A lot of times I see code like:
int s = a / x;
for (int i = 0; i < s; i++)
// do something
If inside the for loop, neither a nor x is modified, can I then simply write:
for (int i = 0; i < a / x; i++)
// do something
and then assume that the compiler optimizes a/x, i.e replaces it with a constant?
The most important part of int s = a / x is the variable name. It gives your syntax semantics, and lets you remember 12 months later why you were dividing one thing by another. You can't name the expression in the for statement, so you lose that self-documenting nature.
const auto monthlyAmount = (int)yearlyAmount / numberOfMonths;
for (auto i = 0; i < monthlyAmount; ++i)
// do something
In this way, extracting the variable isn't for a compiler optimization, it's a human maintainability optimization.
If the compiler can be sure that the variables used in the expression in the middle of your for loop will not change between iterations, it can optimize the calculation to be performed once at the beginning of the loop, instead of every iteration.
However, consider that the variables used are global variables, or references to variables external to the function, and in your for loop you call a function. The function could change these variables. If the compiler is able to see enough of the code at that point, it could find out if this is the case to decide whether to optimize. However, compilers are only willing to look so far (otherwise things would take even longer to compile), so in general you cannot assume the optimization is performed.
The concern for optimization probably stems from the fact that the condition is evaluated before each iteration. If this is a potentially expensive operation and you don't need to do it over and over again, you can extract it out of the loop:
const std::size_t size = s.size(); // just an example
for (std::size_t i = 0; i < size; ++i)
{
}
For inexpensive operations this is probably a premature optimization and the compiler might generate the same code. The only way to be sure is to check the generated assembly code.
The problem with such Questions is that they cannot be generalized. What optimizations the Compiler will perform and what not can only be determined by a case by case analysis.
I'd certainly expect the compiler to do this if one of the following holds true:
1) Both, A and B are local variables, whose addresses are never taken.
2) The code in the loop is completely inlined.
In practice the last requirement isn't as hard as it looks, because if the functions in the body cannot be inlined, their runtime will likely dwarf the time to re-compute the bound anyway
Related
Look at this function:
float process(float in) {
float out = in;
for (int i = 0; i < 31; ++i) {
if (biquads_[i]) {
out = biquads_[i]->filter(out);
}
}
return out;
}
biquads_ is a std::optional<Biquad>[31].
in this case i check for every optional to check if its not empty, and then call the filter function of biquad, if instead I unconditionally call filter function, changing it to multiply by 1 or simply return the input value, would be more efficient?
Most likely it won't make a shread of difference (guessing somewhat though since your question is not entirely clear). For two reasons: 1) unless the code is going to be used in a very hot path, it won't matter even if one way is a few nanoseconds faster than the other. 2) most likely your compilers optimizer will be clever enough to generate code that performs close-to (if not identical to) the same in both cases. Did you test it? Did you benchmark/profile it? If not; do so - with optimization enabled.
Strive to write clear, readable, maintainable code. Worry about micro-optimization later when you actually have a problem and your profiler points to your function as a hot-spot.
I have an example code like this, in which the literal 1 repeats several times.
foo(x - 1);
y = z + 1;
bar[1] = y;
Should I define a constant ONE, and replace the literals with it?
constexpr int ONE = 1;
foo(x - ONE);
y = z + ONE;
bar[ONE] = y;
Would this replacement make any performance improvement and/or reduce machine code size in the favor of reducing code readability? Would the number of repeating of the literal change the answer?
It will not bring you any performance/memory improvements. However, you should try to keep your code clean from magical numbers. So, if there is a repeated constant in your code in several places, and in all those places this constant is the same from logical point of view, it would be better to make it a named constant.
Example:
const int numberOfParticles = 10; //This is just an example, it's better not to use global variables.
void processParticlesPair(int i, int j) {
for (int iteration = 0; iteration < 10; ++iteration) {
//note, that I didn't replace "10" in the line above, because it is not a numberOrParticles,
//but a number of iterations, so it is a different constant from a logical point of view.
