[CIR][CodeGen] support array def after decl with unknown bound

Arrays can be first declared without a known bound, and then defined
with a known bound. clangir crashes on generating CIR for this case.
This patch adds support.

clang/lib/CIR/CodeGen/CIRGenModule.cpp

@@ -501,6 +501,34 @@ void CIRGenModule::setCommonAttributes(GlobalDecl GD, mlir::Operation *GV) {
   assert(!UnimplementedFeature::setCommonAttributes());
 }
 
+/// Given the type of a previously-seen declaration and the type of a new
+/// declaration, determine whether the new declaration can re-declare the
+/// previous declaration.
+///
+/// This typically requires that the given two types are identical. However
+/// there are some exceptions:
+///   - If the given two types are array types with identical element type,
+///     and PreviousTy has unknown array bound, this function returns true.
+static bool isValidRedeclarationType(mlir::Type PreviousTy, mlir::Type NowTy) {
+  if (PreviousTy == NowTy)
+    return true;
+
+  if (isa<mlir::cir::ArrayType>(PreviousTy) &&
+      isa<mlir::cir::ArrayType>(NowTy)) {
+    auto PreviousArrayTy = cast<mlir::cir::ArrayType>(PreviousTy);
+    auto NowArrayTy = cast<mlir::cir::ArrayType>(NowTy);
+
+    if (PreviousArrayTy.getEltType() != NowArrayTy.getEltType())
+      return false;
+
+    // At this point PreviousTy and NowTy cannot have identical sizes because
+    // this would make PreviousTy and NowTy identical.
+    return PreviousArrayTy.getSize() == 0;
+  }
+
+  return false;
+}
+
 /// If the specified mangled name is not in the module,
 /// create and return an mlir GlobalOp with the specified type (TODO(cir):
 /// address space).
@@ -546,7 +574,7 @@ CIRGenModule::getOrCreateCIRGlobal(StringRef MangledName, mlir::Type Ty,
       assert(0 && "not implemented");
 
     // TODO(cir): check TargetAS matches Entry address space
-    if (Entry.getSymType() == Ty &&
+    if (isValidRedeclarationType(Entry.getSymType(), Ty) &&
         !UnimplementedFeature::addressSpaceInGlobalVar())
       return Entry;
 
@@ -909,7 +937,7 @@ void CIRGenModule::buildGlobalVarDefinition(const clang::VarDecl *D,
   // from the type of the global (this happens with unions).
   if (!GV || GV.getSymType() != InitType) {
     // TODO(cir): this should include an address space check as well.
-    assert(0 && "not implemented");
+    setTypeOfGlobal(GV, InitType);
   }
 
   maybeHandleStaticInExternC(D, GV);
@@ -1512,6 +1540,28 @@ mlir::cir::GlobalLinkageKind CIRGenModule::getCIRLinkageForDeclarator(
   return mlir::cir::GlobalLinkageKind::ExternalLinkage;
 }
 
+void CIRGenModule::setTypeOfGlobal(mlir::cir::GlobalOp &Op, mlir::Type Ty) {
+  if (Op.getSymType() == Ty)
+    return;
+
+  Op.setSymType(Ty);
+  auto Sym = cast<mlir::SymbolOpInterface>(Op.getOperation());
+
+  // Replace the types at every use site.
+  auto SymUses = Sym.getSymbolUses(theModule.getOperation());
+  if (!SymUses.has_value())
+    return;
+
+  for (auto SymUse : *SymUses) {
+    auto UseOp = dyn_cast<mlir::cir::GetGlobalOp>(SymUse.getUser());
+    assert(UseOp && "symbol users of GlobalOp is not a GetGlobalOp");
+
+    auto UseOpResultValue = UseOp.getAddr();
+    UseOpResultValue.setType(
+        mlir::cir::PointerType::get(builder.getContext(), Ty));
+  }
+}
+
 /// This function is called when we implement a function with no prototype, e.g.
 /// "int foo() {}". If there are existing call uses of the old function in the
 /// module, this adjusts them to call the new function directly.

clang/lib/CIR/CodeGen/CIRGenModule.h

@@ -567,6 +567,10 @@ class CIRGenModule : public CIRGenTypeCache {
                                            getMLIRVisibilityFromCIRLinkage(L));
   }
 
+  /// Set the type of the given GlobalOp to the given type, and replace the
+  /// types accordingly at every use site of the given GlobalOp.
+  void setTypeOfGlobal(mlir::cir::GlobalOp &Op, mlir::Type Ty);
+
   mlir::cir::GlobalLinkageKind getCIRLinkageVarDefinition(const VarDecl *VD,
                                                           bool IsConstant);
 

clang/test/CIR/CodeGen/array-unknown-bound.cpp

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -emit-cir %s -o - | FileCheck %s
+
+extern int table[];
+// CHECK: cir.global external @table = #cir.const_array<[#cir.int<1> : !s32i, #cir.int<2> : !s32i, #cir.int<3> : !s32i]> : !cir.array<!s32i x 3>
+
+int test() { return table[1]; }
+//      CHECK: cir.func @_Z4testv() -> !s32i extra( {inline = #cir.inline<no>, optnone = #cir.optnone} ) {
+// CHECK-NEXT:    %0 = cir.alloca !s32i, cir.ptr <!s32i>, ["__retval"] {alignment = 4 : i64}
+// CHECK-NEXT:    %1 = cir.get_global @table : cir.ptr <!cir.array<!s32i x 3>>
+
+int table[3] {1, 2, 3};