[DSE] Add stat for remaining stores after DSE.

[fhahn]

Summary

Using the existing NumFastStores statistic can be misleading when
comparing the impact of DSE patches.

For example, consider the case where a store gets removed from a
function before it is inlined into another function. A less
powerful DSE might only remove the store from functions it has
been inlined into, which will result in more stores being removed, but
no difference in the actual number of stores after DSE.

The new stat provides the absolute number of stores surviving after
DSE.

[asbirlea]

I think generally, for accurate stats on how DSE is doing, it would be good to have tests that just run DSE, vs -O3. But this is a fair statistic to have for benchmarks where this is not possible.

[asbirlea]

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

1984

Nit: for the MemorySSA case, this could walk def chains; but perhaps we don't care much about performance when getting stats?

[fhahn]

In D78830#2003168, @asbirlea wrote:

I think generally, for accurate stats on how DSE is doing, it would be good to have tests that just run DSE, vs -O3. But this is a fair statistic to have for benchmarks where this is not possible.

I am not sure if I follow entirely. Are you suggesting to run DSE and compare the total number of stores after compiling to -O3 for benchmarks from test-suite?

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

1984

I've additionally guarded the loop by AreStatisticsEnabled() and kept the loop as is for now. I think it is slightly more beneficial to ensure we comparing the same thing between both versions in a straight-forward way.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Argument.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
+#include "llvm/IR/InstIterator.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PassManager.h"
 #include "llvm/IR/Value.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/DebugCounter.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
+#include "llvm/Transforms/Utils/Local.h"
 #include <algorithm>
 #include <cassert>
 #include <cstddef>
 #include <cstdint>
 #include <iterator>
 #include <map>
 #include <utility>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "dse"
 
+STATISTIC(NumRemainingStores, "Number of stores remaining after DSE");
 STATISTIC(NumRedundantStores, "Number of redundant stores deleted");
 STATISTIC(NumFastStores, "Number of stores deleted");
 STATISTIC(NumFastOther, "Number of other instrs removed");
 STATISTIC(NumCompletePartials, "Number of stores dead by later partials");
 STATISTIC(NumModifiedStores, "Number of stores modified");
 
 DEBUG_COUNTER(MemorySSACounter, "dse-memoryssa",
               "Controls which MemoryDefs are eliminated.");

 //===----------------------------------------------------------------------===//
 // DSE Pass
 //===----------------------------------------------------------------------===//
 PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
   AliasAnalysis &AA = AM.getResult<AAManager>(F);
   const TargetLibraryInfo &TLI = AM.getResult<TargetLibraryAnalysis>(F);
   DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
 
+  bool Changed = false;
   if (EnableMemorySSA) {
     MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
     PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
 
-    if (!eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI))
-      return PreservedAnalyses::all();
+    Changed = eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
   } else {
     MemoryDependenceResults &MD = AM.getResult<MemoryDependenceAnalysis>(F);
 
-    if (!eliminateDeadStores(F, &AA, &MD, &DT, &TLI))
-      return PreservedAnalyses::all();
+    Changed = eliminateDeadStores(F, &AA, &MD, &DT, &TLI);
   }
 
+#ifdef LLVM_ENABLE_STATS
+  for (auto &I : instructions(F))
+    NumRemainingStores += isa<StoreInst>(&I);
+#endif
+
+  if (!Changed)
+    return PreservedAnalyses::all();
+
   PreservedAnalyses PA;
   PA.preserveSet<CFGAnalyses>();
   PA.preserve<GlobalsAA>();
   if (EnableMemorySSA)
     PA.preserve<MemorySSAAnalysis>();
   else
     PA.preserve<MemoryDependenceAnalysis>();
   return PA;
bool runOnFunction(Function &F) override {
     if (skipFunction(F))
       return false;
 
     AliasAnalysis &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
     DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
     const TargetLibraryInfo &TLI =
         getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
 
+    bool Changed = false;
     if (EnableMemorySSA) {
       MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
       PostDominatorTree &PDT =
           getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
 
-      return eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
+      eliminateDeadStoresMemorySSA(F, AA, MSSA, DT, PDT, TLI);
     } else {
       MemoryDependenceResults &MD =
           getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
 
-      return eliminateDeadStores(F, &AA, &MD, &DT, &TLI);
+      Changed = eliminateDeadStores(F, &AA, &MD, &DT, &TLI);
     }
+
+#ifdef LLVM_ENABLE_STATS
+    for (auto &I : instructions(F))
+      NumRemainingStores += isa<StoreInst>(&I);
+#endif
+
+    return Changed;
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     AU.setPreservesCFG();
     AU.addRequired<AAResultsWrapperPass>();
     AU.addRequired<TargetLibraryInfoWrapperPass>();
     AU.addPreserved<GlobalsAAWrapperPass>();
     AU.addRequired<DominatorTreeWrapperPass>();