[Docs] [llvm-mca] Point out a caveat for using llvm-mca markers in source code.
Summary: See: https://bugs.llvm.org/show_bug.cgi?id=42173
Reviewers: andreadb, mattd, RKSimon, spatel
Reviewed By: andreadb
Subscribers: tschuett, gbedwell, llvm-commits, andreadb
Tags: #llvm
Patch by Max Marrone (maxpm)! Thanks!
Differential Revision: https://reviews.llvm.org/D63040
llvm-svn: 362979
diff --git a/llvm/docs/CommandGuide/llvm-mca.rst b/llvm/docs/CommandGuide/llvm-mca.rst
index 5b30a8a5..d8c3b57 100644
--- a/llvm/docs/CommandGuide/llvm-mca.rst
+++ b/llvm/docs/CommandGuide/llvm-mca.rst
@@ -241,8 +241,8 @@
Note that multiple anonymous regions cannot overlap. Also, overlapping regions
cannot have the same name.
-Inline assembly directives may be used from source code to annotate the
-assembly text:
+There is no support for marking regions from high-level source code, like C or
+C++. As a workaround, inline assembly directives may be used:
.. code-block:: c++
@@ -254,6 +254,15 @@
return a;
}
+However, this interferes with optimizations like loop vectorization and may have
+an impact on the code generated. This is because the ``__asm`` statements are
+seen as real code having important side effects, which limits how the code
+around them can be transformed. If users want to make use of inline assembly
+to emit markers, then the recommendation is to always verify that the output
+assembly is equivalent to the assembly generated in the absence of markers.
+The `Clang options to emit optimization reports <https://clang.llvm.org/docs/UsersManual.html#options-to-emit-optimization-reports>`_
+can also help in detecting missed optimizations.
+
HOW LLVM-MCA WORKS
------------------