//Do stuff
}
}
void displayParticles() {
for (int i = 0; i < numberOfParticles; ++i) {
for (int j = 0; j < numberOfParticles; ++j) {
if (i != j) {
processParticlesPair(i, j);
}
}
}
}
Depends. If you just have 1s in your code and you ask if you should replace them: DONT. Keep your code clean. You will not have any performance or memory advantages - even worse, you might increase build time
If the 1, however, is a build-time parameter: Yes, please introduce a constant! But choose a better name than ONE!
Should I define a constant ONE, and replace the literals with it?
No, absolutely not. If you have a name that indicates the meaning of the number (e.g. NumberOfDummyFoos), if its value can change and you want to prevent having to update it in a dozen locations, then you can use a constant for that, but a constant ONE adds absolutely no value over a literal 1.
Would this replacement make any performance improvement and/or reduce machine code size in the favor of reducing code readability?
In any realistic implementation, it does not.
Replacing literals with named constants make only sense,
if the meaning of the constant is special. Replacing 1 with ONE is
just overhead in most cases, and does not add any useful information
to the reader, especially if it is used in different functions (index, part of a calculation etc.). If the entry 1 of an array is somehow special, using a constant THE_SPECIAL_INDEX=1 would make sense.
For the compiler it usually does not make any difference.
In assembly, one constant value generally takes the same amount of memory as any other. Setting a constant value in your source code is more of a convenience for humans than an optimization.
In this case, using ONE in such a way is neither a performance or readability enhancement. That's why you've probably never seen it in source code before ;)
I noticed that Google's C++ style guide cautions against inlining functions with loops or switch statements:
Another useful rule of thumb: it's typically not cost effective to
inline functions with loops or switch statements (unless, in the
common case, the loop or switch statement is never executed).
Other comments on StackOverflow have reiterated this sentiment.
Why are functions with loops or switch statements (or gotos) not suitable for or compatible with inlining. Does this apply to functions that contain any type of jump? Does it apply to functions with if statements? Also (and this might be somewhat unrelated), why is inlining functions that return a value discouraged?
I am particularly interested in this question because I am working with a segment of performance-sensitive code. I noticed that after inlining a function that contains a series of if statements, performance degrades pretty significantly. I'm using GNU Make 3.81, if that's relevant.
Inlining functions with conditional branches makes it more difficult for the CPU to accurately predict the branch statements, since each instance of the branch is independent.
If there are several branch statements, successful branch prediction saves a lot more cycles than the cost of calling the function.
Similar logic applies to unrolling loops with switch statements.
The Google guide referenced doesn't mention anything about functions returning values, so I'm assuming that reference is elsewhere, and requires a different question with an explicit citation.
While in your case, the performance degradation seems to be caused by branch mispredictions, I don't think that's the reason why the Google style guide advocates against inline functions containing loops or switch statements. There are use cases where the branch predictor can benefit from inlining.
A loop is often executed hundreds of times, so the execution time of the loop is much larger than the time saved by inlining. So the performance benefit is negligible (see Amdahl's law). OTOH, inlining functions results in increase of code size which has negative effects on the instruction cache.
In the case of switch statements, I can only guess. The rationale might be that jump tables can be rather large, wasting much more memory in the code segment than is obvious.
I think the keyword here is cost effective. Functions that cost a lot of cycles or memory are typically not worth inlining.
The purpose of a coding style guide is to tell you that if you are reading it you are unlikely to have added an optimisation to a real compiler, even less likely to have added a useful optimisation (measured by other people on realistic programs over a range of CPUs), therefore quite unlikely to be able to out-guess the guys who did. At least, do not mislead them, for example, by putting the volatile keyword in front of all your variables.
Inlining decisions in a compiler have very little to do with 'Making a Simple Branch Predictor Happy'. Or less confused.
First off, the target CPU may not even have branch prediction.
Second, a concrete example:
Imagine a compiler which has no other optimisation (turned on) except inlining. Then the only positive effect of inlining a function is that bookkeeping related to function calls (saving registers, setting up locals, saving the return address, and jumping to and back) are eliminated. The cost is duplicating code at every single location where the function is called.
