[CodeGen] Recognize more cases of zero initialization

If a variable has an initializer, codegen tries to build its value. If
the variable is large in size, building its value requires substantial
resources. It causes strange behavior from user viewpoint: compilation
of huge zero initialized arrays like:

    char data_1[2147483648u] = { 0 };

consumes enormous amount of time and memory.

With this change codegen tries to determine if variable initializer is
equivalent to zero initializer. In this case variable value is not
constructed.

This change fixes PR18978.

Differential Revision: https://reviews.llvm.org/D46241

llvm-svn: 332847

9 files changed, 77 insertions, 41 deletions

diff --git a/clang/include/clang/AST/Expr.h b/clang/include/clang/AST/Expr.h
index 47e8261abf1..a44e98b1d1e 100644
--- a/clang/include/clang/AST/Expr.h
+++ b/clang/include/clang/AST/Expr.h
@@ -537,6 +537,13 @@ public:
   bool isConstantInitializer(ASTContext &Ctx, bool ForRef,
                              const Expr **Culprit = nullptr) const;
 
+  enum SideEffectsKind {
+    SE_NoSideEffects,          ///< Strictly evaluate the expression.
+    SE_AllowUndefinedBehavior, ///< Allow UB that we can give a value, but not
+                               ///< arbitrary unmodeled side effects.
+    SE_AllowSideEffects        ///< Allow any unmodeled side effect.
+  };
+
   /// EvalStatus is a struct with detailed info about an evaluation in progress.
   struct EvalStatus {
     /// Whether the evaluated expression has side effects.
@@ -565,6 +572,11 @@ public:
     bool hasSideEffects() const {
       return HasSideEffects;
     }
+
+    bool hasUnacceptableSideEffect(SideEffectsKind SEK) {
+      return (SEK < SE_AllowSideEffects && HasSideEffects) ||
+             (SEK < SE_AllowUndefinedBehavior && HasUndefinedBehavior);
+    }
   };
 
   /// EvalResult is a struct with detailed info about an evaluated expression.
@@ -591,13 +603,6 @@ public:
   /// side-effects.
   bool EvaluateAsBooleanCondition(bool &Result, const ASTContext &Ctx) const;
 
-  enum SideEffectsKind {
-    SE_NoSideEffects,          ///< Strictly evaluate the expression.
-    SE_AllowUndefinedBehavior, ///< Allow UB that we can give a value, but not
-                               ///< arbitrary unmodeled side effects.
-    SE_AllowSideEffects        ///< Allow any unmodeled side effect.
-  };
-
   /// EvaluateAsInt - Return true if this is a constant which we can fold and
   /// convert to an integer, using any crazy technique that we want to.
   bool EvaluateAsInt(llvm::APSInt &Result, const ASTContext &Ctx,
diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 0e1fb98bf77..3efd8dc6a60 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -10312,12 +10312,6 @@ bool Expr::EvaluateAsBooleanCondition(bool &Result,
          HandleConversionToBool(Scratch.Val, Result);
 }
 
-static bool hasUnacceptableSideEffect(Expr::EvalStatus &Result,
-                                      Expr::SideEffectsKind SEK) {
-  return (SEK < Expr::SE_AllowSideEffects && Result.HasSideEffects) ||
-         (SEK < Expr::SE_AllowUndefinedBehavior && Result.HasUndefinedBehavior);
-}
-
 bool Expr::EvaluateAsInt(APSInt &Result, const ASTContext &Ctx,
                          SideEffectsKind AllowSideEffects) const {
   if (!getType()->isIntegralOrEnumerationType())
@@ -10325,7 +10319,7 @@ bool Expr::EvaluateAsInt(APSInt &Result, const ASTContext &Ctx,
 
   EvalResult ExprResult;
   if (!EvaluateAsRValue(ExprResult, Ctx) || !ExprResult.Val.isInt() ||
-      hasUnacceptableSideEffect(ExprResult, AllowSideEffects))
+      ExprResult.hasUnacceptableSideEffect(AllowSideEffects))
     return false;
 
