Why does replace() return the original string when the pattern is not found?

[egecanakincioglu]

In Arimo's String.replace() method, when the old string is not found in the source, the method returns the original string unchanged instead of crashing. How is this handled at the LLVM IR level?

For example:

String s = "hello world";
String result = s.replace("xyz", "abc");
IO.println(result); // prints "hello world"

Is there a null check somewhere in the generated IR?

[egecanakincioglu]

The strstr C function returns a null pointer when the pattern is not found. In the previous implementation this caused heap corruption because ptr_to_int(null) produced 0, making the prefix length calculation overflow.

The fix adds a runtime null check in LLVM IR:

1. strstr is called and the result pointer is checked with build_is_null()
2. If null → branch to rep.nofound block, jump to rep.end
3. If non-null → branch to rep.found block, do the replacement, jump to rep.end
4. A phi node at rep.end selects between the original str_ptr (no match) and the new buf_ptr (match found)

This gives correct behaviour without any Arimo-level changes.