In a real compiler dozens of other simple optimisations are done and the hope of inlining decisions is that those optimisations will interact (or cascade) nicely. Here is a very simple example:
int f(int s)
{
...;
switch (s) {
case 1: ...; break;
case 2: ...; break;
case 42: ...; return ...;
}
return ...;
}
void g(...)
{
int x=f(42);
...
}
When the compiler decides to inline f, it replaces the RHS of the assignment with the body of f. It substitutes the actual parameter 42 for the formal parameter s and suddenly it finds that the switch is on a constant value...so it drops all the other branches and hopefully the known value will allow further simplifications (ie they cascade).
If you are really lucky all calls to the function will be inlined (and unless f is visible outside) the original f will completely disappear from your code. So your compiler eliminated all the bookkeeping and made your code smaller at compile time. And made the code more local at runtime.
If you are unlucky, the code size grows, locality at runtime decreases and your code runs slower.
It is trickier to give a nice example when it is beneficial to inline loops because one has to assume other optimisations and the interactions between them.
The point is that it is hellishly difficult to predict what happens to a chunk of code even if you know all the ways the compiler is allowed to change it. I can't remember who said it but one should not be able to recognise the executable code produced by an optimising compiler.
I think it might be worth to extend the example provided by #user1666959. I'll answer to provide cleaner example code.
Let's consider such scenario.
/// Counts odd numbers in range [0;number]
size_t countOdd(size_t number)
{
size_t result = 0;
for (size_t i = 0; i <= number; ++i)
{
result += (i % 2);
}
return result;
}
int main()
{
return countOdd(5);
}
If the function is not inlined and uses external linking, it will execute whole loop. Imagine what happens when you inline it.
int main()
{
size_t result = 0;
for (size_t i = 0; i <= 5; ++i)
{
result += (i % 2);
}
return result;
}
Now let's enable loop unfolding optimization. Here we know that it iterates from 0 to 5, so it can be easily unfolded removing unwanted conditions in the code.
int main()
{
size_t result = 0;
// iteration 0
size_t i = 0
result += (i % 2);
// iteration 1
++i
result += (i % 2);
// iteration 2
++i
result += (i % 2);
// iteration 3
++i
result += (i % 2);
// iteration 4
++i
result += (i % 2);
// iteration 5
++i
result += (i % 2);
return result;
}
No conditions, it is faster already but that's not all. We know the value of i, so why not passing it directly?
int main()
{
size_t result = 0;
// iteration 0
result += (0 % 2);
// iteration 1
result += (1 % 2);
// iteration 2
result += (2 % 2);
// iteration 3
result += (3 % 2);
// iteration 4
result += (4 % 2);
// iteration 5
result += (5 % 2);
return result;
}
Even simpler but whait, those operations are constexpr, we can calculate them during compilation.
int main()
{
size_t result = 0;
// iteration 0
result += 0;
// iteration 1
result += 1;
// iteration 2
result += 0;
// iteration 3
result += 1;
// iteration 4
result += 0;
// iteration 5
result += 1;
return result;
}
So now the compiler sees that some of those operations don't have any effects leaving only those, which change the value. After that it removes unnecessary temporary variables and performs as much calculations, as it can during compilation, your code ends up with:
int main()
{
return 3;
}
Of the two following for loop implementations which should run faster or are they equal?
This one:
for(int i=0; i<objectPtr->bound; i++){
//do something that has no affect on objectPtr->bound
}
Or this one:
int myBound = objectPtr->bound;
for(int i=0; i<myBound; i++){
//do something that has no effect on objectPtr->bound or myBound
}
My hunch is the latter is faster but there could be some part of the compilation process that I don't understand that makes them equal in speed. I think the former will have to do an address resolution each loop cycle to determine if the value has changed.
Is there some c++ syntax that can be used to let the compiler know that the value/bound will not change. I know volatile lets the compiler know that the value will always have the possibility of changing so could I use const int for the objectPtr->bound variable to get the compiler to not always check its value each loop iteration?
The compiler might see that the value of objectPtr->bound doesn't change between iterations. In this case, it will automatically extract it into a variable to avoid repeated lookups.
