Reland: [clang][ExprConstant] fix __builtin_object_size for flexible array members

[nickdesaulniers]

Summary

As reported by @kees, GCC treats __builtin_object_size of structures
containing flexible array members (aka arrays with incomplete type) not
just as the sizeof the underlying type, but additionally the size of the
members in a designated initializer list.

Fixes: https://github.com/llvm/llvm-project/issues/62789

[erichkeane]

clang/lib/AST/ExprConstant.cpp

11740

Isn't this a possible null-deref?

[nickdesaulniers]

clang/lib/AST/ExprConstant.cpp

11740

I don't think so; in fact, I can use cast and get rather than dyn_cast_or_null and dyn_cast here.

Just because we have a pointer doesn't mean it's possibly nullptr; I don't think we can reach this code patch for evaluating the __builtin_object_size of a struct with a flexible array member if the LValue doesn't have a corresponding VarDecl.

[nickdesaulniers]

• use stronger casts, combine patches

[erichkeane]

clang/lib/AST/ExprConstant.cpp

11740

Of course that is a possibility. You shouldn't use dyn_cast right before a dereference, this should definitely be using cast/get, since they assert.

What does the LValue have when the dynamic struct does not have an initializer? Or just an empty one?

[nickdesaulniers]

clang/lib/AST/ExprConstant.cpp

11740

s/code patch/code path/

[erichkeane]

Added clang-vendors in case this is a breaking change? It also needs a release note (likely both in the potentially breaking changes section, and the bug fix section).

[nickdesaulniers]

clang/lib/AST/ExprConstant.cpp

11740

What does the LValue have when the dynamic struct does not have an initializer? Or just an empty one?

Tested:

struct foo {
    int x;
    int y[];
};

static struct foo instance = {
    .x = 3,
    // .y = { 5, 10, 15, },
};

unsigned long foo (void) {
    return __builtin_object_size(&instance, 1);
}

With the following values for .y in instance:

1. .y = { 5, 10, 15 }: foo returns 16.
2. .y = {},: foo returns 4.
3. <.y is not specified or explicitly initialized>: foo returns 4.

[nickdesaulniers]

• add release note

[erichkeane]

Thanks for testing those! Can you add them to the object-size.c if they aren't already tested?

[nickdesaulniers]

• add release note on bugfix

[erichkeane]

clang/docs/ReleaseNotes.rst

289

We should probably also release-note in this seciton.

[nickdesaulniers]

• moar tests

[nickdesaulniers]

• slightly reword bug fix release note

[erichkeane]

I'm OK with this, but please give the clang-vendors folks a few days to comment before committing this. Thanks for your patience!

[nickdesaulniers]

Sure, thanks for the quick turnaround time on reviews! 🍻

[nickdesaulniers]

This also changes the behavior of __builtin_dynamic_object_size (see @kees ' full example: https://github.com/llvm/llvm-project/issues/62789#issue-1714764560). s/changes/fixes/. Let me add more tests and notes about that.

[nickdesaulniers]

• test __builtin_dynamic_object_size too, add that to release notes

[nickdesaulniers]

• rebase

[efriedma]

clang/lib/AST/ExprConstant.cpp

11740

This might have been missed in the discussion here, but what actually ensures we have a VarDecl here? I guess most other LValueBases can't actually have a struct type, but a CompoundLiteralExpr can. (Looking briefly at the code, I can't find any other possibilities at the moment, but in any case, the assumption seems pretty fragile.)

[nickdesaulniers]

clang/lib/AST/ExprConstant.cpp

11740

I assume that the code works as such:

struct foo my_foo;
return __builtin_object_size(&foo, 1);

foo is a VarDecl otherwise how do you take the address? What other expression could be passed to __builtin_object_size that isn't a reference to a variable?

