[Subtarget] Remove static global constructor call from the tablegened subtarget feature tables

[craig.topper]

Summary

Subtarget features are stored in a std::bitset that has been subclassed. There is a special constructor to allow the tablegen files to provide a list of bits to initialize the std::bitset to. This constructor isn't constexpr and std::bitset doesn't support many constexpr operations either. This results in a static global constructor being used to initialize the feature bitsets in these files at startup.

To fix this I've introduced a new FeatureBitArray class that holds three 64-bit values representing the initial bit values and taught tablegen to emit hex constants for them based on the feature enum values. This makes the tablegen files less readable than they were before. I can add the list of features back as a comment if we think that's important.

I've added a method to convert from this class into the std::bitset subclass we had before. I considered making the new FeatureBitArray class just implement the std::bitset interface we need instead, but thought I'd see how others felts about that first.

I've simplified the interfaces to SetImpliedBits and ClearImpliedBits a little minimize the number of times we need to convert to the bitset.

This removes about 27K from my local release+asserts build of llc.

I'm open to any suggestions on better ways to do this.

[craig.topper]

Ping

[dblaikie]

utils/TableGen/SubtargetEmitter.cpp

179

Perhaps either use Mask.length() as the loop bound (or a range-based for loop) and/or change the parameter to be a reference to an array of MAX_SUBTARGET_WORDS length? (rather than having an implicit contract where you must pass this function an array of that length, but nothing checks that)] & would just walk off the end of the array otherwise)

273–277

This code looks like the same code over ImpliesMask above? (except for ImpliesMask rather than FeatureList) - perhaps it could be folded into the printFeatureMask function?

[bjope]

llvm/trunk/utils/TableGen/SubtargetEmitter.cpp

185 ↗

(On Diff #188831)

@craig.topper, I see lots of warning when compiling using clang 3.6 after this patch:

lib/Target/AArch64/AArch64GenSubtargetInfo.inc:167:63: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
  { "a35", "Cortex-A35 ARM processors", AArch64::ProcA35, { { 0x20800080800800ULL, 0x0ULL, 0x0ULL, } } },
                                                              ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                                               {                                  }

Adding one more level of braces "{ { { 0x20800080800800ULL, 0x0ULL, 0x0ULL, } } }" seems to work, but would that mean that we no longer run the new constexpr ctor?

[bjope]

llvm/trunk/utils/TableGen/SubtargetEmitter.cpp

185 ↗

(On Diff #188831)

Ok, now I've read this: https://en.cppreference.com/w/cpp/container/array

So as described in the example

std::array<int, 3> a1{ {1, 2, 3} }; // double-braces required in C++11 prior to the CWG 1270 revision
                                    // (not needed in C++11 after the revision and in C++14 and beyond)

I guess my old clang 3.6 compiler doesn't include CWG 1270 then.

Afaict clang 3.6 should still be supported (even if I don't know how many warnings that are accepted when building).
Should we simply generate the double braces here, to work around these warnings?

[craig.topper]

@bjope , thanks! I'll add another set of curly braces.

include/llvm/MC/SubtargetFeature.h

 #define LLVM_MC_SUBTARGETFEATURE_H
 
 #include "llvm/ADT/StringRef.h"
+#include <array>
 #include <bitset>
 #include <initializer_list>
 #include <string>

 class raw_ostream;
 class Triple;
 
-const unsigned MAX_SUBTARGET_FEATURES = 192;
+const unsigned MAX_SUBTARGET_WORDS = 3;
+const unsigned MAX_SUBTARGET_FEATURES = MAX_SUBTARGET_WORDS * 64;
+
 /// Container class for subtarget features.
 /// This is convenient because std::bitset does not have a constructor
 /// with an initializer list of set bits.

   }
 };
 
+/// Class used to store the subtarget bits in the tables created by tablegen.
+/// The std::initializer_list constructor of FeatureBitset can't be done at
+/// compile time and requires a static constructor to run at startup.
+class FeatureBitArray {
+  std::array<uint64_t, MAX_SUBTARGET_WORDS> Bits;
+
+public:
+  constexpr FeatureBitArray(const std::array<uint64_t, MAX_SUBTARGET_WORDS> &B)
+      : Bits(B) {}
+
+  FeatureBitset getAsBitset() const {
+    FeatureBitset Result;
+
+    for (unsigned i = 0; i != MAX_SUBTARGET_WORDS; ++i)
+      Result |= FeatureBitset(Bits[i]) << (64 * i);
+
+    return Result;
+  }
+};
+
 //===----------------------------------------------------------------------===//
 
 /// Used to provide key value pairs for feature and CPU bit flags.

   const char *Key;                      ///< K-V key string
   const char *Desc;                     ///< Help descriptor
   unsigned Value;                       ///< K-V integer value
-  FeatureBitset Implies;                ///< K-V bit mask
+  FeatureBitArray Implies;              ///< K-V bit mask
 
