This new CTU implementation is the natural extension of the normal single TU

analysis. The approach consists of two analysis phases. During the first phase,

we do a normal single TU analysis. During this phase, if we find a foreign

function (that could be inlined from another TU) then we don’t inline that

immediately, we rather mark that to be analysed later.

When the first phase is finished then we start the second phase, the CTU phase.

In this phase, we continue the analysis from those points (exploded nodes)

which had been enqueued during the first phase. We gradually extend the

exploded graph of the single TU analysis with new nodes that are created by the

inlining of foreign functions.

We count the number of analysis steps of the first phase and we limit the

second (ctu) phase with this number.

This new implementation makes it convenient for the users to run the single-TU

and the CTU analysis in one go, they don't need to run the two analysis separately.

Thus, we name this new implementation as **onego** CTU.

Discussion:

https://discourse.llvm.org/t/rfc-much-faster-cross-translation-unit-ctu-analysis-implementation/61728