[clang] [Clang][PGO][UserManual] Specify usage of -b flag (PR #139763)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Madhur Amilkanthwar (madhur13490)

<details>
<summary>Changes</summary>

llvm-profgen cannot accept the perf profiles collected without `-b` and errors out with a message `"Invalid perf script input!"`.

This can also be validated from the code in function `checkPerfScriptType()` in `tools/llvm-profgen/PerfReader.cpp.`

---
Full diff: https://github.com/llvm/llvm-project/pull/139763.diff


1 Files Affected:

- (modified) clang/docs/UsersManual.rst (+3) 


``````````diff
diff --git a/clang/docs/UsersManual.rst b/clang/docs/UsersManual.rst
index 6f804a10748d8..00899dcb8d8c2 100644
--- a/clang/docs/UsersManual.rst
+++ b/clang/docs/UsersManual.rst
@@ -2792,6 +2792,9 @@ usual build cycle when using sample profilers for optimization:
 
      $ llvm-profgen --binary=./code --output=code.prof --perfdata=perf.data
 
+   Please note, ``perf.data`` must be collected with ``-b`` flag to Linux ``perf``
+   for the above step to work.
+
    When using SEP the output is in the textual format corresponding to
    ``llvm-profgen --perfscript``. For example:
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/139763