[patch] Teach GlobalDCE how to remove empty global_ctor entries

[Rafael Espíndola]

OptimizeGlobalCtorsList

start the name with a lowercase.

+bool IsEmptyFunction(Function* F) {
+  BasicBlock &Entry = F->getEntryBlock();
+  return Entry.size() == 1 && isa<ReturnInst>(Entry.front());
 }

There is no way to cast to void in llvm, right? The thing I am afraid
of is if a trapping constant can be hidden in there.


+/// Given a specified llvm.global_ctors list, install the
+/// specified array, returning the new global to use.
+void InstallGlobalCtors(GlobalVariable *GCL,

lowercase. Returning? install?

ParseGlobalCtors

lowercase.

+  if (!GV->hasUniqueInitializer()) return nullptr;

please clang-format.


+    // Init priority must be standard.

Do you know why? At least for the empty case it should be valid to
drop constructors of any priority.


+ ... Call "Remove" for


It is actually a predicate, so maybe ShouldRemove?


This is a case where an integration test might be a good idea.  Do you
have a reasonably size test where this change causes a ctor to  be
deleted at -O2? We might want to test that, so that even if we go with
a different optimization strategy in the future we still have a test
of the end result (the ctor gets deleted somewhere in -O2).



Cheers,
Rafael