[thinlto] Basic thinlto fdo heuristic

Summary:
This patch improves thinlto importer
by importing 3x larger functions that are called from hot block.

I compared performance with the trunk on spec, and there
were about 2% on povray and 3.33% on milc. These results seems
to be consistant and match the results Teresa got with her simple
heuristic. Some benchmarks got slower but I think they are just
noisy (mcf, xalancbmki, omnetpp)- running the benchmarks again with
more iterations to confirm. Geomean of all benchmarks including the noisy ones
were about +0.02%.

I see much better improvement on google branch with Easwaran patch
for pgo callsite inlining (the inliner actually inline those big functions)
Over all I see +0.5% improvement, and I get +8.65% on povray.
So I guess we will see much bigger change when Easwaran patch will land
(it depends on new pass manager), but it is still worth putting this to trunk
before it.

Implementation details changes:
- Removed CallsiteCount.
- ProfileCount got replaced by Hotness
- hot-import-multiplier is set to 3.0 for now,
didn't have time to tune it up, but I see that we get most of the interesting
functions with 3, so there is no much performance difference with higher, and
binary size doesn't grow as much as with 10.0.

Reviewers: eraman, mehdi_amini, tejohnson

Subscribers: mehdi_amini, llvm-commits

Differential Revision: https://reviews.llvm.org/D24638

llvm-svn: 282437

1 files changed, 12 insertions, 3 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 81d9ca5638d..6c43b780870 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -48,6 +48,10 @@ static cl::opt<float>
                       cl::desc("As we import functions, multiply the "
                                "`import-instr-limit` threshold by this factor "
                                "before processing newly imported functions"));
+static cl::opt<float> ImportHotMultiplier(
+    "import-hot-multiplier", cl::init(3.0), cl::Hidden, cl::value_desc("x"),
+    cl::ZeroOrMore, cl::desc("Multiply the `import-instr-limit` threshold for "
+                             "hot callsites"));
 
 static cl::opt<bool> PrintImports("print-imports", cl::init(false), cl::Hidden,
                                   cl::desc("Print imported functions"));
@@ -268,7 +272,7 @@ using EdgeInfo = std::pair<const FunctionSummary *, unsigned /* Threshold */>;
 /// exported from their source module.
 static void computeImportForFunction(
     const FunctionSummary &Summary, const ModuleSummaryIndex &Index,
-    unsigned Threshold, const GVSummaryMapTy &DefinedGVSummaries,
+    const unsigned Threshold, const GVSummaryMapTy &DefinedGVSummaries,
     SmallVectorImpl<EdgeInfo> &Worklist,
     FunctionImporter::ImportMapTy &ImportList,
     StringMap<FunctionImporter::ExportSetTy> *ExportLists = nullptr) {
@@ -281,7 +285,12 @@ static void computeImportForFunction(
       continue;
     }
 
-    auto *CalleeSummary = selectCallee(GUID, Threshold, Index);
+    // FIXME: Also lower the threshold for cold callsites.
+    const auto NewThreshold =
+        Edge.second.Hotness == CalleeInfo::HotnessType::Hot
+            ? Threshold * ImportHotMultiplier
+            : Threshold;
+    auto *CalleeSummary = selectCallee(GUID, NewThreshold, Index);
     if (!CalleeSummary) {
       DEBUG(dbgs() << "ignored! No qualifying callee with summary found.\n");
       continue;
@@ -297,7 +306,7 @@ static void computeImportForFunction(
     } else
       ResolvedCalleeSummary = cast<FunctionSummary>(CalleeSummary);
 
-    assert(ResolvedCalleeSummary->instCount() <= Threshold &&
+    assert(ResolvedCalleeSummary->instCount() <= NewThreshold &&
            "selectCallee() didn't honor the threshold");
 
     auto ExportModulePath = ResolvedCalleeSummary->modulePath();