String copying optimization

[NN---]

Can JIT optimize this case ?

string a = f();
string b = a + "\n"; // g(a + "\n")
// 'a' is not used after this line

It is obvious that sine a is not used later in the method.
Can JIT optimize copying the string here ?

[AlgorithmsAreCool]

I am unclear on what optimization you are asking for. What would the optimized result look like?

[NN---]

String string is immutable, string concatenation results in a new string and memory allocation.

[EgorBo]

I am not sure I understand the desirable behavior, f() allocates a string somehow than we need to append a char to it - it's a different object now, we can't avoid allocating a can we? is f() supposed to be inlined?

[AlgorithmsAreCool]

I think the // 'a' is not used after this line comment in the original snippet is a typo. The only optimization in can see here is to elide the call to String.Concat because it's result b is not used and Concat itself is a pure function.

[EgorBo]

Well, if b is not used the allocation indeed can be elided but it requires a (partial) escape analysis - it's not something JIT does today unfortunately

[acaly]

You probably want to add an overload in addition to string f() as void f(StringBuilder), or use Span<char> if you have an idea on its maximum length. Then you can perform the concat yourself on your buffer. I believe that's how most C# code optimizes this kind of string concatenation today.

[NN---]

This option complicated the code which makes it harder to pass code review.

[danmoseley]

Answered - not possible without escape analysis.