Irreducible loop metadata for more accurate block frequency under PGO.

Summary:
Currently the block frequency analysis is an approximation for irreducible
loops.

The new irreducible loop metadata is used to annotate the irreducible loop
headers with their header weights based on the PGO profile (currently this is
approximated to be evenly weighted) and to help improve the accuracy of the
block frequency analysis for irreducible loops.

This patch is a basic support for this.

Reviewers: davidxl

Reviewed By: davidxl

Subscribers: mehdi_amini, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D39028

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317278 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 23 insertions, 0 deletions

diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index 9d910568bd5..6823fe5fcd7 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -5194,6 +5194,29 @@ the loop identifier metadata node directly:
    !1 = !{!1} ; an identifier for the inner loop
    !2 = !{!2} ; an identifier for the outer loop
 
+'``irr_loop``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^
+
+``irr_loop`` metadata may be attached to the terminator instruction of a basic
+block that's an irreducible loop header (note that an irreducible loop has more
+than once header basic blocks.) If ``irr_loop`` metadata is attached to the
+terminator instruction of a basic block that is not really an irreducible loop
+header, the behavior is undefined. The intent of this metadata is to improve the
+accuracy of the block frequency propagation. For example, in the code below, the
+block ``header0`` may have a loop header weight (relative to the other headers of
+the irreducible loop) of 100:
+
+.. code-block:: llvm
+
+    header0:
+    ...
+    br i1 %cmp, label %t1, label %t2, !irr_loop !0
+
+    ...
+    !0 = !{"loop_header_weight", i64 100}
+
+Irreducible loop header weights are typically based on profile data.
+
 '``invariant.group``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^