Getting calling function in the C frontend

[Per Ekman]

Hi,

Is it possible to get hold of the calling function (in the compiled
program, not in gcc itself) in c-typeck.c:build_function_call()?
Alternatively, what would be a good place to compare the attributes of
a called function with the attributes of the calling function?

What I'm trying to accomplish is adding a "execution context"
attribute to functions and have gcc warn when a function tagged
with execution context X calls a function with execution context Y.

/Pek