[PATCH] D46241: [CodeGen] Recognize more cases of zero initialization

[Serge Pavlov via Phabricator via cfe-commits]

sepavloff updated this revision to Diff 145049.
sepavloff marked an inline comment as done.
sepavloff added a comment.

Small simplification


Repository:
  rC Clang

https://reviews.llvm.org/D46241

Files:
  lib/CodeGen/CGExprConstant.cpp
  test/CodeGenCXX/cxx11-initializer-aggregate.cpp


Index: test/CodeGenCXX/cxx11-initializer-aggregate.cpp
===================================================================
--- test/CodeGenCXX/cxx11-initializer-aggregate.cpp
+++ test/CodeGenCXX/cxx11-initializer-aggregate.cpp
@@ -51,3 +51,27 @@
   // meaningful.
   B b[30] = {};
 }
+
+namespace ZeroInit {
+  enum { Zero, One };
+  constexpr int zero() { return 0; }
+  constexpr int *null() { return nullptr; }
+  struct Filler {
+    int x;
+    Filler();
+  };
+
+  // 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() };
+
+
+  // This variable must be initialized elementwise.
+  Filler data_e1[1024] = {};
+  // CHECK: getelementptr inbounds {{.*}} @_ZN8ZeroInit7data_e1E
+}
Index: lib/CodeGen/CGExprConstant.cpp
===================================================================
--- lib/CodeGen/CGExprConstant.cpp
+++ lib/CodeGen/CGExprConstant.cpp
@@ -1392,20 +1392,43 @@
   return type;
 }
 
+/// Checks if the specified initializer is equivalent to zero initialization.
+static bool isZeroInitializer(ConstantEmitter &CE, const Expr *Init) {
+  QualType InitTy = Init->getType().getCanonicalType();
+  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)) {
+    if (InitTy->isConstantArrayType()) {
+      for (auto I : IL->inits())
+        if (!isZeroInitializer(CE, I))
+          return false;
+      if (const Expr *Filler = IL->getArrayFiller()) {
+        return isZeroInitializer(CE, Filler);
+      }
+      return true;
+    }
+  } else if (!Init->isEvaluatable(CE.CGM.getContext())) {
+    return false;
+  } else if (Init->isNullPointerConstant(CE.CGM.getContext(),
+                                         Expr::NPC_NeverValueDependent)) {
+    return true;
+  } else {
+    llvm::APSInt Value;
+    if (Init->isIntegerConstantExpr(Value, CE.CGM.getContext()))
+      return Value.isNullValue();
+  }
+
+  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();
 


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