[efriedma]

clang/lib/AST/ExprConstant.cpp

11740

__builtin_object_size(&(struct S){}, 1)? Not really a useful construct, but we don't reject it.

[nickdesaulniers]

clang/lib/AST/ExprConstant.cpp

11740

https://reviews.llvm.org/D151148

[saugustine]

This change causes an assertion failure on the following test case. The memset is important for some reason.

saugustine@saugustine-desktop:~/llvm/build $ cat t.c
struct foo {
  unsigned char x;
  unsigned char	data[];
};

extern void *memset (void *__s, int __c, unsigned long __n);

void bar(void)
{
  struct foo bat;
  memset(&bat, 0, sizeof(bat));
}
saugustine@saugustine-desktop:~/llvm/build $ ./bin/clang -c t.c
clang: /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/AST/Decl.cpp:2786: clang::CharUnits clang::VarDecl::getFlexibleArrayInitChars(const clang::ASTContext &) const: Assertion `hasInit() && "Expect initializer to check for flexible array init"' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.	Program arguments: ./bin/clang -c t.c
1.	t.c:11:30: current parser token ')'
2.	t.c:9:1: parsing function body 'bar'
3.	t.c:9:1: in compound statement ('{}')
 #0 0x000055bf947c1d7d llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/Unix/Signals.inc:602:11
 #1 0x000055bf947c21fb PrintStackTraceSignalHandler(void*) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/Unix/Signals.inc:675:1
 #2 0x000055bf947c0496 llvm::sys::RunSignalHandlers() /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/Signals.cpp:104:5
 #3 0x000055bf947c15ee llvm::sys::CleanupOnSignal(unsigned long) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/Unix/Signals.inc:368:1
 #4 0x000055bf946ff954 (anonymous namespace)::CrashRecoveryContextImpl::HandleCrash(int, unsigned long) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:0:7
 #5 0x000055bf946ffd12 CrashRecoverySignalHandler(int) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:391:1
 #6 0x00007f95ab65af90 (/lib/x86_64-linux-gnu/libc.so.6+0x3bf90)
 #7 0x00007f95ab6a9ccc __pthread_kill_implementation ./nptl/./nptl/pthread_kill.c:44:76
 #8 0x00007f95ab65aef2 raise ./signal/../sysdeps/posix/raise.c:27:6
 #9 0x00007f95ab645472 abort ./stdlib/./stdlib/abort.c:81:7
#10 0x00007f95ab645395 _nl_load_domain ./intl/./intl/loadmsgcat.c:1177:9
#11 0x00007f95ab653df2 (/lib/x86_64-linux-gnu/libc.so.6+0x34df2)
#12 0x000055bf9a6a62cf clang::VarDecl::getFlexibleArrayInitChars(clang::ASTContext const&) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/AST/Decl.cpp:0:3
#13 0x000055bf9a89094b determineEndOffset((anonymous namespace)::EvalInfo&, clang::SourceLocation, unsigned int, (anonymous namespace)::LValue const&, clang::CharUnits&)::$_15::operator()(clang::QualType, clang::CharUnits&) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/AST/ExprConstant.cpp:11741:21
#14 0x000055bf9a89024e determineEndOffset((anonymous namespace)::EvalInfo&, clang::SourceLocation, unsigned int, (anonymous namespace)::LValue const&, clang::CharUnits&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/AST/ExprConstant.cpp:11763:5
#15 0x000055bf9a7dbb62 tryEvaluateBuiltinObjectSize(clang::Expr const*, unsigned int, (anonymous namespace)::EvalInfo&, unsigned long&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/AST/ExprConstant.cpp:11848:7
#16 0x000055bf9a7db953 clang::Expr::tryEvaluateObjectSize(unsigned long&, clang::ASTContext&, unsigned int) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/AST/ExprConstant.cpp:16274:3
#17 0x000055bf98a86e07 clang::Sema::checkFortifiedBuiltinMemoryFunction(clang::FunctionDecl*, clang::CallExpr*)::$_1::operator()(unsigned int) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Sema/SemaChecking.cpp:1111:9
#18 0x000055bf98a86913 clang::Sema::checkFortifiedBuiltinMemoryFunction(clang::FunctionDecl*, clang::CallExpr*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Sema/SemaChecking.cpp:1318:21
#19 0x000055bf98fdcb51 clang::Sema::BuildResolvedCallExpr(clang::Expr*, clang::NamedDecl*, clang::SourceLocation, llvm::ArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*, bool, clang::CallExpr::ADLCallKind) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Sema/SemaExpr.cpp:7463:9
#20 0x000055bf98fbbf78 clang::Sema::BuildCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*, bool, bool) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Sema/SemaExpr.cpp:7112:10
#21 0x000055bf98fda08f clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Sema/SemaExpr.cpp:6880:7
#22 0x000055bf988f57cf clang::Parser::ParsePostfixExpressionSuffix(clang::ActionResult<clang::Expr*, true>) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseExpr.cpp:2125:23
#23 0x000055bf988fca6f clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, bool&, clang::Parser::TypeCastState, bool, bool*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseExpr.cpp:1852:9
#24 0x000055bf988f3fe9 clang::Parser::ParseCastExpression(clang::Parser::CastParseKind, bool, clang::Parser::TypeCastState, bool, bool*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseExpr.cpp:683:20
#25 0x000055bf988f24d8 clang::Parser::ParseAssignmentExpression(clang::Parser::TypeCastState) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseExpr.cpp:175:20
#26 0x000055bf988f238f clang::Parser::ParseExpression(clang::Parser::TypeCastState) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseExpr.cpp:126:18
#27 0x000055bf98981928 clang::Parser::ParseExprStatement(clang::Parser::ParsedStmtContext) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseStmt.cpp:525:19
#28 0x000055bf9897fe5e clang::Parser::ParseStatementOrDeclarationAfterAttributes(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*, clang::ParsedAttributes&, clang::ParsedAttributes&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseStmt.cpp:279:14
