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

[Teresa Johnson via llvm-commits]

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@@ -51,5 +53,155 @@ 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 structure of the ContextNode is known to LLVM, because LLVM needs to:
+///   (1) increment countes, and
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teresajohnson wrote:

s/countes/counts/

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