Why is C so fast, and why aren't other languages as fast or faster?

Question

In listening to the StackOverflow podcast, the jab keeps coming up that "real programmers" write in C, and that C is so much faster because it's "close to the machine." Leaving the former assertion for another post, what is special about C that allows it to be faster than other languages? Or put another way: what's to stop other languages from being able to compile down to binary that runs every bit as fast as C?

Answer 1

what's to stop other languages from being able to compile down to binary that runs every bit as fast as C?

Nothing. Modern languages like Java or .NET langs are oriented more on programmer productivity rather than performance. Hardware is cheap now days. Also compilation to intermediate representation gives a lot of bonuses such as security, portability etc. .NET CLR can take advantage of different hardware - for example you don't need to manually optimize/recompile program to use SSE instructions set.

Answer 2

If you spend a month to build something in C that runs in 0.05 seconds, and I spend a day writing the same thing in Java, and it runs in 0.10 seconds, then is C really faster?

But to answer your question, well-written C code will generally run faster than well-written code in other languages because part of writing C code "well" includes doing manual optimizations at a near-machine level.

Although compilers are very clever indeed, they are not yet able to creatively come up with code that competes with hand-massaged algorithms (assuming the "hands" belong to a good C programmer).

Edit:

A lot of comments are along the lines of "I write in C and I don't think about optimizations."

But to take a specific example from this post:

In Delphi I could write this:

function RemoveAllAFromB(a, b: string): string;
var
  before, after :string;
begin
  Result := b;
  if 0 < Pos(a,b) then begin
    before := Copy(b,1,Pos(a,b)-Length(a));
    after := Copy(b,Pos(a,b)+Length(a),Length(b));
    Result := before + after;
    Result := RemoveAllAFromB(a,Result);  //recursive
  end;
end;

and in C I write this:

char *s1, *s2, *result; /* original strings and the result string */
int len1, len2; /* lengths of the strings */
for (i = 0; i < len1; i++) {
   for (j = 0; j < len2; j++) {
     if (s1[i] == s2[j]) {
       break;
     }
   }
   if (j == len2) {  /* s1[i] is not found in s2 */
     *result = s1[i]; 
     result++; /* assuming your result array is long enough */
   }
}

But how many optimizations are there in the C version? We make lots of decisions about implementation that I don't think about in the Delphi version. How is a string implemented? In Delphi I don't see it. In C, I've decided it will be a pointer to an array of ASCII integers, which we call chars. In C, we test for character existence one at a time. In Delphi, I use Pos.

And this is just a small example. In a large program, a C programmer has to make these kinds of low-level decisions with every few lines of code. It adds up to a hand-crafted, hand-optimized executable.

Answer 3

For the most part, every C instruction corresponds to a very few assembler instructions. You are essentially writing higher level machine code, so you have control over almost everything the processor does. Many other compiled languages, such as C++, have a lot of simple looking instructions that can turn into much more code than you think it does (virtual functions, copy constructors, etc..) And interpreted languages like Java or Ruby have another layer of instructions that you never see - the Virtual Machine or Interpreter.

Answer 4

It's not so much about the language as the tools and libraries. The available libraries and compilers for C are much older than for newer languages. You might think this would make them slower, but au contraire.

These libraries were written at a time when processing power and memory were at a premium. They had to be written very efficiently in order to work at all. Developers of C compilers have also had a long time to work in all sorts of clever optimizations for different processors. C's maturity and wide adoption makes for a signficant advantage over other languages of the same age. It also gives C a speed advantage over newer tools that don't emphasize raw performance as much as C had to.

Answer 5

There are a lot of questions in there - mostly ones I am not qualified to answer. But for this last one:

what's to stop other languages from being able to compile down to binary that runs every bit as fast as C?

In a word, Abstraction.

C is only one or 2 levels of abstraction away from machine language. Java and the .Net languages are at a minimum 3 levels of abstraction away from assembler. I'm not sure about Python and Ruby.

Typically, the more programmer toys (complex data types, etc.), the further you are from machine language and the more translation has to be done.

I'm off here and there but that's the basic gist.

