[ADT] Remove StatisticBase and make NoopStatistic empty

In LLVM_ENABLE_STATS=0 builds, `llvm::Statistic` maps to `llvm::NoopStatistic`
but has 3 mostly unused pointers. GlobalOpt considers that the pointers can
potentially retain allocated objects, so GlobalOpt cannot optimize out the
`NoopStatistic` variables (see D69428 for more context), wasting 23KiB for stage
2 clang.

This patch makes `NoopStatistic` empty and thus reclaims the wasted space.  The
clang size is even smaller than applying D69428 (slightly smaller in both .bss and
.text).
```
# This means the D69428 optimization on clang is mostly nullified by this patch.
HEAD+D69428: size(.bss) = 0x0725a8
HEAD+D101211: size(.bss) = 0x072238

# bloaty - HEAD+D69428 vs HEAD+D101211
# With D101211, we also save a lot of string table space (.rodata).
    FILE SIZE        VM SIZE
 --------------  --------------
  -0.0%     -32  -0.0%     -24    .eh_frame
  -0.0%    -336  [ = ]       0    .symtab
  -0.0%    -360  [ = ]       0    .strtab
  [ = ]       0  -0.2%    -880    .bss
  -0.0% -2.11Ki  -0.0% -2.11Ki    .rodata
  -0.0% -2.89Ki  -0.0% -2.89Ki    .text
  -0.0% -5.71Ki  -0.0% -5.88Ki    TOTAL
```

Note: LoopFuse is a disabled pass. For now this patch adds
`#if LLVM_ENABLE_STATS` so `OptimizationRemarkMissed` is skipped in
LLVM_ENABLE_STATS==0 builds.  If these `OptimizationRemarkMissed` are useful in
LLVM_ENABLE_STATS==0 builds, we can replace `llvm::Statistic` with
`llvm::TrackingStatistic`, or use a different abstraction to keep track of the strings.

Similarly, skip the code in `mlir/lib/Pass/PassStatistics.cpp` which
calls `getName`/`getDesc`/`getValue`.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D101211

llvm/include/llvm/ADT/Statistic.h

@@ -46,27 +46,22 @@ class raw_ostream;
 class raw_fd_ostream;
 class StringRef;
 
-class StatisticBase {
+class TrackingStatistic {
 public:
-  const char *DebugType;
-  const char *Name;
-  const char *Desc;
+  const char *const DebugType;
+  const char *const Name;
+  const char *const Desc;
 
-  StatisticBase(const char *DebugType, const char *Name, const char *Desc)
-      : DebugType(DebugType), Name(Name), Desc(Desc) {}
-
-  const char *getDebugType() const { return DebugType; }
-  const char *getName() const { return Name; }
-  const char *getDesc() const { return Desc; }
-};
-
-class TrackingStatistic : public StatisticBase {
-public:
   std::atomic<unsigned> Value;
   std::atomic<bool> Initialized;
 
   TrackingStatistic(const char *DebugType, const char *Name, const char *Desc)
-      : StatisticBase(DebugType, Name, Desc), Value(0), Initialized(false) {}
+      : DebugType(DebugType), Name(Name), Desc(Desc), Value(0),
+        Initialized(false) {}
+
+  const char *getDebugType() const { return DebugType; }
+  const char *getName() const { return Name; }
+  const char *getDesc() const { return Desc; }
 
   unsigned getValue() const { return Value.load(std::memory_order_relaxed); }
 
@@ -132,9 +127,10 @@ protected:
   void RegisterStatistic();
 };
 
