[compiler-rt] [compiler-rt][ctx_profile] Add the instrumented contextual profiling APIs (PR #89838)

[Mircea Trofin via llvm-commits]

================
@@ -51,5 +53,189 @@ class Arena final {
   const uint64_t Size;
 };
 
+/// The contextual profile is a directed tree where each node has one parent. A
+/// node (ContextNode) corresponds to a function activation. The root of the
+/// tree is at a function that was marked as entrypoint to the compiler. A node
+/// stores counter values for edges and a vector of subcontexts. These are the
+/// contexts of callees. The index in the subcontext vector corresponds to the
+/// index of the callsite (as was instrumented via llvm.instrprof.callsite). At
+/// that index we find a linked list, potentially empty, of ContextNodes. Direct
+/// calls will have 0 or 1 values in the linked list, but indirect callsites may
+/// have more.
+///
+/// The ContextNode has a fixed sized header describing it - the GUID of the
+/// function, the size of the counter and callsite vectors. It is also an
+/// (intrusive) linked list for the purposes of the indirect call case above.
+///
+/// Allocation is expected to happen on an Arena. The allocation lays out inline
+/// the counter and subcontexts vectors. The class offers APIs to correctly
+/// reference the latter.
+///
+/// The layout is as follows:
+///
+/// [[statically-declared fields][counters vector][subcontexts vector]]
----------------
mtrofin wrote:

I don't follow. The subcontext vector is of pointers to subcontexts. Maybe I should clarify that.

https://github.com/llvm/llvm-project/pull/89838