-tuning a function, maybe they'd also be willing to run a compiler in a "take 10 hours to crunch on this" mode. Example: http://blog.regehr.org/archives/923 http://blog.regehr.org/archives/923
- stephencanon 13y ago
  Oh, absolutely. That said, it’s also worth keeping in mind the enormous differences in how computers and expert humans currently approach the problem closely parallel the differences in how computers and expert humans play chess; quickly evaluate billions of possible “moves” vs. quickly identify the few most promising “moves” and then slowly evaluate them to pick the best. I fully expect to be regularly beaten by the compiler “someday”, but (a) I believe that day is still several years off and (b) even then, I expect that expert human + compiler will beat compiler alone, just as in chess.
  - zwegner 13y ago
    As a former chess engine author, and current compiler writer, with ideas in search-based compiler algorithms/unbounded compilation times, I hope to accelerate the arrival of that day :)
npsimons 13y ago
For most programmers most of the time, that’s not enough of a payoff to bother. I would argue that for most programmers, they can't beat the compiler; they probably don't have your skills and experience. I'm glad there are still people like you (I've probably used some of your code in embedded projects using VxWorks/PPC), but the truth is that many people like you are writing the compilers (or libraries, as you are). That, and the fact that when you mention profiling, you often get blank stares, plus mentioning algorithmic complexity gets you knotted brows, tends to lead me to believe that the great mass of programmers shouldn't try optimizing, prematurely or otherwise, especially in assembly. Play with it, learn about your whole stack, top to bottom, sure, but very few (such as yourself) can beat a good optimizing compiler.

revscat 13y ago

I haven't dealt with assembly in many years, and if you don't mind my asking, what tools do you use to do this these days?

stephencanon 13y ago
I don’t think the tools really change; architecture reference manuals, your favorite text editor, and an assembler. Processor simulators are sometimes useful for resolving the most perplexing quandaries.
- codemac 13y ago
  With the new AVX 512 announcement from Intel, I was trying to use the sde to see what I could get away with in terms of reducing cycles for certain operations by loading larger values.. Have you used the Intel SDE? Do you know how helpful the -mix histogram is or isn't? Or just general docs on usage?
  - stephencanon 13y ago
    SDE has never been particularly interesting to me, but I can certainly see that it could be useful for some cases. In general, the simulator I want is one that completely models the CPU and generates detailed traces (like the late great SimG4).
    - nkurz 13y ago
      the simulator I want is one that completely models the CPU and generates detailed traces I'd like one also. Lacking that, have you found any tools close enough to this to be useful? Intel's IACA is better than pen and paper, but rarely replaces it. Is there an AMD equivalent? Are PTLsim or MARSSx86 useful? (sorry for presuming x86 if not what you work on)

mjn 13y ago

if your compiler took a day (or even an hour) to compile a small program, you simply wouldn’t use it, no matter how good the resulting code was Some research has been moving in that direction, since there does seem to be a demand for it. After all, if someone is willing to wait for you to spend a week hand-tuning a function, maybe they'd also be willing to run a compiler in a "take 10 hours to crunch on this" mode. Example: http://blog.regehr.org/archives/923 http://blog.regehr.org/archives/923

stephencanon 13y ago
Oh, absolutely. That said, it’s also worth keeping in mind the enormous differences in how computers and expert humans currently approach the problem closely parallel the differences in how computers and expert humans play chess; quickly evaluate billions of possible “moves” vs. quickly identify the few most promising “moves” and then slowly evaluate them to pick the best. I fully expect to be regularly beaten by the compiler “someday”, but (a) I believe that day is still several years off and (b) even then, I expect that expert human + compiler will beat compiler alone, just as in chess.
- zwegner 13y ago
  As a former chess engine author, and current compiler writer, with ideas in search-based compiler algorithms/unbounded compilation times, I hope to accelerate the arrival of that day :)

npsimons 13y ago

For most programmers most of the time, that’s not enough of a payoff to bother. I would argue that for most programmers, they can't beat the compiler; they probably don't have your skills and experience. I'm glad there are still people like you (I've probably used some of your code in embedded projects using VxWorks/PPC), but the truth is that many people like you are writing the compilers (or libraries, as you are). That, and the fact that when you mention profiling, you often get blank stares, plus mentioning algorithmic complexity gets you knotted brows, tends to lead me to believe that the great mass of programmers shouldn't try optimizing, prematurely or otherwise, especially in assembly. Play with it, learn about your whole stack, top to bottom, sure, but very few (such as yourself) can beat a good optimizing compiler.