   Result = ExprResult.Val.getInt();
@@ -10339,7 +10333,7 @@ bool Expr::EvaluateAsFloat(APFloat &Result, const ASTContext &Ctx,
 
   EvalResult ExprResult;
   if (!EvaluateAsRValue(ExprResult, Ctx) || !ExprResult.Val.isFloat() ||
-      hasUnacceptableSideEffect(ExprResult, AllowSideEffects))
+      ExprResult.hasUnacceptableSideEffect(AllowSideEffects))
     return false;
 
   Result = ExprResult.Val.getFloat();
@@ -10417,7 +10411,7 @@ bool Expr::EvaluateAsInitializer(APValue &Value, const ASTContext &Ctx,
 bool Expr::isEvaluatable(const ASTContext &Ctx, SideEffectsKind SEK) const {
   EvalResult Result;
   return EvaluateAsRValue(Result, Ctx) &&
-         !hasUnacceptableSideEffect(Result, SEK);
+         !Result.hasUnacceptableSideEffect(SEK);
 }
 
 APSInt Expr::EvaluateKnownConstInt(const ASTContext &Ctx,
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 7b076ea3e65..ae05df1be58 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1392,20 +1392,40 @@ static QualType getNonMemoryType(CodeGenModule &CGM, QualType type) {
   return type;
 }
 
+/// Checks if the specified initializer is equivalent to zero initialization.
+static bool isZeroInitializer(ConstantEmitter &CE, const Expr *Init) {
+  if (auto *E = dyn_cast_or_null<CXXConstructExpr>(Init)) {
+    CXXConstructorDecl *CD = E->getConstructor();
+    return CD->isDefaultConstructor() && CD->isTrivial();
+  }
+
+  if (auto *IL = dyn_cast_or_null<InitListExpr>(Init)) {
+    for (auto I : IL->inits())
+      if (!isZeroInitializer(CE, I))
+        return false;
+    if (const Expr *Filler = IL->getArrayFiller())
+      return isZeroInitializer(CE, Filler);
+    return true;
+  }
+
+  QualType InitTy = Init->getType();
+  if (InitTy->isIntegralOrEnumerationType() || InitTy->isPointerType()) {
+    Expr::EvalResult Result;
+    if (Init->EvaluateAsRValue(Result, CE.CGM.getContext()) &&
+        !Result.hasUnacceptableSideEffect(Expr::SE_NoSideEffects))
+      return (Result.Val.isInt() && Result.Val.getInt().isNullValue()) ||
+             (Result.Val.isLValue() && Result.Val.isNullPointer());
+  }
+
+  return false;
+}
+
 llvm::Constant *ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &D) {
   // Make a quick check if variable can be default NULL initialized
   // and avoid going through rest of code which may do, for c++11,
   // initialization of memory to all NULLs.
-  if (!D.hasLocalStorage()) {
-    QualType Ty = CGM.getContext().getBaseElementType(D.getType());
-    if (Ty->isRecordType())
-      if (const CXXConstructExpr *E =
-          dyn_cast_or_null<CXXConstructExpr>(D.getInit())) {
-        const CXXConstructorDecl *CD = E->getConstructor();
-        if (CD->isTrivial() && CD->isDefaultConstructor())
-          return CGM.EmitNullConstant(D.getType());
-      }
-  }
+  if (!D.hasLocalStorage() && isZeroInitializer(*this, D.getInit()))
+    return CGM.EmitNullConstant(D.getType());
 
   QualType destType = D.getType();
 
diff --git a/clang/test/CodeGen/const-init.c b/clang/test/CodeGen/const-init.c
index 3fd231b630e..2d699954031 100644
--- a/clang/test/CodeGen/const-init.c
+++ b/clang/test/CodeGen/const-init.c
@@ -167,7 +167,7 @@ void g30() {
     int : 1;
     int x;
   } a = {};
-  // CHECK: @g30.a = internal global %struct.anon.1 <{ i8 undef, i32 0 }>, align 1
+  // CHECK: @g30.a = internal global %struct.anon.1 zeroinitializer, align 1
 #pragma pack()
 }
 
diff --git a/clang/test/CodeGen/designated-initializers.c b/clang/test/CodeGen/designated-initializers.c
index 74532c8fa5b..21077b4c147 100644
--- a/clang/test/CodeGen/designated-initializers.c
+++ b/clang/test/CodeGen/designated-initializers.c
@@ -8,7 +8,7 @@ struct foo {
 // CHECK: @u = global %union.anon zeroinitializer
 union { int i; float f; } u = { };
 
