Use zeroinitializer for (trailing zero portion of) large array initializers

more reliably.

Clang has two different ways it emits array constants (from InitListExprs and
from APValues), and both had some ability to emit zeroinitializer, but neither
was able to catch all cases where we could use zeroinitializer reliably. In
particular, emitting from an APValue would fail to notice if all the explicit
array elements happened to be zero. In addition, for large arrays where only an
initial portion has an explicit initializer, we would emit the complete
initializer (which could be huge) rather than emitting only the non-zero
portion. With this change, when the element would have a suffix of more than 8
zero elements, we emit the array constant as a packed struct of its initial
portion followed by a zeroinitializer constant for the trailing zero portion.

In passing, I found a bug where SemaInit would sometimes walk the entire array
when checking an initializer that only covers the first few elements; that's
fixed here to unblock testing of the rest.

Differential Revision: https://reviews.llvm.org/D47166

llvm-svn: 333044

clang/lib/CodeGen/CGExprConstant.cpp

@@ -635,6 +635,52 @@ static ConstantAddress tryEmitGlobalCompoundLiteral(CodeGenModule &CGM,
   return ConstantAddress(GV, Align);
 }
 
+static llvm::Constant *
+EmitArrayConstant(llvm::ArrayType *PreferredArrayType,
+                  llvm::Type *CommonElementType, unsigned ArrayBound,
+                  SmallVectorImpl<llvm::Constant *> &Elements,
+                  llvm::Constant *Filler) {
+  // Figure out how long the initial prefix of non-zero elements is.
+  unsigned NonzeroLength = ArrayBound;
+  if (Elements.size() < NonzeroLength && Filler->isNullValue())
+    NonzeroLength = Elements.size();
+  if (NonzeroLength == Elements.size()) {
+    while (NonzeroLength > 0 && Elements[NonzeroLength - 1]->isNullValue())
+      --NonzeroLength;
+  }
+
+  if (NonzeroLength == 0)
+    return llvm::ConstantAggregateZero::get(PreferredArrayType);
+
+  // If there's not many trailing zero elements, just emit an array
+  // constant.
+  if (NonzeroLength + 8 >= ArrayBound && CommonElementType) {
+    Elements.resize(ArrayBound, Filler);
+    return llvm::ConstantArray::get(
+        llvm::ArrayType::get(CommonElementType, ArrayBound), Elements);
+  }
+
+  // Add a zeroinitializer array filler if we have trailing zeroes.
+  if (unsigned TrailingZeroes = ArrayBound - NonzeroLength) {
+    assert(Elements.size() >= NonzeroLength &&
+           "missing initializer for non-zero element");
+    Elements.resize(NonzeroLength + 1);
+    auto *FillerType = PreferredArrayType->getElementType();
+    if (TrailingZeroes > 1)
+      FillerType = llvm::ArrayType::get(FillerType, TrailingZeroes);
+    Elements.back() = llvm::ConstantAggregateZero::get(FillerType);
+  }
+
+  // We have mixed types. Use a packed struct.
+  llvm::SmallVector<llvm::Type *, 16> Types;
+  Types.reserve(Elements.size());
+  for (llvm::Constant *Elt : Elements)
+    Types.push_back(Elt->getType());
+  llvm::StructType *SType =
+      llvm::StructType::get(PreferredArrayType->getContext(), Types, true);
+  return llvm::ConstantStruct::get(SType, Elements);
+}
+
 /// This class only needs to handle two cases:
 /// 1) Literals (this is used by APValue emission to emit literals).
 /// 2) Arrays, structs and unions (outside C++11 mode, we don't currently
@@ -834,7 +880,6 @@ public:
   llvm::Constant *EmitArrayInitialization(InitListExpr *ILE, QualType T) {
     llvm::ArrayType *AType =
         cast<llvm::ArrayType>(ConvertType(ILE->getType()));
-    llvm::Type *ElemTy = AType->getElementType();
     unsigned NumInitElements = ILE->getNumInits();
     unsigned NumElements = AType->getNumElements();
 
@@ -845,55 +890,35 @@ public:
     QualType EltType = CGM.getContext().getAsArrayType(T)->getElementType();
 
     // Initialize remaining array elements.
-    llvm::Constant *fillC;
-    if (Expr *filler = ILE->getArrayFiller())
+    llvm::Constant *fillC = nullptr;
+    if (Expr *filler = ILE->getArrayFiller()) {
       fillC = Emitter.tryEmitAbstractForMemory(filler, EltType);
-    else
-      fillC = Emitter.emitNullForMemory(EltType);
-    if (!fillC)
-      return nullptr;
-
-    // Try to use a ConstantAggregateZero if we can.
-    if (fillC->isNullValue() && !NumInitableElts)
-      return llvm::ConstantAggregateZero::get(AType);
+      if (!fillC)
+        return nullptr;
+    }
 
