Slashdot Mirror
Optimizations - Programmer vs. Compiler?
Saravana Kannan asks: "I have been coding in C for a while (10 yrs or so) and tend to use short code snippets. As a simple example, take 'if (!ptr)' instead of 'if (ptr==NULL)'. The reason someone might use the former code snippet is because they believe it would result in smaller machine code if the compiler does not do optimizations or is not smart enough to optimize the particular code snippet. IMHO the latter code snippet is clearer than the former, and I would use it in my code if I know for sure that the compiler will optimize it and produce machine code equivalent to the former code snippet. The previous example was easy. What about code that is more complex? Now that compilers have matured over years and have had many improvements, I ask the Slashdot crowd, what they believe the compiler can be trusted to optimize and what must be hand optimized?"
"How would your answer differ (in terms of the level of trust on the compiler) if I'm talking about compilers for Desktops vs. Embedded systems? Compilers for which of the following platforms do you think is more optimized at present - Desktops (because is more commonly used) or Embedded systems (because of need for maximum optimization)? Would be better if you could stick to free (as in beer) and Open Source compilers. Give examples of code optimizations that you think the compiler can/can't be trusted to do."
Programmer: Hey, compiler. How do you like optimizing?
Compiler: Optimizing? Optimizing? Don't talk to me about optimizing. Here I am, brain the size of a planet, and they've got me optimizing inane snippets of code. Just when you think code couldn't possibly get any worse, it suddenly does. Oh look, a null pointer. I suppose you'll want to see the assembly now. Do you want me to go into an infinite loop or throw an exception right where I'm standing?
Programmer: Yeah, just show me the stack trace, won't you compiler?
A programmer is a machine for converting coffee into code.
I think writing clear and easy to understand code is more important in the long run, especially if other people will have to look at it.
Optimize. Using cryptic, short variable names also shaves valuable microseconds off compile time and run time.
Donald Knuth wrote "We should forget about small efficiencies, about 97% of the time. Premature optimization is the root of all evil."
The sad truth is that, as far as optimization goes, this isn't where attention is most needed.
Before we start worrying about things like saving two cycles here and there, we need to start teaching people how to select the proper algorithm for the task at hand.
There are too many programmers who spend hours turning their code into unreadable mush for the sake of squeezing a few milliseconds out of a loop that runs on the order of O(n!) or O(2^n).
For 99% of the coders out there, all that needs to be known about code optimization is: pick the right algorithms! Couple this with readable code, and you'll have a program that runs several thousand times faster than it'll ever need to and is easy to maintain--and that's probably all you'll ever need.
Obliteracy: Words with explosions
It's better to write clear, legible code that saves a human minutes of reading, than complex code that might save a computer a few milliseconds of processing time per year, because human time costs more than machine time.
Also the clear code will result in fewer misinterpretations, which will mean fewer bugs (especially when the original author is not the one doing maintenance years later), further reducing costs in dollars, man hours, and frustration.
I just checked the U.S. Patent office and sure enough, just minutes after your post, Microsoft patented "if (!ptr)" as a shorthand for "if (ptr==NULL)".
Prepare to be sued.
"What the hell is an aluminum falcon?"
Hard to measure, but what is the tradeoff between increased speed and increased readability (which is a prerequisite for correctness and maintainability)? And if you can estimate that tradeoff, which is more important to the goals of your application?
As a side note, it is far more important to make sure you are using efficient algorithms and data structures than to make minor local optimizations. I've seen programmers use bizarre local optimization tricks in a module that ran in exponential time rather than log time.
Sheesh, evil *and* a jerk. -- Jade
What about code that is more complex? Now that compilers have matured over years and have had many improvements, I ask the Slashdot crowd, what they believe the compiler can be trusted to optimize and what must be hand optimized?
Programmers cost lots more per hour than computer time. Let the compiler optimize and let the programmers concentrated on developing solid maintainable code.
If you make code too clever in an effort to try to pre-optimize, you end up with code that other people have difficulty understanding. This is leads to lower quality code as it evolves if the people that follow you are not as savvy.
Not only that, but the vast majority of code written today is UI-centric or I/O bound. If you want real optimization, design a harddrive/controller combo that gets you 1 GBps off the physical platter (and at a price that consumers can afford).
