[lldb][TypeSystemClang] Create VLAs of explicitly 0-size as ConstantArrayType (#100710)

Depends on #100674

Currently, we treat VLAs declared as `int[]` and `int[0]` identically.
I.e., we create them as `IncompleteArrayType`s. However, the
`DW_AT_count` for `int[0]` *does* exist, and is retrievable without an
execution context. This patch decouples the notion of "has 0 elements"
from "has no known `DW_AT_count`".

This aligns with how Clang represents `int[0]` in the AST (it treats it
as a `ConstantArrayType` of 0 size).

This issue was surfaced when adapting LLDB to
#93069. There, the
`__compressed_pair_padding` type has a `char[0]` member. If we
previously got the `__compressed_pair_padding` out of a module (where
clang represents `char[0]` as a `ConstantArrayType`), and try to merge
the AST with one we got from DWARF (where LLDB used to represent
`char[0]` as an `IncompleteArrayType`), the AST structural equivalence
check fails, resulting in silent ASTImporter failures. This manifested
in a failure in `TestNonModuleTypeSeparation.py`.

**Implementation**
1. Adjust `ParseChildArrayInfo` to store the element counts of each VLA
dimension as an `optional<uint64_t>`, instead of a regular `uint64_t`.
So when we pass this down to `CreateArrayType`, we have a better
heuristic for what is an `IncompleteArrayType`.
2. In `TypeSystemClang::GetBitSize`, if we encounter a
`ConstantArrayType` simply return the size that it was created with. If
we couldn't determine the authoritative bound from DWARF during parsing,
we would've created an `IncompleteArrayType`. This ensures that
`GetBitSize` on arrays with `DW_AT_count 0x0` returns `0` (whereas
previously it would've returned a `std::nullopt`, causing that
`FieldDecl` to just get dropped during printing)

Author: Michael137

lldb/include/lldb/Symbol/SymbolFile.h

@@ -211,7 +211,15 @@ class SymbolFile : public PluginInterface {
   /// The characteristics of an array type.
   struct ArrayInfo {
     int64_t first_index = 0;
-    llvm::SmallVector<uint64_t, 1> element_orders;
+
+    ///< Each entry belongs to a distinct DW_TAG_subrange_type.
+    ///< For multi-dimensional DW_TAG_array_types we would have
+    ///< an entry for each dimension. An entry represents the
+    ///< optional element count of the subrange.
+    ///
+    ///< The order of entries follows the order of the DW_TAG_subrange_type
+    ///< children of this DW_TAG_array_type.
+    llvm::SmallVector<std::optional<uint64_t>, 1> element_orders;
     uint32_t byte_stride = 0;
     uint32_t bit_stride = 0;
   };

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParser.cpp

@@ -37,7 +37,7 @@ DWARFASTParser::ParseChildArrayInfo(const DWARFDIE &parent_die,
     if (attributes.Size() == 0)
       continue;
 
-    uint64_t num_elements = 0;
+    std::optional<uint64_t> num_elements;
     uint64_t lower_bound = 0;
     uint64_t upper_bound = 0;
     bool upper_bound_valid = false;
@@ -91,7 +91,7 @@ DWARFASTParser::ParseChildArrayInfo(const DWARFDIE &parent_die,
       }
     }
 
-    if (num_elements == 0) {
+    if (!num_elements || *num_elements == 0) {
       if (upper_bound_valid && upper_bound >= lower_bound)
         num_elements = upper_bound - lower_bound + 1;
     }

