LowerBitSets: Introduce global layout builder.

The builder is based on a layout algorithm that tries to keep members of
small bit sets together. The new layout compresses Chromium's bit sets to
around 15% of their original size.

Differential Revision: http://reviews.llvm.org/D7796

llvm-svn: 230394

llvm/docs/BitSets.rst

@@ -17,8 +17,10 @@ global variable.
 This will cause a link-time optimization pass to generate bitsets from the
 memory addresses referenced from the elements of the bitset metadata. The pass
 will lay out the referenced globals consecutively, so their definitions must
-be available at LTO time. An intrinsic, :ref:`llvm.bitset.test <bitset.test>`,
-generates code to test whether a given pointer is a member of a bitset.
+be available at LTO time. The `GlobalLayoutBuilder`_ class is responsible for
+laying out the globals efficiently to minimize the sizes of the underlying
+bitsets. An intrinsic, :ref:`llvm.bitset.test <bitset.test>`, generates code
+to test whether a given pointer is a member of a bitset.
 
 :Example:
 
@@ -64,3 +66,5 @@ generates code to test whether a given pointer is a member of a bitset.
       %d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1
       ret void
     }
+
+.. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerBitSets.h

llvm/include/llvm/Transforms/IPO/LowerBitSets.h

@@ -20,6 +20,7 @@
 
 #include <stdint.h>
 #include <limits>
+#include <set>
 #include <vector>
 
 namespace llvm {
@@ -73,6 +74,69 @@ struct BitSetBuilder {
   BitSetInfo build();
 };
 
+/// This class implements a layout algorithm for globals referenced by bit sets
+/// that tries to keep members of small bit sets together. This can
+/// significantly reduce bit set sizes in many cases.
+///
+/// It works by assembling fragments of layout from sets of referenced globals.
+/// Each set of referenced globals causes the algorithm to create a new
+/// fragment, which is assembled by appending each referenced global in the set
+/// into the fragment. If a referenced global has already been referenced by an
+/// fragment created earlier, we instead delete that fragment and append its
+/// contents into the fragment we are assembling.
+///
+/// By starting with the smallest fragments, we minimize the size of the
+/// fragments that are copied into larger fragments. This is most intuitively
+/// thought about when considering the case where the globals are virtual tables
+/// and the bit sets represent their derived classes: in a single inheritance
+/// hierarchy, the optimum layout would involve a depth-first search of the
+/// class hierarchy (and in fact the computed layout ends up looking a lot like
+/// a DFS), but a naive DFS would not work well in the presence of multiple
+/// inheritance. This aspect of the algorithm ends up fitting smaller
+/// hierarchies inside larger ones where that would be beneficial.
+///
+/// For example, consider this class hierarchy:
+///
+/// A       B
+///   \   / | \
+///     C   D   E
+///
+/// We have five bit sets: bsA (A, C), bsB (B, C, D, E), bsC (C), bsD (D) and
+/// bsE (E). If we laid out our objects by DFS traversing B followed by A, our
+/// layout would be {B, C, D, E, A}. This is optimal for bsB as it needs to
+/// cover the only 4 objects in its hierarchy, but not for bsA as it needs to
+/// cover 5 objects, i.e. the entire layout. Our algorithm proceeds as follows:
+///
+/// Add bsC, fragments {{C}}
+/// Add bsD, fragments {{C}, {D}}
+/// Add bsE, fragments {{C}, {D}, {E}}
+/// Add bsA, fragments {{A, C}, {D}, {E}}
+/// Add bsB, fragments {{B, A, C, D, E}}
+///
+/// This layout is optimal for bsA, as it now only needs to cover two (i.e. 3
+/// fewer) objects, at the cost of bsB needing to cover 1 more object.
+///
+/// The bit set lowering pass assigns an object index to each object that needs
+/// to be laid out, and calls addFragment for each bit set passing the object
+/// indices of its referenced globals. It then assembles a layout from the
+/// computed layout in the Fragments field.
+struct GlobalLayoutBuilder {
+  /// The computed layout. Each element of this vector contains a fragment of
+  /// layout (which may be empty) consisting of object indices.
+  std::vector<std::vector<uint64_t>> Fragments;
+
+  /// Mapping from object index to fragment index.
+  std::vector<uint64_t> FragmentMap;
+
+  GlobalLayoutBuilder(uint64_t NumObjects)
+      : Fragments(1), FragmentMap(NumObjects) {}
+
+  /// Add \param F to the layout while trying to keep its indices contiguous.
+  /// If a previously seen fragment uses any of \param F's indices, that
+  /// fragment will be laid out inside \param F.
+  void addFragment(const std::set<uint64_t> &F);
+};
+
 } // namespace llvm
 