#29 0x000055bf9897f4db clang::Parser::ParseStatementOrDeclaration(llvm::SmallVector<clang::Stmt*, 32u>&, clang::Parser::ParsedStmtContext, clang::SourceLocation*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseStmt.cpp:117:20
#30 0x000055bf9898822e clang::Parser::ParseCompoundStatementBody(bool) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseStmt.cpp:1200:11
#31 0x000055bf98989934 clang::Parser::ParseFunctionStatementBody(clang::Decl*, clang::Parser::ParseScope&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseStmt.cpp:2464:21
#32 0x000055bf9887db1f clang::Parser::ParseFunctionDefinition(clang::ParsingDeclarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::LateParsedAttrList*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1471:3
#33 0x000055bf9889f2cb clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::SourceLocation*, clang::Parser::ForRangeInit*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseDecl.cpp:2175:27
#34 0x000055bf9887c99a clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1210:10
#35 0x000055bf9887bedf clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1225:12
#36 0x000055bf9887b7a4 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/Parser.cpp:1040:14
#37 0x000055bf9887967c clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/Parser.cpp:742:12
#38 0x000055bf988744d7 clang::ParseAST(clang::Sema&, bool, bool) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Parse/ParseAST.cpp:163:16
#39 0x000055bf957d311c clang::ASTFrontendAction::ExecuteAction() /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1168:1
#40 0x000055bf9629da04 clang::CodeGenAction::ExecuteAction() /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:1174:5
#41 0x000055bf957d2b1c clang::FrontendAction::Execute() /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1060:7
#42 0x000055bf956fbe48 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:1049:23
#43 0x000055bf9599b5c7 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:264:8
#44 0x000055bf92b09533 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /usr/local/google/home/saugustine/llvm/llvm-project/clang/tools/driver/cc1_main.cpp:249:13
#45 0x000055bf92af4f6a ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/tools/driver/driver.cpp:366:5
#46 0x000055bf92af68dd clang_main(int, char**, llvm::ToolContext const&)::$_0::operator()(llvm::SmallVectorImpl<char const*>&) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/tools/driver/driver.cpp:506:7
#47 0x000055bf92af68ad int llvm::function_ref<int (llvm::SmallVectorImpl<char const*>&)>::callback_fn<clang_main(int, char**, llvm::ToolContext const&)::$_0>(long, llvm::SmallVectorImpl<char const*>&) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:5
#48 0x000055bf955aa499 llvm::function_ref<int (llvm::SmallVectorImpl<char const*>&)>::operator()(llvm::SmallVectorImpl<char const*>&) const /usr/local/google/home/saugustine/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:5
#49 0x000055bf955a68b8 clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const::$_1::operator()() const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Driver/Job.cpp:439:34
#50 0x000055bf955a6885 void llvm::function_ref<void ()>::callback_fn<clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const::$_1>(long) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:45:5
#51 0x000055bf9345b379 llvm::function_ref<void ()>::operator()() const /usr/local/google/home/saugustine/llvm/llvm-project/llvm/include/llvm/ADT/STLFunctionalExtras.h:68:5
#52 0x000055bf946ff76a llvm::CrashRecoveryContext::RunSafely(llvm::function_ref<void ()>) /usr/local/google/home/saugustine/llvm/llvm-project/llvm/lib/Support/CrashRecoveryContext.cpp:427:3
#53 0x000055bf955a6017 clang::driver::CC1Command::Execute(llvm::ArrayRef<std::optional<llvm::StringRef> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >*, bool*) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Driver/Job.cpp:439:7
#54 0x000055bf95540bdf clang::driver::Compilation::ExecuteCommand(clang::driver::Command const&, clang::driver::Command const*&, bool) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Driver/Compilation.cpp:199:15
#55 0x000055bf95540de7 clang::driver::Compilation::ExecuteJobs(clang::driver::JobList const&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&, bool) const /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Driver/Compilation.cpp:253:13
#56 0x000055bf9555b4f8 clang::driver::Driver::ExecuteCompilation(clang::driver::Compilation&, llvm::SmallVectorImpl<std::pair<int, clang::driver::Command const*> >&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/lib/Driver/Driver.cpp:1868:7
#57 0x000055bf92af4a28 clang_main(int, char**, llvm::ToolContext const&) /usr/local/google/home/saugustine/llvm/llvm-project/clang/tools/driver/driver.cpp:542:9
#58 0x000055bf92b2ccdd main /usr/local/google/home/saugustine/llvm/build/tools/clang/tools/driver/clang-driver.cpp:15:3
#59 0x00007f95ab64618a __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:74:3
#60 0x00007f95ab646245 call_init ./csu/../csu/libc-start.c:128:20
#61 0x00007f95ab646245 __libc_start_main ./csu/../csu/libc-start.c:368:5
#62 0x000055bf92af36c1 _start (./bin/clang+0x19096c1)
clang: error: clang frontend command failed with exit code 134 (use -v to see invocation)
clang version 17.0.0 (https://github.com/llvm/llvm-project.git 4faf3aaf28226a4e950c103a14f6fc1d1fdabb1b)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/google/home/saugustine/llvm/build/./bin
clang: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang: note: diagnostic msg: /tmp/t-4b8223.c
clang: note: diagnostic msg: /tmp/t-4b8223.sh
clang: note: diagnostic msg: 