I got in the habit of writing "readable but inefficient" code, taking care that my constructs don't get too sophisticated for the optimizer but then depending on gcc -O3 thoroughly. And then it happened I had to program 8051 clone. Then I learned there are no optimizing compilers for '51, that I'm really tight on CPU cycles, and that I simply don't know HOW to write really efficient C code.
Ended up writing my programs in assembler...
45 5F E1 04 22 CA 29 C4 93 3F 95 05 2B 79 2A B2
As a simple example, take 'if (!ptr)' instead of 'if (ptr==NULL)'.
Both forms resolve to the same opcode. Even under my 6502 compiler.
CMP register,val
JNE
Enjoy,
It's just the normal noises in here.
1) Code for maintainability
2) Profile your code
3) Optimize the bottlenecks
That said, (!ptr) should be just as maintanable as (ptr == NULL) simply because it is a frequently used 'dialect'. As long as these 'shortcuts' are used throughout the entire codebase they should be familiar enough that they don't get in the way of maintainability.
What you're talking about it micro-optimization.
Compilers are pretty good at that, and you should let them do their job.
Programmers should optimize at a higher level: by their choice of algorithms, organizing the program so that memory access is cache-friendly, making sure various objects don't get destroyed and re-created unnecessarily, that sort of thing.
"ptr == 0" must give the same result as "ptr == NULL", always.
...are doomed to repeat the biggest trap in computer programming over and over again:
"Premature optimization is the root of all evil"
If there's only one rule in computer programming a person ever learns, "Hoare's dictum" is the one I would choose.
Almost all modern languages have extensive libraries available to handle common programming tasks and can handle the vast majority of optimizations you speak of automatically. This means that 99.99% of the time you shouldn't be thinking about optimizations at all. Unless you're John Carmack or you're writing a new compiler from scratch (and perhaps you are) or involved in a handful of other activities you're making a big big mistake if your spending any time worrying about these things. There are far more important things to worry about, such as writing code that can be understood by others, can easily be units tested, etc.
A few years ago I used to write C/C++/asm code extensively and used to be obsessed with performance and optimization. Then, one day, I had an epiphany and started writing code that is about 10 times slower than my old code (different in computer language and style) and infinitely easier to understand and expand. The only time I optimize now is at the very very end of development when I have solid profiler results from the final product that show noticable delays for the end user and this only happens rarely.
Of course, this is just my own personal experience and others may see things differently.
With regard to your example, I can't imagine any modern compiler wouldn't treat the two as equivalent.
However, in your example, I actually prefer "if (!ptr)" to "if (ptr == NULL)", for two reasons. First the latter is more error-prone, because you can accidentally end up with "if (ptr = NULL)". One common solution to avoid that problem is to write "if (NULL == ptr)", but that just doesn't read well to me. Another is to turn on warnings, and let your compiler point out code like that -- but that assumes a decent compiler.
The second, and more important, reason is that to anyone who's been writing C for a while, the compact representation is actually clearer because it's an instantly-recognizable idiom. To me, parsing the "ptr == NULL" format requires a few microseconds of thought to figure out what you're doing. "!ptr" requires none. There are a number of common idioms in C that are strange-looking at first, but soon become just another part of your programming vocabulary. IMO, if you're writing code in a given language, you should write it in the style that is most comfortable to other programmers in that language. I think proper use of idiomatic expressions *enhances* maintainability. Don't try to write Pascal in C, or Java in C++, or COBOL in, well, anything, but that's a separate issue :-)
Oh, and my answer to your more general question about whether or not you should try to write code that is easy for the compiler... no. Don't do that. Write code that is clear and readable to programmers and let the compiler do what it does. If profiling shows that a particular piece of code is too slow, then figure out how to optimize it, whether by tailoring the code, dropping down to assembler, or whatever. But not before.
Note to ACs: I usually delete AC replies without reading them. If you want to talk to me, log in.
"Programs should be written for people to read, and only incidentally for machines to execute."
- Structure and Interpretation of Computer Programs
Each compiler is different. Some will optimise things other won't.
In general, however, systems are now fast enought that when in doubt, write the clearest code possible. I mean for most apps, speed is not critical, however for all apps stability and lack of bugs is important and obscure code leads to problems.
