If the script that jsir_gen is acting upon contains non-ASCII characters such as Cyrillic script (even in comments), it mixes up byte-position vs character-position and outputs wrong segments of the script. The issue appears to be that JsPosition when fetching from the JSON returns character position, but then original_source.substr on lines 75-77 of jsir_gen_lib.cc uses absl::string_view.substr which operates on bytes.

For example, if you run jsir_gen on the script:

// тестовые символы кириллицы

let c = atob("SGVsbG8sIHdvcmxkIQ==");

It will output:

// Dataflow etc omitted
================================================================
Computed constants:
================================================================
From [39, 67): `риллицы

let c = atob` -> `Hello, world!`
================================================================

In between the ` ` it has mistakenly captured the newlines and the end of the comment due to the mixup instead of the actual function call.