[clang] [Clang] Handle structs with inner structs and no fields (PR #89126)

[Bill Wendling via cfe-commits]

https://github.com/bwendling created https://github.com/llvm/llvm-project/pull/89126

A struct that declares an inner struct, but no fields, won't have a field count. So getting the offset of the inner struct fails. This happens in both C and C++:

  struct foo {
    struct bar {
      int Quantizermatrix[];
    };
  };

Here 'struct foo' has no fields.

Fixes: https://github.com/llvm/llvm-project/issues/88931

>From 36ddb5811f11a1f6968705005713f34713026dbb Mon Sep 17 00:00:00 2001
From: Bill Wendling <morbo at google.com>
Date: Wed, 17 Apr 2024 12:23:02 -0700
Subject: [PATCH] [Clang] Handle structs with inner structs and no fields

A struct that declares an inner struct, but no fields, won't have a
field count. So getting the offset of the inner struct fails. This
happens in both C and C++:

  struct foo {
    struct bar {
      int Quantizermatrix[];
    };
  };

Here 'struct foo' has no fields.
---
 clang/lib/CodeGen/CGBuiltin.cpp               | 15 ++++++++++++-
 clang/test/CodeGen/attr-counted-by-pr88931.c  | 22 +++++++++++++++++++
 .../test/CodeGen/attr-counted-by-pr88931.cpp  | 21 ++++++++++++++++++
 3 files changed, 57 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/CodeGen/attr-counted-by-pr88931.c
 create mode 100644 clang/test/CodeGen/attr-counted-by-pr88931.cpp

diff --git a/clang/lib/CodeGen/CGBuiltin.cpp b/clang/lib/CodeGen/CGBuiltin.cpp
index a05874e63c73c2..bee12c4469e7f7 100644
--- a/clang/lib/CodeGen/CGBuiltin.cpp
+++ b/clang/lib/CodeGen/CGBuiltin.cpp
@@ -829,6 +829,9 @@ const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberField(
   unsigned FieldNo = 0;
   bool IsUnion = RD->isUnion();
 
+  if (RD->isImplicit())
+    return nullptr;
+
   for (const Decl *D : RD->decls()) {
     if (const auto *Field = dyn_cast<FieldDecl>(D);
         Field && (Name.empty() || Field->getNameAsString() == Name) &&
@@ -844,7 +847,17 @@ const FieldDecl *CodeGenFunction::FindFlexibleArrayMemberField(
       if (const FieldDecl *Field =
               FindFlexibleArrayMemberField(Ctx, Record, Name, Offset)) {
         const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(RD);
-        Offset += Layout.getFieldOffset(FieldNo);
+        if (Layout.getFieldCount()) {
+          // A struct that holds only an inner struct won't have any fields. E.g.
+          //
+          //     struct foo {
+          //         struct bar {
+          //             int count;
+          //             int array[];
+          //         };
+          //     };
+          Offset += Layout.getFieldOffset(FieldNo);
+        }
         return Field;
       }
 
diff --git a/clang/test/CodeGen/attr-counted-by-pr88931.c b/clang/test/CodeGen/attr-counted-by-pr88931.c
new file mode 100644
index 00000000000000..baa1bbc8397456
--- /dev/null
+++ b/clang/test/CodeGen/attr-counted-by-pr88931.c
@@ -0,0 +1,22 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 4
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O2 -Wno-missing-declarations -emit-llvm -o - %s | FileCheck %s
+
+struct foo {
+  struct bar {
+    int count;
+    int array[] __attribute__((counted_by(count)));
+  };
+};
+
+void init(void * __attribute__((pass_dynamic_object_size(0))));
+
+// CHECK-LABEL: define dso_local void @test1(
+// CHECK-SAME: ptr noundef [[P:%.*]]) local_unnamed_addr #[[ATTR0:[0-9]+]] {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    [[ARRAY:%.*]] = getelementptr inbounds i8, ptr [[P]], i64 4
+// CHECK-NEXT:    tail call void @init(ptr noundef nonnull [[ARRAY]], i64 noundef -1) #[[ATTR2:[0-9]+]]
+// CHECK-NEXT:    ret void
+//
+void test1(struct bar *p) {
+  init(p->array);
+}
diff --git a/clang/test/CodeGen/attr-counted-by-pr88931.cpp b/clang/test/CodeGen/attr-counted-by-pr88931.cpp
new file mode 100644
index 00000000000000..2a8cc1d07e50d9
--- /dev/null
+++ b/clang/test/CodeGen/attr-counted-by-pr88931.cpp
@@ -0,0 +1,21 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 4
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -O2 -Wall -emit-llvm -o - %s | FileCheck %s
+
+struct foo {
+  struct bar {
+    int array[];
+    bar();
+  };
+};
+
+void init(void * __attribute__((pass_dynamic_object_size(0))));
+
+// CHECK-LABEL: define dso_local void @_ZN3foo3barC1Ev(
+// CHECK-SAME: ptr noundef nonnull align 4 dereferenceable(1) [[THIS:%.*]]) unnamed_addr #[[ATTR0:[0-9]+]] align 2 {
+// CHECK-NEXT:  entry:
+// CHECK-NEXT:    tail call void @_Z4initPvU25pass_dynamic_object_size0(ptr noundef nonnull [[THIS]], i64 noundef -1) #[[ATTR2:[0-9]+]]
+// CHECK-NEXT:    ret void
+//
+foo::bar::bar() {
+  init(array);
+}