 #endif

llvm/lib/Transforms/IPO/LowerBitSets.cpp

@@ -118,6 +118,35 @@ BitSetInfo BitSetBuilder::build() {
   return BSI;
 }
 
+void GlobalLayoutBuilder::addFragment(const std::set<uint64_t> &F) {
+  // Create a new fragment to hold the layout for F.
+  Fragments.emplace_back();
+  std::vector<uint64_t> &Fragment = Fragments.back();
+  uint64_t FragmentIndex = Fragments.size() - 1;
+
+  for (auto ObjIndex : F) {
+    uint64_t OldFragmentIndex = FragmentMap[ObjIndex];
+    if (OldFragmentIndex == 0) {
+      // We haven't seen this object index before, so just add it to the current
+      // fragment.
+      Fragment.push_back(ObjIndex);
+    } else {
+      // This index belongs to an existing fragment. Copy the elements of the
+      // old fragment into this one and clear the old fragment. We don't update
+      // the fragment map just yet, this ensures that any further references to
+      // indices from the old fragment in this fragment do not insert any more
+      // indices.
+      std::vector<uint64_t> &OldFragment = Fragments[OldFragmentIndex];
+      Fragment.insert(Fragment.end(), OldFragment.begin(), OldFragment.end());
+      OldFragment.clear();
+    }
+  }
+
+  // Update the fragment map to point our object indices to this fragment.
+  for (uint64_t ObjIndex : Fragment)
+    FragmentMap[ObjIndex] = FragmentIndex;
+}
+
 namespace {
 
 struct LowerBitSets : public ModulePass {
@@ -485,27 +514,66 @@ bool LowerBitSets::buildBitSets(Module &M) {
     // Build the list of bitsets and referenced globals in this disjoint set.
     std::vector<MDString *> BitSets;
     std::vector<GlobalVariable *> Globals;
+    llvm::DenseMap<MDString *, uint64_t> BitSetIndices;
+    llvm::DenseMap<GlobalVariable *, uint64_t> GlobalIndices;
     for (GlobalClassesTy::member_iterator MI = GlobalClasses.member_begin(I);
          MI != GlobalClasses.member_end(); ++MI) {
-      if ((*MI).is<MDString *>())
+      if ((*MI).is<MDString *>()) {
+        BitSetIndices[MI->get<MDString *>()] = BitSets.size();
         BitSets.push_back(MI->get<MDString *>());
-      else
+      } else {
+        GlobalIndices[MI->get<GlobalVariable *>()] = Globals.size();
         Globals.push_back(MI->get<GlobalVariable *>());
+      }
+    }
+
+    // For each bitset, build a set of indices that refer to globals referenced
+    // by the bitset.
+    std::vector<std::set<uint64_t>> BitSetMembers(BitSets.size());
+    if (BitSetNM) {
+      for (MDNode *Op : BitSetNM->operands()) {
+        // Op = { bitset name, global, offset }
+        if (!Op->getOperand(1))
+          continue;
+        auto I = BitSetIndices.find(cast<MDString>(Op->getOperand(0)));
+        if (I == BitSetIndices.end())
+          continue;
+
+        auto OpGlobal = cast<GlobalVariable>(
+            cast<ConstantAsMetadata>(Op->getOperand(1))->getValue());
+        BitSetMembers[I->second].insert(GlobalIndices[OpGlobal]);
+      }
     }
 
