[llvm] [llvm][AsmPrinter] Call graph section format. (PR #159866)

[Paul Kirth via llvm-commits]

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+# .callgraph Section Layout
+
+The `.callgraph` section is used to store call graph information for each function. The section contains a series of records, with each record corresponding to a single function.
+
+## Per Function Record Layout
+
+Each record in the `.callgraph` section has the following binary layout:
+
+| Field                                  | Type          | Size (bits) | Description                                                                                             |
+| -------------------------------------- | ------------- | ----------- | ------------------------------------------------------------------------------------------------------- |
+| Format Version                         | `uint8_t`     | 8           | The version of the record format. The current version is 0.                                             |
+| Flags                                  | `uint8_t`     | 8           | A bitfield where: Bit 0 is set if the function is a potential indirect call target; Bit 1 is set if there are direct callees; Bit 2 is set if there are indirect callees. The remaining 5 bits are reserved. |
+| Function Entry PC                      | `uintptr_t`   | 32/64       | The address of the function's entry point.                                                              |
+| Function Type ID                       | `uint64_t`    | 64          | The type ID of the function. This field is non-zero if the function is a potential indirect call target and its type is known. |
+| Number of Unique Direct Callees        | `ULEB128`     | Variable    | The number of unique direct call destinations from this function. This field is only present if there is at least one direct callee. |
+| Direct Callees Array                   | `uintptr_t[]` | Variable    | An array of unique direct callee entry point addresses.                                                 |
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ilovepi wrote:

this is optional too, right?

https://github.com/llvm/llvm-project/pull/159866