-class NoopStatistic : public StatisticBase {
+class NoopStatistic {
 public:
-  using StatisticBase::StatisticBase;
+  NoopStatistic(const char * /*DebugType*/, const char * /*Name*/,
+                const char * /*Desc*/) {}
 
   unsigned getValue() const { return 0; }
 

llvm/lib/Transforms/Scalar/LoopFuse.cpp

@@ -372,11 +372,13 @@ private:
   bool reportInvalidCandidate(llvm::Statistic &Stat) const {
     using namespace ore;
     assert(L && Preheader && "Fusion candidate not initialized properly!");
+#if LLVM_ENABLE_STATS
     ++Stat;
     ORE.emit(OptimizationRemarkAnalysis(DEBUG_TYPE, Stat.getName(),
                                         L->getStartLoc(), Preheader)
              << "[" << Preheader->getParent()->getName() << "]: "
              << "Loop is not a candidate for fusion: " << Stat.getDesc());
+#endif
     return false;
   }
 };
@@ -1533,6 +1535,7 @@ private:
     assert(FC0.Preheader && FC1.Preheader &&
            "Expecting valid fusion candidates");
     using namespace ore;
+#if LLVM_ENABLE_STATS
     ++Stat;
     ORE.emit(RemarkKind(DEBUG_TYPE, Stat.getName(), FC0.L->getStartLoc(),
                         FC0.Preheader)
@@ -1540,6 +1543,7 @@ private:
              << "]: " << NV("Cand1", StringRef(FC0.Preheader->getName()))
              << " and " << NV("Cand2", StringRef(FC1.Preheader->getName()))
              << ": " << Stat.getDesc());
+#endif
   }
 
   /// Fuse two guarded fusion candidates, creating a new fused loop.

mlir/include/mlir/Pass/Pass.h

@@ -139,10 +139,6 @@ public:
 
     /// Assign the statistic to the given value.
     Statistic &operator=(unsigned value);
-
-  private:
-    /// Hide some of the details of llvm::Statistic that we don't use.
-    using llvm::Statistic::getDebugType;
   };
 
   /// Returns the main statistics for this pass instance.

mlir/lib/Pass/PassStatistics.cpp

@@ -64,6 +64,7 @@ static void printResultsAsList(raw_ostream &os, OpPassManager &pm) {
 
     // If this is not an adaptor, add the stats to the list if there are any.
     if (!adaptor) {
+#if LLVM_ENABLE_STATS
       auto statistics = pass->getStatistics();
       if (statistics.empty())
         return;
@@ -76,6 +77,7 @@ static void printResultsAsList(raw_ostream &os, OpPassManager &pm) {
         for (auto &it : llvm::enumerate(pass->getStatistics()))
           passEntry[it.index()].value += it.value()->getValue();
       }
+#endif
       return;
     }
 
@@ -103,6 +105,7 @@ static void printResultsAsList(raw_ostream &os, OpPassManager &pm) {
 /// Print the results in pipeline mode that mirrors the internal pass manager
 /// structure.
 static void printResultsAsPipeline(raw_ostream &os, OpPassManager &pm) {
+#if LLVM_ENABLE_STATS
   std::function<void(unsigned, Pass *)> printPass = [&](unsigned indent,
                                                         Pass *pass) {
     if (auto *adaptor = dyn_cast<OpToOpPassAdaptor>(pass)) {
@@ -132,6 +135,7 @@ static void printResultsAsPipeline(raw_ostream &os, OpPassManager &pm) {
   };
   for (Pass &pass : pm.getPasses())
     printPass(/*indent=*/0, &pass);
+#endif
 }
 
 static void printStatistics(OpPassManager &pm, PassDisplayMode displayMode) {

mlir/test/Pass/pipeline-stats.mlir

@@ -1,3 +1,4 @@
+// REQUIRES: asserts
 // RUN: mlir-opt %s -verify-each=true -pass-pipeline='func(test-stats-pass,test-stats-pass)' -pass-statistics -pass-statistics-display=list 2>&1 | FileCheck -check-prefix=LIST %s
 // RUN: mlir-opt %s -verify-each=true -pass-pipeline='func(test-stats-pass,test-stats-pass)' -pass-statistics -pass-statistics-display=pipeline 2>&1 | FileCheck -check-prefix=PIPELINE %s