Check for NULL Destination-Type when creating ArrayConstant

[bviyer]

Summary

While emitting Array Constant, if the destination type is null-pointer, it will cause an assert. This patch will check if the destination type is null, and if so then it will just return nullptr as the array constant (not something that is derived from destination type).
A test case is also attached.

[rjmccall]

I think the right fix here is to ensure that DestType is non-null at a higher level. Older branches of the compiler seem to be able to correctly emit this, probably because the initializer generated for this field ends up having type a[0]. Maybe we've just done some refactor that tries to optimize that but doesn't handle flexible array members correctly.

[erik.pilkington]

test/CodeGenCXX/empty-struct-init-list.cpp

2–5

You should add -emit-llvm, or else CodeGen won't even run. Also, expected-no-diagnostics only means anything when -verify is included on the run line.

[bviyer]

Fixed code as suggested by John McCall.

[bviyer]

Forgot to pass the -emit-llvm to the test case.

[rjmccall]

test/CodeGenCXX/empty-struct-init-list.cpp

12

Please make this a FileCheck test that tests the actual compiler output.

[bviyer]

Added FileCheck as requested by John McCall
Made comments start with "//" instead of /*
Removed expected-no-diagnostics from the test case (as requested by Erik)

[rjmccall]

lib/CodeGen/CGExprConstant.cpp

2123

It definitely means that it's an incomplete array type, i.e. a flexible array member.

test/CodeGenCXX/empty-struct-init-list.cpp

12

I'm sorry, I should've been more specific. Please include a test line that shows the complete constant generated to initialize @d.

[bviyer]

John, I have updated the test case as you requested (I think). I am a bit new to Clang, so apologize if I mistook your request.

[rjmccall]

Sure, that's fine.

lib/CodeGen/CGExprConstant.cpp

for (unsigned I = 0; I < NumInitElts; ++I) {
 
       if (I == 0)
         CommonElementType = C->getType();
       else if (C->getType() != CommonElementType)
         CommonElementType = nullptr;
       Elts.push_back(C);
     }
 
+    // This means that the array type is probably "IncompleteType" or some
+    // type that is not ConstantArray.
+    if (CAT == nullptr && CommonElementType == nullptr && !NumInitElts) {
+      const ArrayType *AT = CGM.getContext().getAsArrayType(DestType);
+      CommonElementType = CGM.getTypes().ConvertType(AT->getElementType());
+      llvm::ArrayType *AType = llvm::ArrayType::get(CommonElementType,
+                                                    NumElements);
+      return llvm::ConstantAggregateZero::get(AType);
+    }
+
     return EmitArrayConstant(CGM, CAT, CommonElementType, NumElements, Elts,
                              Filler);
   }
   case APValue::MemberPointer:
     return CGM.getCXXABI().EmitMemberPointer(Value, DestType);
   }
   llvm_unreachable("Unknown APValue kind");
 }

test/CodeGenCXX/empty-struct-init-list.cpp

+// RUN: %clang_cc1 -std=c++11 -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -std=c++14 -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -std=c++17 -emit-llvm -o - %s | FileCheck %s
+
+// CHECK: struct.a
+typedef struct { } a;
+typedef struct {
+  a b[];
+} c;
+
+// CHECK: global %struct.c zeroinitializer, align 1
+c d{ };