********************

[nickdesaulniers]

• rebase, merge D151148, fix @saugustine's case, move helper out of line

[nickdesaulniers]

• prefer OBJECT_SIZE_BUILTIN to builtin_object_size to additionally test dynamic_builtin_object_size

[efriedma]

LGTM

clang/docs/ReleaseNotes.rst

- Indirect edges of asm goto statements under certain circumstances may now be
   guaranteed (and necessary to support outputs along indirect edges, which is
   now supported as of this release). This change is more consistent with the
   behavior of GCC.
 
   .. code-block:: c
 
     foo: asm goto ("# %0 %1"::"i"(&&foo)::foo);
 
+- ``__builtin_object_size`` and ``__builtin_dynamic_object_size`` now add the
+  ``sizeof`` the elements specified in designated initializers of flexible
+  array members for structs that contain them. This change is more consistent
+  with the behavior of GCC.
+
 C++ Specific Potentially Breaking Changes
 -----------------------------------------
 - Clang won't search for coroutine_traits in std::experimental namespace any more.
   Clang will only search for std::coroutine_traits for coroutines then.
 
 ABI Changes in This Version
 ---------------------------
 

 - Address a false positive in ``-Wpacked`` when applied to a non-pod type using
   Clang ABI >= 15.
   (`#62353: <https://github.com/llvm/llvm-project/issues/62353>`_,
   fallout from the non-POD packing ABI fix in LLVM 15).
 - Clang constexpr evaluator now prints subobject's name instead of its type in notes
   when a constexpr variable has uninitialized subobjects after its constructor call.
   (`#58601 <https://github.com/llvm/llvm-project/issues/58601>`_)
 
