Sнаđошƒаӽ Sнаđошƒаӽ - 1 year ago 65
C++ Question

Compiler/JIT optimisation of bounds check of for loop in Java and C++

I have learned from this answer on

loops in C# that: "The compiler/JIT has optimisations for this scenario as long as you use
in the condition:"

for(int i = 0 ; i < arr.Length ; i++) {
Console.WriteLine(arr[i]); // skips bounds check

This tempted me to wonder if java compiler has such optimizations.

for(int i=0; i<arr.length; i++) {
System.out.println(arr[i]); // is bounds check skipped here?

I think it does, well, does it? Does the same happen when using

But what if I have to use the value of
inside the body of the for loop, considering now
to be an ArrayList? So in that case will not hoisting
help, since
is a method call? For example may be something like this:

int len = myList.size(); // hoisting for using inside the loop
for(int i = 0; i < myList.size(); i++) { // not using hoisted value for optimization

if(someOtherVariable == len) {

Though I haven't obtained an answer for java, I am still adding a second part to this question.

Q: Does C++ (C++98/C++11) have such optimisations, e.g. for
? For instance, which is better performance-wise?

for (int i = 0; i < myvector.size(); ++i)
cout << myvector[i] << " "; // is bounds checking skipped here, like in C#?


// does this manual optimisation help improve performance, or does it make?
int size = myvector.size();
for (int i = 0; i < size; ++i)
cout << myvector[i] << " ";

That said, does such optimisation exist for
as well?

Answer Source


Since Java 7, the compiler eliminates bounds checking for primitive arrays if it can prove that out-of-bound access is not possible. Before Java 7, JIT or AOT compilers can do that. For JIT and AOT compilers, it is not limited to for (int i = 0; i < arr.length; i++), it can move bounds checks outside the loop for loops such as for (int i = 0; i < 10000000; i++), reducing it to a single check. If that check fails, it will run a version of the code with full bounds checking to throw the exception at the correct place.

For collections it's much more complicated because the bounds are checked by the called method, not at the place of the call. Generally, it cannot be eliminated from the bytecode but JIT and AOT compilers can eliminate it if they can inline the methods (which depends on how the object is instantiated and where it is stored, among other things, because all non-private methods in Java are virtual so the compiler needs to make sure it won't need virtual call) but I don't know if they actually do.


C++ does not check bounds in operator []. It does check bounds when you use at. at is inlined so it depends on the specific compiler and its flags but generally, the compiler can remove bounds checking if it can prove that out-of-bound access is not possible. It could also move the bounds check outside the loop but it still needs to guarantee that the exception will be thrown at the correct place so I don't know if any does.

Both examples for C++ will be equivalent in a compiler that does Dead Code Elimination, assuming you don't use int size after the for loop. The latter may be faster if you would use some methods that are not inlined. (You can also put the definition of size inside the initialization: for (int i = 0, size = myvector.size(); i < size; ++i))