     // Copy initializer elements.
     SmallVector<llvm::Constant*, 16> Elts;
-    Elts.reserve(std::max(NumInitableElts, NumElements));
+    if (fillC && fillC->isNullValue())
+      Elts.reserve(NumInitableElts + 1);
+    else
+      Elts.reserve(NumElements);
 
-    bool RewriteType = false;
-    bool AllNullValues = true;
+    llvm::Type *CommonElementType = nullptr;
     for (unsigned i = 0; i < NumInitableElts; ++i) {
       Expr *Init = ILE->getInit(i);
       llvm::Constant *C = Emitter.tryEmitPrivateForMemory(Init, EltType);
       if (!C)
         return nullptr;
-      RewriteType |= (C->getType() != ElemTy);
+      if (i == 0)
+        CommonElementType = C->getType();
+      else if (C->getType() != CommonElementType)
+        CommonElementType = nullptr;
       Elts.push_back(C);
-      if (AllNullValues && !C->isNullValue())
-        AllNullValues = false;
     }
 
-    // If all initializer elements are "zero," then avoid storing NumElements
-    // instances of the zero representation.
-    if (AllNullValues)
-      return llvm::ConstantAggregateZero::get(AType);
-
-    RewriteType |= (fillC->getType() != ElemTy);
-    Elts.resize(NumElements, fillC);
-
-    if (RewriteType) {
-      // FIXME: Try to avoid packing the array
-      std::vector<llvm::Type*> Types;
-      Types.reserve(Elts.size());
-      for (unsigned i = 0, e = Elts.size(); i < e; ++i)
-        Types.push_back(Elts[i]->getType());
-      llvm::StructType *SType = llvm::StructType::get(AType->getContext(),
-                                                            Types, true);
-      return llvm::ConstantStruct::get(SType, Elts);
-    }
-
-    return llvm::ConstantArray::get(AType, Elts);
+    return EmitArrayConstant(AType, CommonElementType, NumElements, Elts,
+                             fillC);
   }
 
   llvm::Constant *EmitRecordInitialization(InitListExpr *ILE, QualType T) {
@@ -1895,34 +1920,28 @@ llvm::Constant *ConstantEmitter::tryEmitPrivate(const APValue &Value,
 
     // Emit array filler, if there is one.
     llvm::Constant *Filler = nullptr;
-    if (Value.hasArrayFiller())
+    if (Value.hasArrayFiller()) {
       Filler = tryEmitAbstractForMemory(Value.getArrayFiller(),
                                         CAT->getElementType());
+      if (!Filler)
+        return nullptr;
+    }
 
     // Emit initializer elements.
     llvm::Type *CommonElementType =
         CGM.getTypes().ConvertType(CAT->getElementType());
-
-    // Try to use a ConstantAggregateZero if we can.
-    if (Filler && Filler->isNullValue() && !NumInitElts) {
-      llvm::ArrayType *AType =
-          llvm::ArrayType::get(CommonElementType, NumElements);
-      return llvm::ConstantAggregateZero::get(AType);
-    }
+    llvm::ArrayType *PreferredArrayType =
+        llvm::ArrayType::get(CommonElementType, NumElements);
 
     SmallVector<llvm::Constant*, 16> Elts;
-    Elts.reserve(NumElements);
-    for (unsigned I = 0; I < NumElements; ++I) {
-      llvm::Constant *C = Filler;
-      if (I < NumInitElts) {
-        C = tryEmitPrivateForMemory(Value.getArrayInitializedElt(I),
-                                    CAT->getElementType());
-      } else if (!Filler) {
-        assert(Value.hasArrayFiller() &&
-               "Missing filler for implicit elements of initializer");
-        C = tryEmitPrivateForMemory(Value.getArrayFiller(),
-                                    CAT->getElementType());
-      }
+    if (Filler && Filler->isNullValue())
+      Elts.reserve(NumInitElts + 1);
+    else
+      Elts.reserve(NumElements);
+
+    for (unsigned I = 0; I < NumInitElts; ++I) {
+      llvm::Constant *C = tryEmitPrivateForMemory(
+          Value.getArrayInitializedElt(I), CAT->getElementType());
       if (!C) return nullptr;
 
       if (I == 0)
@@ -1932,20 +1951,8 @@ llvm::Constant *ConstantEmitter::tryEmitPrivate(const APValue &Value,
       Elts.push_back(C);
     }
 
-    if (!CommonElementType) {
-      // FIXME: Try to avoid packing the array
-      std::vector<llvm::Type*> Types;
-      Types.reserve(NumElements);
-      for (unsigned i = 0, e = Elts.size(); i < e; ++i)
-        Types.push_back(Elts[i]->getType());
-      llvm::StructType *SType =
-        llvm::StructType::get(CGM.getLLVMContext(), Types, true);
-      return llvm::ConstantStruct::get(SType, Elts);
-    }
-
-    llvm::ArrayType *AType =
-      llvm::ArrayType::get(CommonElementType, NumElements);
-    return llvm::ConstantArray::get(AType, Elts);
+    return EmitArrayConstant(PreferredArrayType, CommonElementType, NumElements,
+                             Elts, Filler);
   }
   case APValue::MemberPointer:
     return CGM.getCXXABI().EmitMemberPointer(Value, DestType);

clang/lib/Sema/SemaInit.cpp

@@ -751,6 +751,9 @@ InitListChecker::FillInEmptyInitializations(const InitializedEntity &Entity,
         ElementEntity.getKind() == InitializedEntity::EK_VectorElement)
       ElementEntity.setElementIndex(Init);
 
