From 41bf444e612949d72df12f343648bca989d328a6 Mon Sep 17 00:00:00 2001 From: Matt Davis Date: Mon, 10 Jun 2019 20:38:56 +0000 Subject: [PATCH] [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 --- llvm/docs/CommandGuide/llvm-mca.rst | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) 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 @@ Example of overlapping regions: 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 @@ assembly text: 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 `_ +can also help in detecting missed optimizations. + HOW LLVM-MCA WORKS ------------------ -- 2.7.4