Also, for things that are time critical, it's generall just one or two little parts that make all the difference. You only need to worry about optimizing those inner loops where all the time is spent. Use a profiler, since programmers generally suck at identifying what needs optimising.
Keep it easy to read and maintain, unless speed is critical in a certian part. Then you can go nuts on hand optimization, but document it well.
LLVM is an aggressive compiler that is able to do many cool things. Best yet, it has a demo page here: http://llvm.org/demo, where you can try two different things and see how they compile.
:)
One of the nice things about this is that the code is printed in a simple abstract assembly language that is easy to read and understand.
The compiler itself is very cool too btw, check it out.
-Chris
Donald Knuth was quoating Tony Hoare when he said that.
*sigh* back to work...
Grrr, you named the algorithm that must not be named! Cursed be the name of the fool who thought it would be a good algorithm for introductory students - I've lost count of the number of people convinced that this satan-spawned algorithm is faster than an insertion sort (it's not) and that there's no reason for them to learn to use the qsort() function. N.B., not to implement a quick sort, but to simply call a standard library routine.
The most frustrating thing is that, if you must use the algorithm that must not be named, the bidirectional form of the algorithm is much faster (in practice) than the unidirectional form yet really no more complex to code than the latter if you have any potential as a software developer.
For every complex problem there is an answer that is clear, simple, and wrong. -- H L Mencken
...then the code isn't important enough to optimize. Plain and simple.
Never try to optimize anything unless you have measured the speed of the code before optimizing and have measured it again after optimizing.
Optimized code is almost always harder to understand, contains more possible code paths, and more likely to contain bugs than the most straightforward code. It's only worth it if it's really faster...
And you simply cannot tell whether it's faster unless you actually time it. It's absolutely mindboggling how often a change you are certain will speed up the code has no effect, or a truly negligible effect, or slows it down.
This has always been true. In these days of heavily optimized compilers and complex CPUs that are doing branch prediction and God knows what all, it is truer than ever. You cannot tell whether code is fast just by glancing at it. Well, maybe there are processor gurus who can accurately visualize the exact flow of all the bits through the pipeline, but I'm certainly not one of them.
A corollary is that since the optimized code is almost always trickier, harder to understand, and often contains more logic paths than the most straightforward code, you shouldn't optimize unless you are committed to spending the time to write a careful unit-test fixture that exercises everything tricky you've done, and write good comments in the code.
"How to Do Nothing," kids activities, back in print!
When I wrote my ray-tracer for the final project of my graphics class, I used gcc -o3 and it optimized my code into Pov-ray, which was sweet. I was done with the project in like ten minutes.
Plus I got extra credit for implementing phong shading. I didn't even try to do phong shading.
I make my code easy to read for my own sanity. I've lived out this bash.org quote way too many times.
"Upon attaching the waterblock to my penis, I began to notice that I know nothing about computers." -- JRockway
And believe me it is a pain in the a$$. Our company did the verification for the code in the microprocessor that controls the locks to the bathroom door on a 777, if the crapper tank is full then the door locks to make sure there isn't an overflow and thus frozen turd/urine meteors that fall from the sky. Every byte of the code MUST be excercised including all error conditions.
So how many dumps does it take to fill up the crapper tank? I'd hate to be the last QA engineer in line to use the crapper. Also what happens when that last engineer fills up the crapper, does the bathroom door look thus trapping him inside?
Premature Optimization is the DEVIL! I repeat, it is the gosh darn DEVIL! Don't do it. Write clear code so that I don't have to spend days trying to figure out what you are trying to do.
The biggest mistake I see in my professional (and unprofessional) life is programmers who try to optimize their code is all sorts of "733+" ways, trying to "trick" the compiler into removing 1 or 2 lines of assembly, yet completely disregard that they are using a map instead of a hash_map, or doing a linear search when they could do a binary search, or doing the same lookup multiple times, when they could do it just once. It's just silly, and goes to show that lots of programmers don't know how to optimize effectively.
Compilers are good. They optimize code well. Don't try to help them out unless you know your code has a definite bottleneck in a tight loop that needs hand tuning. Focus on using correct algorithms and designing your code from a high level to process data efficiently. Write your code in a clear and easy to read manner, so that you or some other programmer can easily figure out what's going on a few months down the line when you need to add fixes or new functionality. These are the ways to build efficient and maintainable systems, not by writing stuff that you could enter in an obfuscated code contest.
