[compiler-rt] [llvm] [ctxprof] Prepare profile format for flat profiles (PR #129626)

[Mircea Trofin via llvm-commits]

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@@ -114,9 +114,14 @@ class ContextNode final {
 };
 
 /// Abstraction for the parameter passed to `__llvm_ctx_profile_fetch`.
+/// `startContextSection` is called before any context roots are sent for
+/// writing. Then one or more `writeContextual` calls are made; finally,
+/// `endContextSection` is called.
 class ProfileWriter {
 public:
+  virtual void startContextSection() = 0;
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mtrofin wrote:

I thought of that but yes, seemed overkill. It's a low-level interface, really meant to go between compiler-rt and whatever service handles writing the profile.

https://github.com/llvm/llvm-project/pull/129626