Update - There are some good comments on this post with more details.

Answer 6

There is a trade off the C designers have made. That's to say, they made the decision to put speed above safety. C won't

• Check array index bound
• Check for uninitialized variable values
• Check for memory leaks
• Check for null pointer dereference

When you index into an array, in Java it takes some method call in the virtual machine, bound checking and other sanity checks. That is valid and absolutely fine, because it adds safety where it's due. But in C, even pretty trivial things are not put in safety. For example, C doesn't require memcpy to check whether the regions to copy overlap. It's not designed as a language to program a big business application.

But these design decisions are not bugs in the C language. They are by design, as it allows compilers and library writers to get every bit of performance out of the computer. Here is the spirit of C how the C Rationale document explains it:

C code can be non-portable. Although it strove to give programmers the opportunity to write truly portable programs, the Committee did not want to force programmers into writing portably, to preclude the use of C as a ``high-level assembler'': the ability to write machine-specific code is one of the strengths of C.

Keep the spirit of C. The Committee kept as a major goal to preserve the traditional spirit of C. There are many facets of the spirit of C, but the essence is a community sentiment of the underlying principles upon which the C language is based. Some of the facets of the spirit of C can be summarized in phrases like

• Trust the programmer.
• Don't prevent the programmer from doing what needs to be done.
• Keep the language small and simple.
• Provide only one way to do an operation.
• Make it fast, even if it is not guaranteed to be portable.

The last proverb needs a little explanation. The potential for efficient code generation is one of the most important strengths of C. To help ensure that no code explosion occurs for what appears to be a very simple operation, many operations are defined to be how the target machine's hardware does it rather than by a general abstract rule. An example of this willingness to live with what the machine does can be seen in the rules that govern the widening of char objects for use in expressions: whether the values of char objects widen to signed or unsigned quantities typically depends on which byte operation is more efficient on the target machine.

Answer 7

Some C++ algorithms are faster than C, and some implementations of algorithms or design patterns in other languages can be faster than C.

When people say that C is fast, and then move on to talking about some other language, they are generally using C's performance as a benchmark.

Answer 8

There isn't much that's special about C. That's one of the reasons why it's fast.

Newer languages which have support for garbage collection, dynamic typing and other facilities which make it easier for the programmer to write programs.

The catch is, there is additional processing overhead which will degrade the performance of the application. C doesn't have any of that, which means that there is no overhead, but that means that the programmer needs to be able to allocate memory and free them to prevent memory leaks, and must deal with static typing of variables.

That said, many languages and platforms, such as Java (with its Java Vitual Machine) and .NET (with its Common Language Runtime) have improved performance over the years with advents such as just-in-time compilation which produces native machine code from bytecode to achieve higher performance.

Answer 9

The lack of abstraction is what makes C faster. If you write an output statement you know exactly what is happening. If you write an output statement in java it is getting compiled to a class file which then gets run on a virtual machine introducing a layor of abstraction. The lack of object oriented features as a part of the language also increases it's speed do to less code being generated. If you use C as an object oriented language then you are doing all the coding for things such as classes, inharitence, etc. This means rather then make something generalized enough for everyone with the amount of code and the performance penelty that requires you only write what you need to get the job done.

Answer 10

The main factors are that it's a statically-typed language and that's compiled to machine code. Also, since it's a low-level language, it generally doesn't do anything you don't tell it to.

These are some other factors that come to mind.

• Variables are not automatically initialized
• No bounds checking on arrays
• Unchecked pointer manipulation
• No integer overflow checking
• Statically-typed variables
• Function calls are static (unless you use function pointers)
• Compiler writers have had lots of time to improve the optimizing code. Also, people program in C for the purpose of getting the best performance, so there's pressure to optimize the code.
• Parts of the language specification are implementation-defined, so compilers are free to do things in the most optimal way

Most static-typed languages could be compiled just as fast or faster than C though, especially if they can make assumptions that C can't because of pointer aliasing, etc.

Answer 11

I guess you forgot that Assembly language is also a language :)

But seriously, C programs are faster only when the programmer knows what he's doing. You can easily write a C program that runs slower than programs written in other languages that do the same job.

