What coding techniques do you use for optimising C programs?

Question

Some years ago I was on a panel that was interviewing candidates for a relatively senior embedded C programmer position.

One of the standard questions that I asked was about optimisation techniques. I was quite surprised that some of the candidates didn't have answers.

So, in the interests of putting together a list for posterity - what techniques and constructs do you normally use when optimising C programs?

Answers to optimisation for speed and size both accepted.

Answer 1

Collecting profiles of code execution get you 50% of the way there. The other 50% deals with analyzing these reports.

Further, if you use GCC or VisualC++, you can use "profile guided optimization" where the compiler will take info from previous executions and reschedule instructions to make the CPU happier.

Answer 2

My favorite technique is to use a good profiler. Without a good profile telling you where the bottleneck lies, no tricks and techniques are going to help you.

Answer 3

most common techniques I encountered are:

• loop unrolling
• loop optimization for better cache prefetch (i.e. do N operations in M cycles instead of NxM singular operations)
• data aligning
• inline functions
• hand-crafted asm snippets

As for general recommendations, most of them are already sounded:

• choose better algos
• use profiler
• don't optimize if it doesn't give 20-30% performance boost

Answer 4

• First and foremost, use a better/faster algorithm. There is no point optimizing code that is slow by design.
• When optimizing for speed, trade memory for speed: lookup tables of precomputed values, binary trees, write faster custom implementation of system calls...
• When trading speed for memory: use in-memory compression

Answer 5

Avoid using the heap. Use obstacks or pool-allocator for identical sized objects. Put small things with short lifetime onto the stack. alloca still exists.

Answer 6

For low-level optimization:

1. START_TIMER/STOP_TIMER macros from ffmpeg (clock-level accuracy for measurement of any code).
2. Oprofile, of course, for profiling.
3. Enormous amounts of hand-coded assembly (just do a wc -l on x264's /common/x86 directory, and then remember most of the code is templated).
4. Careful coding in general; shorter code is usually better.
5. Smart low-level algorithms, like the 64-bit bitstream writer I wrote that uses only a single if and no else.
6. Explicit write-combining.
7. Taking into account important weird aspects of processors, like Intel's cacheline split issue.
8. Finding cases where one can losslessly or near-losslessly make an early termination, where the early-termination check costs much less than the speed one gains from it.
9. Actually inlined assembly for tasks which are far more suited to the x86 SIMD unit, such as median calculations (requires compile-time check for MMX support).

Answer 7

Pre-mature optimization is the root of all evil! ;)

Answer 8

As my applications usually don't need much CPU time by design, I focus on the size my binaries on disk and in memory. What I do mostly is looking out for statically sized arrays and replacing them with dynamically allocated memory where it's worth the additional effort of free'ing the memory later. To cut down the size of the binary, I look for big arrays that are initialized at compile time and put the initializiation to runtime.

char buf[1024] = { 0, };
/* becomes: */
char buf[1024];
memset(buf, 0, sizeof(buf));

This will remove the 1024 zero-bytes from the binaries .DATA section and will instead create the buffer on the stack at runtime and the fill it with zeros.

EDIT: Oh yeah, and I like to cache things. It's not C specific but depending on what you're caching, it can give you a huge boost in performance.

PS: Please let us know when your list is finished, I'm very curious. ;)

Answer 9

Another thing that was not mentioned:

• Know your requirements: don't optimize for situations that will unlikely or never happen, concentrate on the most bang for the buck

Answer 10

Inline functions! Inspired by the profiling fans here I profiled an application of mine and found a small function that does some bitshifting on MP3 frames. It makes about 90% of all function calls in my applcation, so I made it inline and voila - the program now uses half of the CPU time it did before.

Answer 11

First things first - don't optimise too early. It's not uncommon to spend time carefully optimising a chunk of code only to find that it wasn't the bottleneck that you thought it was going to be. Or, to put it another way "Before you make it fast, make it work"

Investigate whether there's any option for optimising the algorithm before optimising the code. It'll be easier to find an improvement in performance by optimising a poor algorithm than it is to optimise the code, only then to throw it away when you change the algorithm anyway.