+1 Insightful, -1 Troll. What can I say, I'm an Insightful Troll.
- How often are functions called (and branches taken)
- Which functions take most of the time
- See the assembler code for each line with a mouse click (no need to guess anymore)
callgrind/kcachegrind is by far the easiest profiling solution I ever tried, and it seems answer more or less all of your questions.I think that most people forget that the reason that i, j, k, etc. are used for loop counters is that unless otherwise declared, I..N default to INTEGER in FORTRAN. This convention just carried over as programmers migrated from FORTRAN to other languages and has been passed down through the ages.
(S(SKK)(SKK))(S(SKK)(SKK))
HAL! Open the bathroom door!
I'm sorry Dave, you shouldn't have had that last burrito.
one product
one customer
420,000 lines
260 staff
no competition
no trade shows
no salespeople selling new features that have never been discussed
It's interesting to talk about their attention to detail, but to hold it up as a model for all software development neglects to consider that they are working under an entirely different set of constraints from most everyone else.
I think the example is fine; you just displayed an assumption that highlights one of the quirks of C.
! means "not" or "inverse of"; it is a boolean function. The variable ptr is a pointer; it is a reference to data, which means it isn't really data itself. !ptr shouldn't compute; a boolean operator should only work on boolean data. But C logical comparators are designed to work on everything. You are just supposed to know that 0 == NULL == false. This supposition is totally arbitrary and doesn't hold up in any language with strong typing.
This is what makes C difficult for beginners. Bad code compiles even though it has logical flaws, and ends up failing in mysterious ways.
The second case makes more sense. Equality is an operator that should work on all types of data. NULL is necessary if you are going to abstract data through the use of pointers or objects. Doing away with NULL would be equivalent to eliminating true and false and using 1 and 0 instead. Or eliminating strings and using sequences of ASCII codes. These substitutions are technically correct but in reality they make code unreadable.
Ten years of programming in the language and you:
1) Don't know when two things are obviously equivalent to any non-brain dead compiler.
2) Think something other than readability matters.
3) Think the non-idiomatic way of doing something is more readable.
But I'm sure I'm just repeating the comments I can't be bothered reading.
No, you're adopting a black or white approach. You are, in essence, saying that you don't need to comment at all. The original poster was saying that comments needed to be everywhere, on everything. I believe in a middle ground approach.
I comment things that are non intuitive. I comment things that I *think* may be non intuitive. I comment things that I think someone else might have some difficulty understanding, because I happened to be deep into a code burn and consequently wrote something pretty tight, pretty sweet, but also pretty obfuscated. Finally, I comment things that I think *I* may not understand when I go back and look at the code again 3 months from now.
I don't comment every single line... I don't comment simple data structures, loops "/* this is a for loop using the integer variable I */" etc which would be stupid. I do however disassemble the complex portions of my code, describe how I'm dispatching events and best of all *why* I decided to do things a certain way instead of a different way.
I have, however, been handed 30k lines of code with zero documentation and not a single comment anywhere in it, with absolutely no clue at all how it worked and no access to the original programmer and been told "We need such and such fixed|updated|added by friday" and had to spend the entire week basically tracing every single line of code to figure out that the original programmer must have been smoking crack with NO indication of why he wrote things how he did and NO help when he decided to be exceedingly "clever"
in his code. That time was wasted.
Would it have killed him to simply put a comment block explaining his event dispatch model? Or to tell me what his functions and methods did and best of all why they did it?
There *is* a middle ground, believe it or not.
-- Gary F.
"I ask the Slashdot crowd, what they believe the compiler can be trusted to optimize and what must be hand optimized? Give examples of code optimizations that you think the compiler can/can't be trusted to do."
/.
Somehow 99% of the readers took this to mean "What is the difference between NULL and the zero bit pattern and do you think it is a good idea to write clear code and do the profile/algorithm change cycle until there is nothing left to optimize or should I write low level optimized code from the start?"
sigh.. I've only found two comments with code so far after going through hundreds of posts. This is possibly the worst signal to noise ratio I've witnessed on