The reason why C is faster is because it is designed in this way. It lets you do a lot of "lower level" stuff that helps the compiler to optimize the code. Or, shall we say, you the programmer are responsible for optimizing the code. But it's often quite tricky and error prone.

Other languages, like others already mentioned, focus more on productivity of the programmer. It is commonly believed that programmer time is much more expensive than machine time (even in the old days). So it makes a lot of sense to minimize the time programmers spend on writing and debugging programs instead of the running time of the programs. To do that, you will sacrifice a bit on what you can do to make the program faster because a lot of things are automated.

Answer 12

It is not so much that C is fast as that C's cost model is transparent. If a C program is slow, it is slow in an obvious way: by executing a lot of statements. Compared with the cost of operations in C, high-level operations on objects (especially reflection) or strings can have costs that are not obvious.

Two languages that generally compile to binaries which are just as fast as C are Standard ML (using the MLton compiler) and Objective Caml. If you check out the Great Language Shootout you'll find that for some benchmarks, like binary trees, the OCaml version is faster than C. (I didn't find any MLton entries.) But don't take the shootout too seriously; it is, as it says, a game, the the results often reflect how much effort people have put in tuning the code.

Answer 13

Amazing to see the old "C/C++ must be faster than Java because Java is interpreted" myth is still alive and kicking. There are articles going back a few years, as well as more recent ones, that explain with concepts or measurements why this simply isn't always the case.

Current virtual machine implementations (and not just the JVM, by the way) can take advantage of information gathered during program execution to dynamically tune the code as it runs, using a variety of techniques:

• rendering frequent methods to machine code,
• inlining small methods,
• adjustment of locking

and a variety of other adjustments based on knowing what the code is actually doing, and on the actual characteristics of the environment in which it's running.

Answer 14

This is actually a bit of a perpetuated falsehood. While it is true that C programs are frequently faster, this is not always the case, especially if the C programmer isn't very good at it.

One big glaring hole that people tend to forget about is when the program has to block for some sort of IO, such as user input in any GUI program. In these cases, it doesn't really matter what language you use since you are limited by the rate at which data can come in rather than how fast you can process it. In this case, it doesn't matter much if you are using C, Java, C# or even Perl; you just cannot go any faster than the data can come in.

The other major thing is that using garbage collection and not using proper pointers allows the virtual machine to make a number of optimizations not available in other languages. For instance, the JVM is capable of moving objects around on the heap to defragment it. This makes future allocations much faster since the next index can simply be used rather than looking it up in a table. Modern JVMs also don't have to actually deallocate memory; instead, they just move the live objects around when they GC and the spent memory from the dead objects is recovered essentially for free.

This also brings up an interesting point about C and even more so in C++. There is something of a design philosophy of "If you don't need it, you don't pay for it." The problem is that if you do want it, you end up paying through the nose for it. For instance, the vtable implementation in Java tends to be a lot better than C++ implementations, so virtual function calls are a lot faster. On the other hand, you have no choice but to use virtual functions in Java and they still cost something, but in programs that use a lot of virtual functions, the reduced cost adds up.

Answer 15

C is not always faster.

C is slower than, for example Modern Fortran.

C is often slower than Java for some things. ( Especially after the JIT compiler has had a go at your code)

C lets pointer aliassing happen, which means some good optimizations are not possible. Particularly when you have multiple execution units, this causes data fetch stalls. Ow.

The assumption that pointer arithmetic works really causes slow bloated performance on some CPU families (PIC particularly!) It used to suck the big on segmented x86.

Basically, when you get a vector unit, or a parallelizing compiler, C stinks and modern Fortran runs faster.

C programmer tricks like thunking ( modifying the executable on the fly) cause CPU prefetch stalls.

You get the drift ?

Any our good friend, the x86, executes an instruction set that these days bears little relationship to the actual CPU archientecture. Shadow registers, load-store optimizers, all in the CPU. So C is then close to the virtual metal. The real metal, Intel don't let you see. (Historically VLIW CPU's were a bit of a bust so, maybe that's no so bad.)