 Bug Fixes in This Version
 -------------------------
 
 - Fix segfault while running clang-rename on a non existing file.
   (`#36471 <https://github.com/llvm/llvm-project/issues/36471>`_)
 - Fix crash when diagnosing incorrect usage of ``_Nullable`` involving alias
   templates.
   (`#60344 <https://github.com/llvm/llvm-project/issues/60344>`_)
 - Fix confusing warning message when ``/clang:-x`` is passed in ``clang-cl``

 - A default argument for a non-type template parameter is evaluated and checked
   at the point where it is required. This fixes:
   (`#62224 <https://github.com/llvm/llvm-project/issues/62224>`_) and
   (`#62596 <https://github.com/llvm/llvm-project/issues/62596>`_)
 - Fix an assertion when instantiating the body of a Class Template Specialization
   when it had been instantiated from a partial template specialization with different
   template arguments on the containing class. This fixes:
   (`#60778 <https://github.com/llvm/llvm-project/issues/60778>`_).
+- Match GCC's behavior for ``__builtin_object_size`` and
+  ``__builtin_dynamic_object_size`` on structs containing flexible array
+  members.
+  (`#62789 <https://github.com/llvm/llvm-project/issues/62789>`_).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a bug where attribute annotations on type specifiers (enums, classes,
   structs, unions, and scoped enums) were not properly ignored, resulting in

clang/lib/AST/ExprConstant.cpp

 static bool determineEndOffset(EvalInfo &Info, SourceLocation ExprLoc,
                                unsigned Type, const LValue &LVal,
                                CharUnits &EndOffset) {
   bool DetermineForCompleteObject = refersToCompleteObject(LVal);
 
   auto CheckedHandleSizeof = [&](QualType Ty, CharUnits &Result) {
     if (Ty.isNull() || Ty->isIncompleteType() || Ty->isFunctionType())
       return false;
-    return HandleSizeof(Info, ExprLoc, Ty, Result);
+    bool Ret = HandleSizeof(Info, ExprLoc, Ty, Result);
+    if (Ty->isStructureType() &&
+        Ty->getAsStructureType()->getDecl()->hasFlexibleArrayMember()) {
+      const auto *VD =
+          cast<VarDecl>(LVal.getLValueBase().get<const ValueDecl *>());
+      Result += VD->getFlexibleArrayInitChars(Info.Ctx);
+    }
+    return Ret;
   };
 
   // We want to evaluate the size of the entire object. This is a valid fallback
   // for when Type=1 and the designator is invalid, because we're asked for an
   // upper-bound.
   if (!(Type & 1) || LVal.Designator.Invalid || DetermineForCompleteObject) {
     // Type=3 wants a lower bound, so we can't fall back to this.
     if (Type == 3 && !DetermineForCompleteObject)

clang/test/CodeGen/object-size.c

void test31(void) {
 
   // CHECK: call i64 @llvm.objectsize.i64.p0(ptr %{{.*}}, i1 false, i1 true, i1
   gi = OBJECT_SIZE_BUILTIN(&ds0[9].snd[0], 1);
 
   // CHECK: call i64 @llvm.objectsize.i64.p0(ptr %{{.*}}, i1 false, i1 true, i1
   gi = OBJECT_SIZE_BUILTIN(&dsv[9].snd[0], 1);
 }
 
+// CHECK-LABEL: @test32
+static struct DynStructVar D32 = {
+  .fst = {},
+  .snd = { 0, 1, 2, },
+};
+unsigned long test32(void) {
+  // CHECK: ret i64 19
+  return OBJECT_SIZE_BUILTIN(&D32, 1);
+}
+// CHECK-LABEL: @test33
+static struct DynStructVar D33 = {
+  .fst = {},
+  .snd = {},
+};
+unsigned long test33(void) {
+  // CHECK: ret i64 16
+  return OBJECT_SIZE_BUILTIN(&D33, 1);
+}
+// CHECK-LABEL: @test34
+static struct DynStructVar D34 = {
+  .fst = {},
+};
+unsigned long test34(void) {
+  // CHECK: ret i64 16
+  return OBJECT_SIZE_BUILTIN(&D34, 1);
+}
+
 // CHECK-LABEL: @PR30346
 void PR30346(void) {
   struct sa_family_t {};
   struct sockaddr {
     struct sa_family_t sa_family;
     char sa_data[14];
   };