And work out why you need to optimise in the first place. What are you trying to achieve? If you're trying, say, to improve the response time to some event work out if there is an opportunity to change the order of execution to minimise the time critical areas. For example when trying to improve the response to some external interrupt can you do any preparation in the dead time between events?

Once you've decided that you need to optimise the code, which bit do you optimise? Use a profiler. Focus your attention (first) on the areas that are used most often.

So what can you do about those areas?

• minimise condition checking. Checking conditions (eg. terminating conditions for loops) is time that isn't being spent on actual processing. Condition checking can be minimised with techniques like loop-unrolling.
• In some circumstances condition checking can also be eliminated by using function pointers. For example if you are implementing a state machine you may find that implementing the handlers for individual states as small functions (with a uniform prototype) and storing the "next state" by storing the function pointer of the next handler is more efficient than using a large switch statement with the handler code implemented in the individual case statements. YMMV.
• minimise function calls. Function calls usually carry a burden of context saving (eg. writing local variables contained in registers to the stack, saving the stack pointer), so if you don't have to make a call this is time saved. One option (if you're optimising for speed and not space) is to make use of inline functions.
• If function calls are unavoidable minimise the data that is being passed to the functions. For example passing pointers is likely to be more efficient than passing structures.
• When optimising for speed choose datatypes that are the native size for your platform. For example on a 32bit processor it is likely to be more efficient to manipulate 32bit values than 8 or 16 bit values. (side note - it is worth checking that the compiler is doing what you think it is. I've had situations where I've discovered that my compiler insisted on doing 16 bit arithmetic on 8 bit values with all of the to and from conversions to go with them)
• Find data that can be precalculated, and either calculate during initialisation or (better yet) at compile time. For example when implementing a CRC you can either calculate your CRC values on the fly (using the polynomial directly) which is great for size (but dreadful for performance), or you can generate a table of all of the interim values - which is a much faster implementation, to the detriment of the size.
• Localise your data. If you're manipulating a blob of data often your processor may be able to speed things up by storing it all in cache. And your compiler may be able to use shorter instructions that are suited to more localised data (eg. instructions that use 8 bit offsets instead of 32 bit)
• In the same vein, localise your functions. For the same reasons.
• Work out the assumptions that you can make about the operations that you're performing and find ways of exploiting them. For example, on an 8 bit platform if the only operation that at you're doing on a 32 bit value is an increment you may find that you can do better than the compiler by inlining (or creating a macro) specifically for this purpose, rather than using a normal arithmetic operation.
• Avoid expensive instructions - division is a prime example.
• The "register" keyword can be your friend (although hopefully your compiler has a pretty good idea about your register usage). If you're going to use "register" it's likely that you'll have to declare the local variables that you want "register"ed first.
• Be consistent with your data types. If you are doing arithmetic on a mixture of data types (eg. shorts and ints, doubles and floats) then the compiler is adding implicit type conversions for each mismatch. This is wasted cpu cycles that may not be necessary.

Most of the options listed above can be used as part of normal practice without any ill effects. However if you're really trying to eke out the best performance: - Investigate where you can (safely) disable error checking. It's not recommended, but it will save you some space and cycles. - Hand craft portions of your code in assembler. This of course means that your code is no longer portable but where that's not an issue you may find savings here. Be aware though that there is potentially time lost moving data into and out of the registers that you have at your disposal (ie. to satisfy the register usage of your compiler). Also be aware that your compiler should be doing a pretty good job on its own. (of course there are exceptions)

Answer 12

On most of embedded system i worked there was no profiling tools, so it's nice to say use profiler but not very practical.