If you program in C on a high-performance DSP (maybe a TI DSP ?), the compiler has to do some tricky stuff to unroll the C across the multiple parallel execution units. So there C isn't close to the metal, but it is close to the compiler. Which will do whole program optimization. Weird.

A finally, some CPU's (www.ajile.com) run Java bytecodes in hardware. C would a PITA to use on that CPU.

Answer 16

Even the difference between C and C++ can at times be great.

When you are allocating memory for an object, invoking constructors, aligning memory on word boundaries, etc. the program winds up going through a lot of overhead that is abstracted away from the programmer.

C forces you to take a look at each thing that your program is doing, generally at a very fine level of detail. This makes it harder (although not by any means impossible) to write code that does a lot of tasks that are unnecessary to the immediate goal at hand.

So where in, for instance a BASIC program you would use the INPUT keyword to read a string form STDIN and automatically allocate memory for its variable, in C the programmer will typically have already allocated memory and can control things like whether the program blocks for I/O or not, and if it stops reading input after it has the information it needs or continues reading characters to the end of the line.

C also performs a lot less error-checking than other languages, presuming the programmer knows what they're doing. So whereas in PHP if you declare a string $myStr = getInput(); and go on to reference $myStr[20], but the input was only 10 characters long, PHP will catch this and safely return to you a blank string. C assumes that you've either allocated enough memory to hold data past the end of the string or that you know what information comes after the string and are trying to reference that instead. These small factors have a huge impact on overhead in aggregate.

Answer 17

With modern optimizing compilers, it's highly unlikely that a pure C program is going to be all that much faster than compiled .net code, if at all. With the productivity enhancement that frameworks like .net provide the developer, you can do things in a day that used to take weeks or months in regular C. Coupled with the cheap cost of hardware compared to a developer's salary, it's just WAY cheaper to write the stuff in a high-level language and throw hardware at any slowness.

The reason Jeff and Joel talk about C being the "real programmer" language is because there is no hand-holding in C. You must allocate your own memory, deallocate that memory, do your own bounds-checking, etc. There's no such thing as new object(); There's no garbage collection, classes, OOP, entity frameworks, LINQ, properties, attributes, fields, or anything like that. You have to know things like pointer arithmetic and how to dereference a pointer. And, for that matter, know and understand what a pointer is. You have to know what a stack frame is and what the instruction pointer is. You have to know the memory model of the CPU architecture you're working on. There is a lot of implicit understanding of the architecture of a microcomputer (usually the microcomputer you're working on) when programming in C that simply is not present nor necessary when programming in something like C# or Java. All of that information has been off-loaded to the compiler (or VM) programmer.

Answer 18

I didn't see it already, so I'll say it: C tends to be faster because almost everything else is written in C.

Java is built on C, Python is built on C (or Java, or .NET, etc.), Perl is, etc. The OS is written in C, the virtual machines are written in C, the compilers are written in C, the interpreters are written in C. Some things are still written in Assembly language, which tends to be even faster. More and more things are being written in something else, which is itself written in C.

Each statement that you write in other languages (not Assembly) is typically implemented underneath as several statements in C, which are compiled down to native machine code. Since those other languages tend to exist in order to obtain a higher level of abstraction than C, those extra statements required in C tend to be focused on adding safety, adding complexity, and providing error handling. Those are often good things, but they have a cost, and its names are speed and size.

Personally, I have written in literally dozens of languages spanning most of the available spectrum, and I personally have sought the magic that you hint at:

How can I have my cake and eat it, too? How can I play with high-level abstractions in my favorite language, then drop down to the nitty gritty of C for speed?

After a couple of years of research, my answer is Python (on C). You might want to give it a look. By the way, you can also drop down to Assembly from Python, too (with some minor help from a special library).

On the other hand, bad code can be written in any language. Therefore, C (or Assembly) code is not automatically faster. Likewise, some optimization tricks can bring portions of higher-level language code close to the performance level of raw C. But, for most applications, your program spends most of its time waiting on people or hardware, so the difference really does not matter.

Enjoy.