There is no way to explicitly tell the compiler that the value of an expression doesn't change. If the compiler can't detect it itself, you can copy the value into a variable as you did.
Declaring local variables as const is not necessary. The compiler can see that you don't assign to the variable in a loop and deduce that its value doesn't change.
I have the following looking code in VC++:
for (int i = (a - 1) * b; i < a * b && i < someObject->someFunction(); i++)
{
// ...
}
As far as I know compilers optimize all these arithmetic operations and they won't be executed on each loop, but I'm not sure if they can tell that the function above also returns the same value each time and it doesn't need to be called each time.
Is it a better practice to save all calculations into variables, or just rely on compiler optimizations to have a more readable code?
int start = (a - 1) * b;
int expra = a * b;
int exprb = someObject->someFunction();
for (int i = startl i < expra && i < exprb; i++)
{
// ...
}
Short answer: it depends. If the compiler can deduce that running someObject->someFunction() every time and caching the result once both produce the same effects, it is allowed (but not guaranteed) to do so. Whether this static analysis is possible depends on your program: specifically, what the static type of someObject is and what its dynamic type is expected to be, as well as what someFunction() actually does, whether it's virtual, and so on.
In general, if it only needs to be done once, write your code in such a way that it can only be done once, bypassing the need to worry about what the compiler might be doing:
int start = (a - 1) * b;
int expra = a * b;
int exprb = someObject->someFunction();
for (int i = start; i < expra && i < exprb; i++)
// ...
Or, if you're into being concise:
for (int i = (a - 1) * b, expra = a * b, exprb = someObject->someFunction();
i < expra && i < exprb; i++)
// ...
From my experience VC++ compiler won't optimize the function call out unless it can see the function implementation at the point of compiling the calling code. So moving the call outside the loop is a good idea.
If a function resides within the same compilation unit as its caller, the compiler can often deduce some facts about it - e.g. that its output might not change for subsequent calls. In general, however, that is not the case.
In your example, assigning variables for these simple arithmetic expressions does not really change anything with regards to the produced object code and, in my opinion, makes the code less readable. Unless you have a bunch of long expressions that cannot reasonably be put within a line or two, you should avoid using temporary variables - if for no other reason, then just to reduce namespace pollution.
Using temporary variables implies a significant management overhead for the programmer, in order to keep them separate and avoid unintended side-effects. It also makes reusing code snippets harder.
On the other hand, assigning the result of the function to a variable can help the compiler optimise your code better by explicitly avoiding multiple function calls.
Personally, I would go with this:
int expr = someObject->someFunction();
for (int i = (a - 1) * b; i < a * b && i < expr; i++)
{
// ...
}
The compiler cannot make any assumption on whether your function will return the same value at each time. Let's imagine that your object is a socket, how could the compiler possibly know what will be its output?
Also, the optimization that a compiler can make in such loops strongly depends on the whether a and b are declared as const or not, and whether or not they are local. With advanced optimization schemes, it may be able to infer that a and b are neither modified in the loop nor in your function (again, you might imagine that your object holds some reference to them).
Well, in short: go for the second version of your code!
It is very likely that the compiler will call the function each time.
If you are concerned with the readability of code, what about using:
int maxindex = min (expra, exprb);
for (i=start; i<maxindex; i++)
IMHO, long lines does not improve readability.
Writing short lines and doing multiple step to get a result, does not impact the performance, this is exactly why we use compilers.
Effectively what you might be asking is whether the compiler will inline the function someFunction() and whether it will see that someObject is the same instance in each loop, and if it does both it will potentially "cache" the return value and not keep re-evaluating it.
Much of this may depend on what optimisation settings you use, with VC++ as well as any other compiler, although I am not sure VC++ gives you quite as many flags as gnu.
I often find it incredible that programmers rely on compilers to optimise things they can easily optimise themselves. Just move the expression to the first section of the for-loop if you know it will evaluate the same each time:
Just do this and don't rely on the compiler:
for (int i = (a - 1) * b, iMax = someObject->someFunction();
i < a * b && i < iMax; ++i)
{
// body
}