This PR removes the old ElimDeadBranches pass and shifts the new one
past lambda lifting.
The reason for dropping the old one is its general unsoundness and the
fact that we want to do refactorings on the IR part. The reason for
shifting the current pass past lambda lifting, is that its analysis is
imprecise in the presence of local function symbols. I experimented with
the exact placement for a while and it seems like it is optimal here.
Overall we observe a slight regression in the amount of C code
generated, likely because we don't propagate information into lambdas
before lifting them anymore. But generally measure a slight performance
improvement in general.
This PR is a followup of #11381 and enforces the invariants on ordering
of closed terms and constants required by the EmitC pass properly by
toposorting before saving the declarations into the Environment.
The current reuse analysis is greedy in that every function attempts to
reuse a value. However, this means that if the last use is an owned
argument, it will be `inc`'d prior to this last use, in order to prevent
reuse from happening in the callee. In many cases, it makes more sense
to give the callee the chance to reuse it instead. The benchmark results
indicate that this is a much better default.
This PR moves the implementation of `lean_add_extern`/`addExtern` from
C++ into Lean. I believe is the last C++ helper function from the
library/compiler directory being relied upon by the new compiler. I put
it into its own file and duplicated some code because this function
needs to execute in CoreM, whereas the other IR functions live in their
own monad stack. After the C++ compiler is removed, we can move the IR
functions into CoreM.