   /// Compare routine for std::lower_bound
   bool operator<(StringRef S) const {

lib/MC/SubtargetFeature.cpp

 
 /// For each feature that is (transitively) implied by this feature, set it.
 static
-void SetImpliedBits(FeatureBitset &Bits, const SubtargetFeatureKV &FeatureEntry,
+void SetImpliedBits(FeatureBitset &Bits, const FeatureBitset &Implies,
                     ArrayRef<SubtargetFeatureKV> FeatureTable) {
   for (const SubtargetFeatureKV &FE : FeatureTable) {
-    if (FeatureEntry.Value == FE.Value) continue;
-
-    if (FeatureEntry.Implies.test(FE.Value)) {
+    if (Implies.test(FE.Value)) {
       Bits.set(FE.Value);
-      SetImpliedBits(Bits, FE, FeatureTable);
+      SetImpliedBits(Bits, FE.Implies.getAsBitset(), FeatureTable);
     }
   }
 }
 
 /// For each feature that (transitively) implies this feature, clear it.
 static
-void ClearImpliedBits(FeatureBitset &Bits,
-                      const SubtargetFeatureKV &FeatureEntry,
+void ClearImpliedBits(FeatureBitset &Bits, unsigned Value,
                       ArrayRef<SubtargetFeatureKV> FeatureTable) {
   for (const SubtargetFeatureKV &FE : FeatureTable) {
-    if (FeatureEntry.Value == FE.Value) continue;
-
-    if (FE.Implies.test(FeatureEntry.Value)) {
+    if (FE.Implies.getAsBitset().test(Value)) {
       Bits.reset(FE.Value);
-      ClearImpliedBits(Bits, FE, FeatureTable);
+      ClearImpliedBits(Bits, FE.Value, FeatureTable);
     }
   }
 }

     if (Bits.test(FeatureEntry->Value)) {
       Bits.reset(FeatureEntry->Value);
       // For each feature that implies this, clear it.
-      ClearImpliedBits(Bits, *FeatureEntry, FeatureTable);
+      ClearImpliedBits(Bits, FeatureEntry->Value, FeatureTable);
     } else {
       Bits.set(FeatureEntry->Value);
 
       // For each feature that this implies, set it.
-      SetImpliedBits(Bits, *FeatureEntry, FeatureTable);
+      SetImpliedBits(Bits, FeatureEntry->Implies.getAsBitset(), FeatureTable);
     }
   } else {
     errs() << "'" << Feature << "' is not a recognized feature for this target"

       Bits.set(FeatureEntry->Value);
 
       // For each feature that this implies, set it.
-      SetImpliedBits(Bits, *FeatureEntry, FeatureTable);
+      SetImpliedBits(Bits, FeatureEntry->Implies.getAsBitset(), FeatureTable);
     } else {
       Bits.reset(FeatureEntry->Value);
 
       // For each feature that implies this, clear it.
-      ClearImpliedBits(Bits, *FeatureEntry, FeatureTable);
+      ClearImpliedBits(Bits, FeatureEntry->Value, FeatureTable);
     }
   } else {
     errs() << "'" << Feature << "' is not a recognized feature for this target"

     // If there is a match
     if (CPUEntry) {
       // Set base feature bits
-      Bits = CPUEntry->Implies;
+      FeatureBitset CPUImplies = CPUEntry->Implies.getAsBitset();
+      Bits = CPUImplies;
 
       // Set the feature implied by this CPU feature, if any.
       for (auto &FE : FeatureTable) {
-        if (CPUEntry->Implies.test(FE.Value))
-          SetImpliedBits(Bits, FE, FeatureTable);
+        if (CPUImplies.test(FE.Value))
+          SetImpliedBits(Bits, FE.Implies.getAsBitset(), FeatureTable);
       }
     } else {
       errs() << "'" << CPU << "' is not a recognized processor for this target"

utils/TableGen/SubtargetEmitter.cpp

   CodeGenSchedModels &SchedModels;
   std::string Target;
 
-  void Enumeration(raw_ostream &OS);
-  unsigned FeatureKeyValues(raw_ostream &OS);
-  unsigned CPUKeyValues(raw_ostream &OS);
+  void Enumeration(raw_ostream &OS, DenseMap<Record *, unsigned> &FeatureMap);
+  unsigned FeatureKeyValues(raw_ostream &OS,
+                            const DenseMap<Record *, unsigned> &FeatureMap);
+  unsigned CPUKeyValues(raw_ostream &OS,
+                        const DenseMap<Record *, unsigned> &FeatureMap);
   void FormItineraryStageString(const std::string &Names,
                                 Record *ItinData, std::string &ItinString,
                                 unsigned &NStages);

