"Include What you use"

1. why this tool is compiler-dependent and not "cross-platform" for compilers. Why from the beginning the creaters of the tool didn't make it compiler-independent ? is it related to the special implementation it has or something like that ?

Reason is simple, clang has is more modern fresh and has better modular architecture. As a result is is much easier to create tools using clang modules.

That is why clang was first which had address sanitizer and have more cool sanitizers. And this is why when someone creates new tool for C++ stars from clang.

2. If i want to take a version of the tool compatible for llvm and i want to make it compatible with gcc(since i'm working with gcc). What i have to do for that ?

clang was created since Apple was not happy with gcc. So when it was written it supposed to be as much compatible with gcc as possible, since there was lots of code which was verified with gcc.

Since clang is mature now and provides new own features, there might be small differences with gcc (for example different bugs in implementations of new C++ standards), but in most common code there should be no problem.

IWYU should work without problems on both compilers. My coworker used it on large project build with 3 compilers (clang, gcc and VS) and it worked like a charm.

The tool itself needs parts of the compiler! It is sitting somewhere between reading the source and parsing it. LLVM provides an API which is used for the tool. The tool itself is not standalone but a plugin to the clang/llvm. As this, it needs the clang/llvm.

The modifications which will be done by the tool are fully compatible to every c++ compiler. And also the plugin in combination with clang/llvm should be able to parse more or less every code base independent of used other compilers. There might be some strange macros which are supported by other tool chains, which llvm might be struggle with. But that should be a rare case at all.