lldb/source/Plugins/SymbolFile/DWARF/DWARFASTParserClang.cpp

@@ -1395,20 +1395,20 @@ DWARFASTParserClang::ParseArrayType(const DWARFDIE &die,
   uint64_t array_element_bit_stride = byte_stride * 8 + bit_stride;
   CompilerType clang_type;
   if (array_info && array_info->element_orders.size() > 0) {
-    uint64_t num_elements = 0;
     auto end = array_info->element_orders.rend();
     for (auto pos = array_info->element_orders.rbegin(); pos != end; ++pos) {
-      num_elements = *pos;
-      clang_type = m_ast.CreateArrayType(array_element_type, num_elements,
-                                         attrs.is_vector);
+      clang_type = m_ast.CreateArrayType(
+          array_element_type, /*element_count=*/*pos, attrs.is_vector);
+
+      uint64_t num_elements = pos->value_or(0);
       array_element_type = clang_type;
       array_element_bit_stride = num_elements
                                      ? array_element_bit_stride * num_elements
                                      : array_element_bit_stride;
     }
   } else {
-    clang_type =
-        m_ast.CreateArrayType(array_element_type, 0, attrs.is_vector);
+    clang_type = m_ast.CreateArrayType(
+        array_element_type, /*element_count=*/std::nullopt, attrs.is_vector);
   }
   ConstString empty_name;
   TypeSP type_sp =

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -2233,30 +2233,31 @@ TypeSystemClang::CreateBlockPointerType(const CompilerType &function_type) {
 
 #pragma mark Array Types
 
-CompilerType TypeSystemClang::CreateArrayType(const CompilerType &element_type,
-                                              size_t element_count,
-                                              bool is_vector) {
-  if (element_type.IsValid()) {
-    ASTContext &ast = getASTContext();
+CompilerType
+TypeSystemClang::CreateArrayType(const CompilerType &element_type,
+                                 std::optional<size_t> element_count,
+                                 bool is_vector) {
+  if (!element_type.IsValid())
+    return {};
 
-    if (is_vector) {
-      return GetType(ast.getExtVectorType(ClangUtil::GetQualType(element_type),
-                                          element_count));
-    } else {
+  ASTContext &ast = getASTContext();
 
-      llvm::APInt ap_element_count(64, element_count);
-      if (element_count == 0) {
-        return GetType(
-            ast.getIncompleteArrayType(ClangUtil::GetQualType(element_type),
-                                       clang::ArraySizeModifier::Normal, 0));
-      } else {
-        return GetType(ast.getConstantArrayType(
-            ClangUtil::GetQualType(element_type), ap_element_count, nullptr,
-            clang::ArraySizeModifier::Normal, 0));
-      }
-    }
-  }
-  return CompilerType();
+  // Unknown number of elements; this is an incomplete array
+  // (e.g., variable length array with non-constant bounds, or
+  // a flexible array member).
+  if (!element_count)
+    return GetType(
+        ast.getIncompleteArrayType(ClangUtil::GetQualType(element_type),
+                                   clang::ArraySizeModifier::Normal, 0));
+
+  if (is_vector)
+    return GetType(ast.getExtVectorType(ClangUtil::GetQualType(element_type),
+                                        *element_count));
+
+  llvm::APInt ap_element_count(64, *element_count);
+  return GetType(ast.getConstantArrayType(ClangUtil::GetQualType(element_type),
+                                          ap_element_count, nullptr,
+                                          clang::ArraySizeModifier::Normal, 0));
 }
 
 CompilerType TypeSystemClang::CreateStructForIdentifier(
@@ -4767,6 +4768,7 @@ TypeSystemClang::GetBitSize(lldb::opaque_compiler_type_t type,
   clang::QualType qual_type(GetCanonicalQualType(type));
   const clang::Type::TypeClass type_class = qual_type->getTypeClass();
   switch (type_class) {
+  case clang::Type::ConstantArray:
   case clang::Type::FunctionProto:
   case clang::Type::Record:
     return getASTContext().getTypeSize(qual_type);
@@ -5457,9 +5459,9 @@ TypeSystemClang::GetNumChildren(lldb::opaque_compiler_type_t type,
   case clang::Type::IncompleteArray:
     if (auto array_info =
             GetDynamicArrayInfo(*this, GetSymbolFile(), qual_type, exe_ctx))
-      // Only 1-dimensional arrays are supported.
+      // FIXME: Only 1-dimensional arrays are supported.
       num_children = array_info->element_orders.size()
-                         ? array_info->element_orders.back()
+                         ? array_info->element_orders.back().value_or(0)
                          : 0;
     break;
 

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.h

@@ -498,7 +498,8 @@ class TypeSystemClang : public TypeSystem {
   // Array Types
 
   CompilerType CreateArrayType(const CompilerType &element_type,
-                               size_t element_count, bool is_vector);
+                               std::optional<size_t> element_count,
+                               bool is_vector);
 