 //
 // Enumeration - Emit the specified class as an enumeration.
 //
-void SubtargetEmitter::Enumeration(raw_ostream &OS) {
+void SubtargetEmitter::Enumeration(raw_ostream &OS,
+                                   DenseMap<Record *, unsigned> &FeatureMap) {
   // Get all records of class and sort
   std::vector<Record*> DefList =
     Records.getAllDerivedDefinitions("SubtargetFeature");

 
     // Get and emit name
     OS << "  " << Def->getName() << " = " << i << ",\n";
+
+    // Save the index for this feature.
+    FeatureMap[Def] = i;
   }
 
   // Close enumeration and namespace

   OS << "} // end namespace " << Target << "\n";
 }
 
+static void printFeatureMask(raw_ostream &OS, ArrayRef<uint64_t> Mask) {
+  OS << "{ { ";
+  for (unsigned i = 0; i != MAX_SUBTARGET_WORDS; ++i) {
+    OS << "0x";
+    OS.write_hex(Mask[i]);
+    OS << "ULL, ";
+  }
+  OS << "} }";
+}
+
 //
 // FeatureKeyValues - Emit data of all the subtarget features.  Used by the
 // command line.
 //
-unsigned SubtargetEmitter::FeatureKeyValues(raw_ostream &OS) {
+unsigned SubtargetEmitter::FeatureKeyValues(
+    raw_ostream &OS, const DenseMap<Record *, unsigned> &FeatureMap) {
   // Gather and sort all the features
   std::vector<Record*> FeatureList =
                            Records.getAllDerivedDefinitions("SubtargetFeature");

 
     RecVec ImpliesList = Feature->getValueAsListOfDefs("Implies");
 
-    OS << "{";
-    for (unsigned j = 0, M = ImpliesList.size(); j < M;) {
-      OS << " " << Target << "::" << ImpliesList[j]->getName();
-      if (++j < M) OS << ",";
+    uint64_t ImpliesMask[MAX_SUBTARGET_WORDS] = {};
+    for (unsigned j = 0, M = ImpliesList.size(); j < M; ++j) {
+      unsigned Bit = FeatureMap.lookup(ImpliesList[j]);
+      ImpliesMask[Bit / 64] |= 1ULL << (Bit % 64);
     }
-    OS << " } },\n";
+
+    printFeatureMask(OS, ImpliesMask);
+
+    OS << " },\n";
     ++NumFeatures;
   }
 

 // CPUKeyValues - Emit data of all the subtarget processors.  Used by command
 // line.
 //
-unsigned SubtargetEmitter::CPUKeyValues(raw_ostream &OS) {
+unsigned
+SubtargetEmitter::CPUKeyValues(raw_ostream &OS,
+                               const DenseMap<Record *, unsigned> &FeatureMap) {
   // Gather and sort processor information
   std::vector<Record*> ProcessorList =
                           Records.getAllDerivedDefinitions("Processor");

        << "\"" << Name << "\", "
        << "\"Select the " << Name << " processor\", 0, ";
 
-    OS << "{";
-    for (unsigned j = 0, M = FeatureList.size(); j < M;) {
-      OS << " " << Target << "::" << FeatureList[j]->getName();
-      if (++j < M) OS << ",";
+    uint64_t FeatureMask[MAX_SUBTARGET_WORDS] = {};
+    for (unsigned j = 0, M = FeatureList.size(); j < M; ++j) {
+      unsigned Bit = FeatureMap.lookup(FeatureList[j]);
+      FeatureMask[Bit / 64] |= 1ULL << (Bit % 64);
     }
-    OS << " } },\n";
+
+    printFeatureMask(OS, FeatureMask);
+
+    // The {{}} is for the "implies" section of this data structure.
+    OS << " },\n";
   }
 
   // End processor table

   OS << "\n#ifdef GET_SUBTARGETINFO_ENUM\n";
   OS << "#undef GET_SUBTARGETINFO_ENUM\n\n";
 
+  DenseMap<Record *, unsigned> FeatureMap;
+
   OS << "namespace llvm {\n";
-  Enumeration(OS);
+  Enumeration(OS, FeatureMap);
   OS << "} // end namespace llvm\n\n";
   OS << "#endif // GET_SUBTARGETINFO_ENUM\n\n";
 

 #if 0
   OS << "namespace {\n";
 #endif
-  unsigned NumFeatures = FeatureKeyValues(OS);
+  unsigned NumFeatures = FeatureKeyValues(OS, FeatureMap);
   OS << "\n";
-  unsigned NumProcs = CPUKeyValues(OS);
+  unsigned NumProcs = CPUKeyValues(OS, FeatureMap);
   OS << "\n";
   EmitSchedModel(OS);
   OS << "\n";