-    // Order bitsets and globals by name for determinism. TODO: We may later
-    // want to use a more sophisticated ordering that lays out globals so as to
-    // minimize the sizes of the bitsets.
+    // Order the sets of indices by size. The GlobalLayoutBuilder works best
+    // when given small index sets first.
+    std::stable_sort(
+        BitSetMembers.begin(), BitSetMembers.end(),
+        [](const std::set<uint64_t> &O1, const std::set<uint64_t> &O2) {
+          return O1.size() < O2.size();
+        });
+
+    // Create a GlobalLayoutBuilder and provide it with index sets as layout
+    // fragments. The GlobalLayoutBuilder tries to lay out members of fragments
+    // as close together as possible.
+    GlobalLayoutBuilder GLB(Globals.size());
+    for (auto &&MemSet : BitSetMembers)
+      GLB.addFragment(MemSet);
+
+    // Build a vector of globals with the computed layout.
+    std::vector<GlobalVariable *> OrderedGlobals(Globals.size());
+    auto OGI = OrderedGlobals.begin();
+    for (auto &&F : GLB.Fragments)
+      for (auto &&Offset : F)
+        *OGI++ = Globals[Offset];
+
+    // Order bitsets by name for determinism.
     std::sort(BitSets.begin(), BitSets.end(), [](MDString *S1, MDString *S2) {
       return S1->getString() < S2->getString();
     });
-    std::sort(Globals.begin(), Globals.end(),
-              [](GlobalVariable *GV1, GlobalVariable *GV2) {
-                return GV1->getName() < GV2->getName();
-              });
 
     // Build the bitsets from this disjoint set.
-    buildBitSetsFromGlobals(M, BitSets, Globals);
+    buildBitSetsFromGlobals(M, BitSets, OrderedGlobals);
   }
 
   return true;

llvm/test/Transforms/LowerBitSets/layout.ll

@@ -0,0 +1,35 @@
+; RUN: opt -S -lowerbitsets < %s | FileCheck %s
+
+target datalayout = "e-p:32:32"
+
+; Tests that this set of globals is laid out according to our layout algorithm
+; (see GlobalLayoutBuilder in include/llvm/Transforms/IPO/LowerBitSets.h).
+; The chosen layout in this case is a, e, b, d, c.
+
+; CHECK: private constant { i32, i32, i32, i32, i32 } { i32 1, i32 5, i32 2, i32 4, i32 3 }
+@a = constant i32 1
+@b = constant i32 2
+@c = constant i32 3
+@d = constant i32 4
+@e = constant i32 5
+
+!0 = !{!"bitset1", i32* @a, i32 0}
+!1 = !{!"bitset1", i32* @b, i32 0}
+!2 = !{!"bitset1", i32* @c, i32 0}
+
+!3 = !{!"bitset2", i32* @b, i32 0}
+!4 = !{!"bitset2", i32* @d, i32 0}
+
+!5 = !{!"bitset3", i32* @a, i32 0}
+!6 = !{!"bitset3", i32* @e, i32 0}
+
+!llvm.bitsets = !{ !0, !1, !2, !3, !4, !5, !6 }
+
+declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
+
+define void @foo() {
+  %x = call i1 @llvm.bitset.test(i8* undef, metadata !"bitset1")
+  %y = call i1 @llvm.bitset.test(i8* undef, metadata !"bitset2")
+  %z = call i1 @llvm.bitset.test(i8* undef, metadata !"bitset3")
+  ret void
+}

llvm/unittests/Transforms/IPO/LowerBitSets.cpp

@@ -62,3 +62,30 @@ TEST(LowerBitSets, BitSetBuilder) {
     }
   }
 }
+
+TEST(LowerBitSets, GlobalLayoutBuilder) {
+  struct {
+    uint64_t NumObjects;
+    std::vector<std::set<uint64_t>> Fragments;
+    std::vector<uint64_t> WantLayout;
+  } GLBTests[] = {
+    {0, {}, {}},
+    {4, {{0, 1}, {2, 3}}, {0, 1, 2, 3}},
+    {3, {{0, 1}, {1, 2}}, {0, 1, 2}},
+    {4, {{0, 1}, {1, 2}, {2, 3}}, {0, 1, 2, 3}},
+    {4, {{0, 1}, {2, 3}, {1, 2}}, {0, 1, 2, 3}},
+    {6, {{2, 5}, {0, 1, 2, 3, 4, 5}}, {0, 1, 2, 5, 3, 4}},
+  };
+
+  for (auto &&T : GLBTests) {
+    GlobalLayoutBuilder GLB(T.NumObjects);
+    for (auto &&F : T.Fragments)
+      GLB.addFragment(F);
+
+    std::vector<uint64_t> ComputedLayout;
+    for (auto &&F : GLB.Fragments)
+      ComputedLayout.insert(ComputedLayout.end(), F.begin(), F.end());
+
+    EXPECT_EQ(T.WantLayout, ComputedLayout);
+  }
+}