[PATCH] D37085: [MachineOutliner] Add missed optimization remarks based off outliner cost model

[Jessica Paquette via Phabricator via llvm-commits]

paquette created this revision.
Herald added a subscriber: fhahn.

This adds missed optimization remarks which report which (possibly viable) candidates were not outlined because they would be more expensive than not outlining them. Other remarks will come in separate patches.

In the case of a code size regression, if the project was built with the outliner, it'd be nice to know if it was a change in the outliner's behaviour that caused that regression. Having remarks will make that a lot easier, since it'll let us use things like the opt-viewer + optdiff.


https://reviews.llvm.org/D37085

Files:
  lib/CodeGen/MachineOutliner.cpp


Index: lib/CodeGen/MachineOutliner.cpp
===================================================================
--- lib/CodeGen/MachineOutliner.cpp
+++ lib/CodeGen/MachineOutliner.cpp
@@ -42,10 +42,13 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/Analysis/OptimizationDiagnosticInfo.h"
+#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/MachineOptimizationRemarkEmitter.h"
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/Support/Allocator.h"
@@ -64,6 +67,7 @@
 #define DEBUG_TYPE "machine-outliner"
 
 using namespace llvm;
+using namespace ore;
 
 STATISTIC(NumOutlined, "Number of candidates outlined");
 STATISTIC(FunctionsCreated, "Number of functions created");
@@ -829,6 +833,10 @@
   size_t FnIdx = 0;
   size_t MaxLen = 0;
 
+  // Count the number of candidates we considered for outlining. Used to give
+  // remarks an identifier.
+  size_t NumCandidatesEvaluated = 0;
+
   // FIXME: Visit internal nodes instead of leaves.
   for (SuffixTreeNode *Leaf : ST.LeafVector) {
     assert(Leaf && "Leaves in LeafVector cannot be null!");
@@ -851,6 +859,10 @@
     if (StringLen < 2)
       continue;
 
+    // We've found something we might want to outline. Keep track of an ID for
+    // remarks.
+    NumCandidatesEvaluated++;
+
     size_t CallOverhead = 0;
     size_t SequenceOverhead = StringLen;
 
@@ -895,8 +907,33 @@
     size_t OutliningCost = CallOverhead + FrameOverhead + SequenceOverhead;
     size_t NotOutliningCost = SequenceOverhead * Parent.OccurrenceCount;
 
-    if (NotOutliningCost <= OutliningCost)
+    if (NotOutliningCost <= OutliningCost) {
+      // Emit a remark explaining why we didn't outline this candidate.
+      for (const std::pair<MachineBasicBlock::iterator,
+                           MachineBasicBlock::iterator> &C : CandidateClass) {
+        MachineBasicBlock *MBB = C.first->getParent();
+        MachineFunction *MF = MBB->getParent();
+
+        // FIXME: It would be nice to be able to store the emitters somewhere so
+        // that we don't have to emit them whenever we visit a machine function.
+        MachineOptimizationRemarkEmitter MORE(*MF, nullptr);
+        MachineOptimizationRemarkMissed R(DEBUG_TYPE, "NotOutliningCheaper",
+                                          MF->getFunction()->getSubprogram(),
+                                          MBB);
+        R << "Candidate " << NV("SequenceID", (uint64_t)NumCandidatesEvaluated)
+          << " [" << MNV("StartInstr", *C.first) << " ... "
+          << MNV("EndInstr", *C.second)
+          << "] (Length: " << NV("Length", (uint64_t)StringLen)
+          << ") not outlined from " << NV("SourceFunction", MF->getName())
+          << "; Outlining would take more instructions than not. "
+             " (OutliningCost: "
+          << NV("OutliningCost", (uint64_t)OutliningCost)
+          << ", NotOutliningCost: "
+          << NV("NotOutliningCost", (uint64_t)NotOutliningCost) << ")";
+        MORE.emit(R);
+      }
       continue;
+    }
 
     size_t Benefit = NotOutliningCost - OutliningCost;
 


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