-// CHECK: @u2 = global { i32, [4 x i8] } { i32 0, [4 x i8] undef }
+// CHECK: @u2 = global %union.anon.0 zeroinitializer
 union { int i; double f; } u2 = { };
 
 // CHECK: @u3 = global  %union.anon.1 zeroinitializer
diff --git a/clang/test/CodeGen/union-init2.c b/clang/test/CodeGen/union-init2.c
index bf20696a22c..0b68c211122 100644
--- a/clang/test/CodeGen/union-init2.c
+++ b/clang/test/CodeGen/union-init2.c
@@ -5,7 +5,7 @@
 union x {long long b;union x* a;} r = {.a = &r};
 
 
-// CHECK: global { [3 x i8], [5 x i8] } { [3 x i8] zeroinitializer, [5 x i8] undef }
+// CHECK: global %union.z zeroinitializer
 union z {
   char a[3];
   long long b;
diff --git a/clang/test/CodeGenCXX/cxx11-initializer-aggregate.cpp b/clang/test/CodeGenCXX/cxx11-initializer-aggregate.cpp
index 8bf35966f5b..0fb247a31c7 100644
--- a/clang/test/CodeGenCXX/cxx11-initializer-aggregate.cpp
+++ b/clang/test/CodeGenCXX/cxx11-initializer-aggregate.cpp
@@ -51,3 +51,30 @@ 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}};
+
+  // This variable must be initialized elementwise.
+  Filler data_e1[1024] = {};
+  // CHECK: getelementptr inbounds {{.*}} @_ZN8ZeroInit7data_e1E
+}
diff --git a/clang/test/CodeGenCXX/cxx1z-initializer-aggregate.cpp b/clang/test/CodeGenCXX/cxx1z-initializer-aggregate.cpp
index f59ec51136f..ab1c6383bd3 100644
--- a/clang/test/CodeGenCXX/cxx1z-initializer-aggregate.cpp
+++ b/clang/test/CodeGenCXX/cxx1z-initializer-aggregate.cpp
@@ -17,14 +17,14 @@ namespace Constant {
 
   C c1 = {};
   C c2 = {1};
-  // CHECK: @_ZN8Constant2c1E = global { i8 } zeroinitializer, align 1
+  // CHECK: @_ZN8Constant2c1E = global %"struct.Constant::C" zeroinitializer, align 1
   // CHECK: @_ZN8Constant2c2E = global { i8 } { i8 1 }, align 1
 
   // Test packing bases into tail padding.
   D d1 = {};
   D d2 = {1, 2, 3};
   D d3 = {1};
-  // CHECK: @_ZN8Constant2d1E = global { i32, i8, i8 } zeroinitializer, align 4
+  // CHECK: @_ZN8Constant2d1E = global %"struct.Constant::D" zeroinitializer, align 4
   // CHECK: @_ZN8Constant2d2E = global { i32, i8, i8 } { i32 1, i8 2, i8 3 }, align 4
   // CHECK: @_ZN8Constant2d3E = global { i32, i8, i8 } { i32 1, i8 0, i8 0 }, align 4
 
diff --git a/clang/test/SemaCXX/large-array-init.cpp b/clang/test/SemaCXX/large-array-init.cpp
deleted file mode 100644
index ba734280597..00000000000
--- a/clang/test/SemaCXX/large-array-init.cpp
+++ /dev/null
@@ -1,10 +0,0 @@
-// RUN: %clang_cc1 -S -o %t.ll -mllvm -debug-only=exprconstant %s 2>&1 | \
-// RUN:     FileCheck %s
-// REQUIRES: asserts
-
-struct S { int i; };
-
-static struct S arr[100000000] = {{ 0 }};
-// CHECK: The number of elements to initialize: 1.
-
-struct S *foo() { return arr; }