First rule in speed optimization is - find your critical path.
Usually you will find that this path is not so long and not so complex. It's hard to say in generic way how to optimize this it's depend on what are you doing and what is in your power to do. For example you want usually avoid memcpy on critical path, so ever you need to use DMA or optimize, but what if you hw does not have DMA ? check if memcpy implementation is a best one if not rewrite it.
Do not use dynamic allocation at all in embedded but if you do for some reason don't do it in critical path.
Organize your thread priorities correctly, what is correctly is real question and it's clearly system specific.
We use very simple tools to analyze the bottle-necks, simple macro that store the time-stamp and index. Few (2-3) runs in 90% of cases will find where you spend your time.
And the last one is code review a very important one. In most case we avoid performance problem during code review very effective way :)

Answer 13

basics/general:

• Do not optimize when you have no problem.
• Know your platform/CPU...
• ...know it thoroughly
• know your ABI
• Let the compiler do the optimization, just help it with the job.

some things that have actually helped:

Opt for size/memory:

• Use bitfields for storing bools
• re-use big global arrays by overlaying with a union (be careful)

Opt for speed (be careful):

• use precomputed tables where possible
• place critical functions/data in fast memory
• Use dedicated registers for often used globals
• count to-zero, zero flag is free

Answer 14

Difficult to summarize ...

• Data structures:

  • Splitting of a data structure depending on case of usage is extremely important. It is common to see a structure that holds data that is accessed based on a flow control. This situation can lower significantly the cache usage.
  • To take into account cache line size and prefetch rules.
  • To reorder the members of the structure to obtain a sequential access to them from your code

• Algorithms:

  • Take time to think about your problem and to find the correct algorithm.
  • Know the limitations of the algorithm you choose (a radix-sort/quick-sort for 10 elements to be sorted might not be the best choice).

• Low level:

  • As for the latest processors it is not recommended to unroll a loop that has a small body. The processor provides its own detection mechanism for this and will short-circuit whole section of its pipeline.
  • Trust the HW prefetcher. Of course if your data structures are well designed ;)
  • Care about your L2 cache line misses.
  • Try to reduce as much as possible the local working set of your application as the processors are leaning to smaller caches per cores (C2D enjoyed a 3MB per core max where iCore7 will provide a max of 256KB per core + 8MB shared to all cores for a quad core die.).

The most important of all: Measure early, Measure often and never ever makes assumptions, base your thinking and optimizations on data retrieved by a profiler (please use PTU).

Another hint, performance is key to the success of an application and should be considered at design time and you should have clear performance targets.

This is far from being exhaustive but should provide an interesting base.

Answer 15

I agree with the other posters:

• Use a profiling tool to find your bottlenecks
• Don't Optimise unless you know you have a problem.
• Select the right algorithm (for example, don't use Bubblesort. Ever!)
• Let the compiler optimise for you, but using certain 'patterns' can help it do its job.
• Benchmark to ensure your 'improvements' really are, under various input data

This link describes many low level bit algorithms that I've found useful in the past: BitHacks.

Answer 16

I would recommend optimizing using more efficient algorithms and not do it as an afterthought but code it that way from the start. Let the compiler work out the details on the small things as it knows more about the target processor than you do.

For one, I rarely use loops to look things up, I add items to a hashtable and then use the hashtable to lookup the results.

For example you have a string to lookup and then 50 possible values. So instead of doing 50 strcmps, you add all 50 strings to a hashtable and give each a unique number ( you only have to do this once ). Then you lookup the target string in the hashtable and have one large switch with all 50 cases ( or have functions pointers ).

When looking up things with common sets of input ( like css rules ), I use fast code to keep track of the only possible solitions and then iterate thought those to find a match. Once I have a match I save the results into a hashtable ( as a cache ) and then use the cache results if I get that same input set later.

My main tools for faster code are:

hashtable - for quick lookups and for caching results

qsort - it's the only sort I use

bsp - for looking up things based on area ( map rendering etc )

Answer 17

1. Measure performance.
2. Use realistic and non-trivial benchmarks. Remember that "everything is fast for small N".
3. Use a profiler to find hotspots.
4. Reduce number of dynamic memory allocations, disk accesses, database accesses, network accesses, and user/kernel transitions, because these often tend to be hotspots.
5. Measure performance.

