[CSSPGO][llvm-profgen] Reimplement CS profile generator using context trie

Our investigation showed ProfileMap's key is the bottleneck of the memory consumption for CS profile generation on some large services. This patch tries to optimize it by storing the CS function samples using the context trie tree structure instead of the context frame array ref. Parts of code in `ContextTrieNode` are reused.

Our experiment on one internal service showed that the context key's memory can be reduced from 80GB to 300MB.

To be compatible with non-CS profiles, the profile writer still needs to use ProfileMap as input, so rebuild the ProfileMap using the context trie in `postProcessProfiles`.

The optimization is not complete yet, next step is to reimplement Pre-inliner or profile trimmer, after that, ProfileMap should be small to be written.

Reviewed By: hoy, wenlei

Differential Revision: https://reviews.llvm.org/D125246

Author: wlei-llvm

llvm/include/llvm/Transforms/IPO/SampleContextTracker.h

@@ -105,6 +105,7 @@ class SampleContextTracker {
   // deterministically.
   using ContextSamplesTy = std::set<FunctionSamples *, ProfileComparer>;
 
+  SampleContextTracker() = default;
   SampleContextTracker(SampleProfileMap &Profiles,
                        const DenseMap<uint64_t, StringRef> *GUIDToFuncNameMap);
   // Query context profile for a specific callee with given name at a given
@@ -122,6 +123,8 @@ class SampleContextTracker {
   // Get all context profile for given function.
   ContextSamplesTy &getAllContextSamplesFor(const Function &Func);
   ContextSamplesTy &getAllContextSamplesFor(StringRef Name);
+  ContextTrieNode *getOrCreateContextPath(const SampleContext &Context,
+                                          bool AllowCreate);
   // Query base profile for a given function. A base profile is a merged view
   // of all context profiles for contexts that are not inlined.
   FunctionSamples *getBaseSamplesFor(const Function &Func,
@@ -146,8 +149,6 @@ class SampleContextTracker {
   ContextTrieNode *getContextFor(const DILocation *DIL);
   ContextTrieNode *getCalleeContextFor(const DILocation *DIL,
                                        StringRef CalleeName);
-  ContextTrieNode *getOrCreateContextPath(const SampleContext &Context,
-                                          bool AllowCreate);
   ContextTrieNode *getTopLevelContextNode(StringRef FName);
   ContextTrieNode &addTopLevelContextNode(StringRef FName);
   ContextTrieNode &promoteMergeContextSamplesTree(ContextTrieNode &NodeToPromo);