[llvm] Introduction of typified section in ExtBinary format (PR #166553)

[Snehasish Kumar via llvm-commits]

snehasish wrote:

We need to address format compatibility. The typified sections introduce a new on-disk representation, but older LLVM readers treat unknown ExtBinary sections as skippable. If we emit typified profiles using the existing profile version, an older reader may therefore accept the file while silently ignoring the actual profile data rather than rejecting it as unsupported.

I recommend bumping the ExtBinary profile version as part of this change. The compatibility model should be:

New readers continue to support existing/older profile versions.

The new typified representation is associated with the new profile version, so older readers reject it cleanly.

While the typified format is still under development, writing it should remain opt-in behind an explicit flag.

The default writer should continue emitting the existing format until the new representation is considered ready to become the default.

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Also it would be great to rename this work to something like "Composite Profile". I believe the word typified means exemplify or characterize which isn't the meaning we are trying to convey here.

https://github.com/llvm/llvm-project/pull/166553