+    if (Init >= NumInits && ILE->hasArrayFiller())
+      return;
+
     Expr *InitExpr = (Init < NumInits ? ILE->getInit(Init) : nullptr);
     if (!InitExpr && Init < NumInits && ILE->hasArrayFiller())
       ILE->setInit(Init, ILE->getArrayFiller());

clang/test/CodeGen/init.c

@@ -72,6 +72,16 @@ struct a7 {
 struct a7 test7 = { .b = 0, .v = "bar" };
 
 
+// CHECK-DAG: @huge_array = global {{.*}} <{ i32 1, i32 0, i32 2, i32 0, i32 3, [999999995 x i32] zeroinitializer }>
+int huge_array[1000000000] = {1, 0, 2, 0, 3, 0, 0, 0};
+
+// CHECK-DAG: @huge_struct = global {{.*}} { i32 1, <{ i32, [999999999 x i32] }> <{ i32 2, [999999999 x i32] zeroinitializer }> }
+struct Huge {
+  int a;
+  int arr[1000 * 1000 * 1000];
+} huge_struct = {1, {2, 0, 0, 0}};
+
+
 // PR279 comment #3
 char test8(int X) {
   char str[100000] = "abc"; // tail should be memset.

clang/test/CodeGenCXX/cxx11-initializer-aggregate.cpp

@@ -11,6 +11,13 @@ namespace NonAggregateCopyInAggregateInit { // PR32044
   struct C { A &&p; } c{{1}};
 }
 
+namespace NearlyZeroInit {
+  // CHECK-DAG: @_ZN14NearlyZeroInit1aE = global {{.*}} <{ i32 1, i32 2, i32 3, [120 x i32] zeroinitializer }>
+  int a[123] = {1, 2, 3};
+  // CHECK-DAG: @_ZN14NearlyZeroInit1bE = global {{.*}} { i32 1, <{ i32, [2147483647 x i32] }> <{ i32 2, [2147483647 x i32] zeroinitializer }> }
+  struct B { int n; int arr[1024 * 1024 * 1024 * 2u]; } b = {1, {2}};
+}
+
 // CHECK-LABEL: define {{.*}}@_Z3fn1i(
 int fn1(int x) {
   // CHECK: %[[INITLIST:.*]] = alloca %struct.A
@@ -51,3 +58,35 @@ namespace NonTrivialInit {
   // meaningful.
   B b[30] = {};
 }
+
+namespace ZeroInit {
+  enum { Zero, One };
+  constexpr int zero() { return 0; }
+  constexpr int *null() { return nullptr; }
+  struct Filler {
+    int x;
+    Filler();
+  };
+  struct S1 {
+    int x;
+  };
+
+  // These declarations, if implemented elementwise, require huge
+  // amout of memory and compiler time.
+  unsigned char data_1[1024 * 1024 * 1024 * 2u] = { 0 };
+  unsigned char data_2[1024 * 1024 * 1024 * 2u] = { Zero };
+  unsigned char data_3[1024][1024][1024] = {{{0}}};
+  unsigned char data_4[1024 * 1024 * 1024 * 2u] = { zero() };
+  int *data_5[1024 * 1024 * 512] = { nullptr };
+  int *data_6[1024 * 1024 * 512] = { null() };
+  struct S1 data_7[1024 * 1024 * 512] = {{0}};
+  char data_8[1000 * 1000 * 1000] = {};
+  int (&&data_9)[1000 * 1000 * 1000] = {0};
+  unsigned char data_10[1024 * 1024 * 1024 * 2u] = { 1 };
+  unsigned char data_11[1024 * 1024 * 1024 * 2u] = { One };
+  unsigned char data_12[1024][1024][1024] = {{{1}}};
+
+  // This variable must be initialized elementwise.
+  Filler data_e1[1024] = {};
+  // CHECK: getelementptr inbounds {{.*}} @_ZN8ZeroInit7data_e1E
+}

clang/test/SemaCXX/aggregate-initialization.cpp

@@ -180,3 +180,9 @@ namespace IdiomaticStdArrayInitDoesNotWarn {
 
 #pragma clang diagnostic pop
 }
+
+namespace HugeArraysUseArrayFiller {
+  // All we're checking here is that initialization completes in a reasonable
+  // amount of time.
+  struct A { int n; int arr[1000 * 1000 * 1000]; } a = {1, {2}};
+}