In addition, you should measure performance.

Answer 18

If possible, compare with 0, not with arbitrary numbers, especially in loops, because comparison with 0 is often implemented with separate, faster assembler commands.

For example, if possible, write

for (i=n; i!=0; --i) { ... }

instead of

for (i=0; i!=n; ++i) { ... }

Answer 19

As everybody else has said: profile, profile profile.

As for actual techniques, one that I don't think has been mentioned yet:

Hot & Cold Data Separation: Staying within the CPU's cache is incredibly important. One way of helping to do this is by splitting your data structures into frequently accessed ("hot") and rarely accessed ("cold") sections.

An example: Suppose you have a structure for a customer that looks something like:

struct Customer
{
    int ID;
    int AccountNumber;
    char Name[128];
    char Address[256];
};

Customer customers[1000];

Now, lets assume that you want to access the ID and AccountNumber a lot, but not so much the name and address. What you'd do is to split it into two:

struct CustomerAccount
{
    int ID;
    int AccountNumber;
    CustomerData *pData;
};

strut CustomerData
{
    char Name[128];
    char Address[256];
}

CustomerAccount customers[1000];

In this way, when you're looping through your "customers" array, each entry is only 12 bytes and so you can fit many more entries in the cache. This can be a huge win if you can apply it to situations like the inner loop of a rendering engine.

Answer 20

Sometimes you have to decide whether it is more space or more speed that you are after, which will lead to almost opposite optimizations. For example, to get the most out of you space, you pack structures e.g. #pragma pack(1) and use bit fields in structures. For more speed you pack to align with the processors preference and avoid bitfields.

Another trick is picking the right re-sizing algorithms for growing arrays via realloc, or better still writing your own heap manager based on your particular application. Don't assume the one that comes with the compiler is the best possible solution for every application.

Answer 21

These days, the most important things in optimzation are:

• respecting the cache - try to access memory in simple patterns, and don't unroll loops just for fun. Use arrays instead of data structures with lots of pointer chasing and it'll probably be faster for small amounts of data. And don't make anything too big.
• avoiding latency - try to avoid divisions and stuff that's slow if other calculations depend on them immediately. Memory accesses that depend on other memory accesses (ie, a[b[c]]) are bad.
• avoiding unpredictabilty - a lot of if/elses with unpredictable conditions, or conditions that introduce more latency, will really mess you up. There's a lot of branchless math tricks that are useful here, but they increase latency and are only useful if you really need them. Otherwise, just write simple code and don't have crazy loop conditions.

Don't bother with optimizations that involve copy-and-pasting your code (like loop unrolling), or reordering loops by hand. The compiler usually does a better job than you at doing this, but most of them aren't smart enough to undo it.

Answer 22

If someone doesn't have an answer to that question, it could be they don't know much.

It could also be that they know a lot. I know a lot (IMHO :-), and if I were asked that question, I would be asking you back: Why do you think that's important?

The problem is, any a-priori notions about performance, if they are not informed by a specific situation, are guesses by definition.

I think it is important to know coding techniques for performance, but I think it is even more important to know not to use them, until diagnosis reveals that there is a problem and what it is.

Now I'm going to contradict myself and say, if you do that, you learn how to recognize the design approaches that lead to trouble so you can avoid them, and to a novice, that sounds like premature optimization.

To give you a concrete example, this is a C application that was optimized.

Answer 23

Great lists. I will just add one tip I didn't saw in the above lists that in some case can yield huge optimisation for minimal cost.

• bypass linker

  if you have some application divided in two files, say main.c and lib.c, in many cases you can just add a \#include "lib.c" in your main.c That will completely bypass linker and allow for much more efficient optimisation for compiler.

The same effect can be achieved optimizing dependencies between files, but the cost of changes is usually higher.

Answer 24

Sometimes Google is the best algorithm optimization tool. When I have a complex problem, a bit of searching reveals some guys with PhD's have found a mapping between this and a well-known problem and have already done most of the work.