Answer 19

The fastest running code would be carefully hand crafted machine code. Assembler will be almost as good. Both are very low level and it takes a lot of writing code to do things. C is a little above assembler. You still have the ability to control things at a very low level in the actual machine, but there is enough abstraction make writing it faster and easier then assembler. Other languages such as C# and JAVA are even more abstract. While Assembler and machine code are called low level languages, C# and JAVA (and many others) are called high level languages. C is sometimes called a middle level language.

Answer 20

Just step through the machine code in your IDE, and you'll see why it's faster (if it's faster). It leaves out a lot of hand-holding. Chances are your Cxx can also be told to leave it out too, in which case it should be about the same.

Compiler optimizations are overrated, as are almost all perceptions about language speed.

Optimization of generated code only makes a difference in hotspot code, that is, tight algorithms devoid of function calls (explicit or implicit). Anywhere else, it achieves very little.

Answer 21

I know plenty of people have said it in a long winded way, but:

C is faster because it does less (for you).

Answer 22

1) As others have said, C does less for you. No initializing variables, no array bounds checking, no memory management, etc. Those features in other languages cost memory and CPU cycles that C doesn't spend.

2) Answers saying that C is less abstracted and therefore faster are only half correct I think. Technically speaking, if you had a "sufficiently advanced compiler" for language X, then language X could approach or equal the speed of C. The difference with C is that since it maps so obviously (if you've taken an architecture course) and directly to assembly language that even a naive compiler can do a decent job. For something like Python, you need a very advanced compiler to predict the probable types of objects and generate machine code on the fly -- C's semantics are simple enough that a simple compiler can do well.

Answer 23

Back in the good ole days, there were just two types of languages: compiled and interpreted.

Compiled languages utilized a "compiler" to read the language syntax and convert it into identical assembly language code, which could than just directly on the CPU. Interpreted languages used a couple of different schemes, but essentially the language syntax was converted into an intermediate form, and then run in a "interpreter", an environment for executing the code.

Thus, in a sense, there was another "layer" -- the interpreter -- between the code and the machine. And, as always the case in a computer, more means more resources get used. Interpreters were slower, because they had to perform more operations.

More recently, we've seen more hybrid languages like Java, that employ both a compiler and an interpreter to make them work. It's complicated, but a JVM is faster, more sophisticated and way more optimized than the old interpreters, so it stands a much better change of performing (over time) closer to just straight compiled code. Of course, the newer compilers also have more fancy optimizing tricks so they tend to generate way better code than they used to as well. But most optimizations, most often (although not always) make some type of trade-off such that they are not always faster in all circumstances. Like everything else, nothing comes for free, so the optimizers must get their boast from somewhere (although often times it using compile-time CPU to save runtime CPU).

Getting back to C, it is a simple language, that can be compiled into fairly optimized assembly and then run directly on the target machine. In C, if you increment an integer, it's more than likely that it is only one assembler step in the CPU, in Java however, it could end up being a lot more than that (and could include a bit of garbage collection as well :-) C offers you an abstraction that is way closer to the machine (assembler is the closest), but you end up having to do way more work to get it going and it is not as protected, easy to use or error friendly. Most other languages give you a higher abstraction and take care of more of the underlying details for you, but in exchange for their advanced functionality they require more resources to run. As you generalize some solutions, you have to handle a broader range of computing, which often requires more resources.

Paul.

Answer 24

Actually, in certain applications (numerics) even C can be beaten, and I don't mean assembly language, but the old, oft-ridiculed Fortran. The reason is, Fortran guarantees no pointer aliasing.

Answer 25

Many of these answers give valid reasons for why C is, or is not, faster (either in general or in specific scenarios). It's undeniable that:

• Many other languages provide automatic features that we take for granted. Bounds checking, run-time type checking, and automatic memory management, for example, don't come for free. There is at least some cost associated with these features, which we may not think about—or even realize—while writing code that uses these features.
• The step from source to machine is often not as direct in other languages as it is in C.
• OTOH, to say that compiled C code executes faster than other code written in other languages is a generalization that isn't always true. Counter-examples are easy to find (or contrive).

