Add a pass to generate synthetic function entry counts.

Summary:
This pass synthesizes function entry counts by traversing the callgraph
and using the relative block frequencies of the callsites. The intended
use of these counts is in inlining to determine hot/cold callsites in
the absence of profile information.

The pass is split into two files with the code that propagates the
counts in a callgraph in a Utils file. I plan to add support for
propagation in the thinlto link phase and the propagation code will be
shared and hence this split. I did not add support to the old PM since
hot callsite determination in inlining is not possible in old PM
(although we could use hot callee heuristic with synthetic counts in the
old PM it is not worth the effort tuning it)

Reviewers: davidxl, silvas

Subscribers: mgorny, mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D41604

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@322110 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 2 deletions

diff --git a/lib/IR/MDBuilder.cpp b/lib/IR/MDBuilder.cpp
index 6d77a8f2d60..9d467fb9f6d 100644
--- a/lib/IR/MDBuilder.cpp
+++ b/lib/IR/MDBuilder.cpp
@@ -58,10 +58,14 @@ MDNode *MDBuilder::createUnpredictable() {
 }
 
 MDNode *MDBuilder::createFunctionEntryCount(
-    uint64_t Count, const DenseSet<GlobalValue::GUID> *Imports) {
+    uint64_t Count, bool Synthetic,
+    const DenseSet<GlobalValue::GUID> *Imports) {
   Type *Int64Ty = Type::getInt64Ty(Context);
   SmallVector<Metadata *, 8> Ops;
-  Ops.push_back(createString("function_entry_count"));
+  if (Synthetic)
+    Ops.push_back(createString("synthetic_function_entry_count"));
+  else
+    Ops.push_back(createString("function_entry_count"));
   Ops.push_back(createConstant(ConstantInt::get(Int64Ty, Count)));
   if (Imports) {
     SmallVector<GlobalValue::GUID, 2> OrderID(Imports->begin(), Imports->end());