[CodeGenPrepare] Implement releaseMemory

[svenvh]

Summary

Release BlockFrequencyInfo and BranchProbabilityInfo results,
and other per function information
immediately afterwards, instead of holding onto the memory until the
next CodeGenPrepare::runOnFunction call.

Co-authored-by: Erik Hogeman <[e-mail]>

[arsenm]

llvm/lib/CodeGen/CodeGenPrepare.cpp

383

There are a few other maps, do those need to be cleared as well?

[svenvh]

llvm/lib/CodeGen/CodeGenPrepare.cpp

383

I was wondering about those too. They weren't problematic for my use case, but if you think it's worth I can move the various clear()s into releaseMemory too.

[svenvh]

Also clear some members that were cleared at the start of runOnFunction.

llvm/lib/CodeGen/CodeGenPrepare.cpp

public:
   static char ID; // Pass identification, replacement for typeid
 
   CodeGenPrepare() : FunctionPass(ID) {
     initializeCodeGenPreparePass(*PassRegistry::getPassRegistry());
   }
 
   bool runOnFunction(Function &F) override;
 
+  void releaseMemory() override {
+    BPI.reset();
+    BFI.reset();
+  }
+
   StringRef getPassName() const override { return "CodeGen Prepare"; }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     // FIXME: When we can selectively preserve passes, preserve the domtree.
     AU.addRequired<ProfileSummaryInfoWrapperPass>();
     AU.addRequired<TargetLibraryInfoWrapperPass>();
     AU.addRequired<TargetPassConfig>();
     AU.addRequired<TargetTransformInfoWrapperPass>();