All of this notwithstanding, there is something else I have noticed that, I think, affects the comparative performance of C vs. many other languages more greatly than any other factor. To wit:

Other languages often make it easier to write code that executes more slowly. Often, it's even encouraged by the design philosophies of the language. Corollary: a C programmer is more likely to write code that doesn't perform unnecessary operations.

As an example, consider a simple Windows program in which a single main window is created. A C version would populate a WNDCLASS[EX] structure which would be passed to RegisterClass[Ex], then call CreateWindow[Ex] and enter a message loop. Highly simplified and abbreviated code follows:

WNDCLASS wc;
MSG      msg;

wc.style         = 0;
wc.lpfnWndProc   = &WndProc;
wc.cbClsExtra    = 0;
wc.cbWndExtra    = 0;
wc.hInstance     = hInstance;
wc.hIcon         = NULL;
wc.hCursor       = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_BTNFACE + 1);
wc.lpszMenuName  = NULL;
wc.lpszClassName = "MainWndCls";

RegisterClass(&wc);

CreateWindow("MainWndCls", "", WS_OVERLAPPEDWINDOW | WS_VISIBLE,
             CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

while(GetMessage(&msg, NULL, 0, 0)){
    TranslateMessage(&msg);
    DispatchMessage(&msg);
}

An equivalent program in C# could be just one line of code:

Application.Run(new Form());

This one line of code provides all of the functionality that nearly 20 lines of C code did, and adds some things we left out, such as error checking. The richer, fuller library (compared to those used in a typical C project) did a lot of work for us, freeing our time to write many more snippets of code that look short to us but involve many steps behind the scenes.

But a rich library enabling easy and quick code bloat isn't really my point. My point is more apparent when you start examining what actually happens when our little one-liner actually executes. For fun sometime, enable .NET source access in Visual Studio 2008 or higher, and step into the simple one-linef above. One of the fun little gems you'll come across is this comment in the getter for Control.CreateParams:

// In a typical control this is accessed ten times to create and show a control.
// It is a net memory savings, then, to maintain a copy on control.
// 
if (createParams == null) {
    createParams = new CreateParams(); 
}

Ten times. The information roughly equivalent to the sum of what's stored in a WNDCLASSEX structure and what's passed to CreateWindowEx is retrieved from the Control class ten times before it's stored in a WNDCLASSEX structure and passed on to RegisterClassEx and CreateWindowEx.

All in all, the number of instructions executed to perform this very basic task is 2–3 orders of magnitude more in C# than in C. Part of this is due to the use of a feature-rich library, which is necessarily generalized, versus our simple C code which does exactly what we need and nothing more. But part of it is due to the fact that the modularized, object-oriented nature of .NET framework, lends itself to a lot of repetition of execution that often is avoided by a procedural approach.

I'm not trying to pick on C# or the .NET framework. Nor am I saying that modularization, generalization, library/language features, OOP, etc. are bad things. I used to do most of my development in C, later in C++, and most lately in C#. Similarly, before C, I used mostly assembly. And with each step "higher" my language goes, I write better, more maintainable, more robust programs in less time. They do, however, tend to execute a little more slowly.

Answer 26

Don't take someones word for it, look at the dissassembly for both C and your language-of-choice in any performance critical part of your code. I think you can just look in the disassembly window at runtime in Visual Studio to see disassembled .Net. Should be possible if tricky for Java using windbg, though if you do it with .Net many of the issues would be the same.

I don't like to write in C if I don't need to, but I think many of the claims made in these answers that tout the speed of languages other than C can be put aside by simply disassembling the same routine in C and in your higher level language of choice, especially if lots of data is involved as is common in performance critical applications. Fortran may be an exception in its area of expertise, don't know. Is it higher level than C?

First time I did compared JITed code with native code resolved any and all questions whether .Net code could run comparably to C code. The extra level of abstraction and all the safety checks come with a significant cost. Same costs would probably apply to Java, but don't take my word for it, try it on something where performance is critical. (Anyone know enough about JITed Java to locate a compiled procedure in memory? It should certainly be possible)