Function calls (in hot code paths) can make the code really slow due to the function call overhead alone but the need to step into these function is minimal.
In order to add useful debug functionality, it would be useful to make these key places faster by forcing the code to be inlined.
Specifically, forcing inlining of wtfThreadData() in currentThread() should allow for currentThread to have comparable speed to isMainThread().
Actually this isn't needed anymore. I did the necessary speed ups in https://bugs.webkit.org/show_bug.cgi?id=64577 without it.