   // Enumeration Types
   CompilerType CreateEnumerationType(llvm::StringRef name,

lldb/test/API/lang/c/struct_types/main.c

@@ -1,3 +1,4 @@
+// clang-format off
 struct things_to_sum {
     int a;
     int b;
@@ -18,7 +19,7 @@ int main (int argc, char const *argv[])
     }; //% self.expect("frame variable pt.padding[0]", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ["pt.padding[0] = "])
        //% self.expect("frame variable pt.padding[1]", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ["pt.padding[1] = "])
        //% self.expect_expr("pt.padding[0]", result_type="char")
-       //% self.expect("image lookup -t point_tag", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ['padding[]'])
+       //% self.expect("image lookup -t point_tag", DATA_TYPES_DISPLAYED_CORRECTLY, substrs = ['padding[0]'])
 
     struct {} empty;
     //% self.expect("frame variable empty", substrs = ["empty = {}"])

lldb/test/Shell/SymbolFile/DWARF/vla.cpp

@@ -0,0 +1,80 @@
+// RUN: %clangxx_host -gdwarf -std=c++11 -o %t %s
+// RUN: %lldb %t \
+// RUN:   -o run \
+// RUN:   -o "frame var --show-types f" \
+// RUN:   -o "frame var vla0" \
+// RUN:   -o "frame var fla0" \
+// RUN:   -o "frame var fla1" \
+// RUN:   -o "frame var vla01" \
+// RUN:   -o "frame var vla10" \
+// RUN:   -o "frame var vlaN" \
+// RUN:   -o "frame var vlaNM" \
+// RUN:   -o exit | FileCheck %s
+
+struct Foo {
+  static constexpr int n = 1;
+  int m_vlaN[n];
+
+  int m_vla0[0];
+};
+
+int main() {
+  Foo f;
+  f.m_vlaN[0] = 60;
+
+  // CHECK:      (lldb) frame var --show-types f
+  // CHECK-NEXT: (Foo) f = {
+  // CHECK-NEXT:   (int[1]) m_vlaN = {
+  // CHECK-NEXT:     (int) [0] = 60
+  // CHECK-NEXT:   }
+  // CHECK-NEXT:   (int[0]) m_vla0 = {}
+  // CHECK-NEXT: }
+
+  int vla0[0] = {};
+
+  // CHECK:      (lldb) frame var vla0
+  // CHECK-NEXT: (int[0]) vla0 = {}
+
+  int fla0[] = {};
+
+  // CHECK:      (lldb) frame var fla0
+  // CHECK-NEXT: (int[0]) fla0 = {}
+
+  int fla1[] = {42};
+
+  // CHECK:      (lldb) frame var fla1
+  // CHECK-NEXT: (int[1]) fla1 = ([0] = 42)
+
+  int vla01[0][1];
+
+  // CHECK:      (lldb) frame var vla01
+  // CHECK-NEXT: (int[0][1]) vla01 = {}
+
+  int vla10[1][0];
+
+  // CHECK:      (lldb) frame var vla10
+  // CHECK-NEXT: (int[1][0]) vla10 = ([0] = int[0]
+
+  int n = 3;
+  int vlaN[n];
+  for (int i = 0; i < n; ++i)
+    vlaN[i] = -i;
+
+  // CHECK:      (lldb) frame var vlaN
+  // CHECK-NEXT: (int[]) vlaN = ([0] = 0, [1] = -1, [2] = -2)
+
+  int m = 2;
+  int vlaNM[n][m];
+  for (int i = 0; i < n; ++i)
+    for (int j = 0; j < m; ++j)
+      vlaNM[i][j] = i + j;
+
+  // FIXME: multi-dimensional VLAs aren't well supported
+  // CHECK:      (lldb) frame var vlaNM
+  // CHECK-NEXT: (int[][]) vlaNM = {
+  // CHECK-NEXT:   [0] = ([0] = 0, [1] = 1, [2] = 1)
+  // CHECK-NEXT:   [1] = ([0] = 1, [1] = 1, [2] = 2)
+  // CHECK-NEXT: }
+
+  __builtin_debugtrap();
+}