Can't build V8 with iterator checks in libcxx

[Chris Palmer]

Hi all,

I'm doing an experiment: turning on increased checks in Chromium's copy of libcxx (LLVM's C++ library). I use this diff:

diff --git a/build/config/c++/BUILD.gn b/build/config/c++/BUILD.gn
index f1c18b9f35fd..3718fc40773d 100644
--- a/build/config/c++/BUILD.gn
+++ b/build/config/c++/BUILD.gn
@@ -68,11 +68,12 @@ config("runtime_library") {
     # libc++ has two levels of debug mode. Setting _LIBCPP_DEBUG to zero
     # enables most assertions. Setting it to one additionally enables iterator
     # debugging. See https://libcxx.llvm.org/docs/DesignDocs/DebugMode.html
-    if (enable_iterator_debugging) {
-      defines += [ "_LIBCPP_DEBUG=1" ]
-    } else if (is_debug || dcheck_always_on) {
-      defines += [ "_LIBCPP_DEBUG=0" ]
-    }
+    #if (enable_iterator_debugging) {
+    #  defines += [ "_LIBCPP_DEBUG=1" ]
+    #} else if (is_debug || dcheck_always_on) {
+    #  defines += [ "_LIBCPP_DEBUG=0" ]
+    #}
+    defines += [ "_LIBCPP_DEBUG=1" ]
   }

_LIBCPP_DEBUG=1 turns on debugging checks in many std:: classes, and also checks in iterators. (_LIBCPP_DEBUG=0 turns on debugging checks but not in iterators.)

When building, I get this explosion:

ninja: Entering directory `out/Release'
[1/1] Regenerating ninja files
[39041/39294] ACTION //v8:run_mksnapshot_default(//build/toolchain/linux:clang_x64)
FAILED: gen/v8/embedded.S snapshot_blob.bin
python ../../v8/tools/run.py ./mksnapshot --turbo_instruction_scheduling --target_os=linux --target_arch=x64 --embedded_src gen/v8/embedded.S --embedded_variant Default --random-seed 314159265 --startup_blob snapshot_blob.bin --no-native-code-counters
../../buildtools/third_party/libc++/trunk/include/__hash_table:337: _LIBCPP_ASSERT '__get_const_db()->__dereferenceable(this)' failed. Attempted to dereference a non-dereferenceable unordered container iterator
Return code is -6
ninja: build stopped: subcommand failed.

Unfortunately, I don't know how to get you a better error message. Any clues on that? Assuming the check itself is correct, it looks like there's a bug somewhere in V8.

[seth.b...@microsoft.com]

Hi Chris,

This build step is just running a program: run.py executes whatever is on the command line after it. To get a better error details, you can re-run ./mksnapshot in a debugger with all of the provided arguments. When doing so, I see a failure at the following call stack:

00 mksnapshot!abort

01 mksnapshot!std::__1::__libcpp_abort_debug_function

02 mksnapshot!std::__1::__hash_iterator<std::__1::__hash_node<std::__1::__hash_value_type<int,v8::internal::compiler::Node *>,void *> *>::operator->

03 mksnapshot!std::__1::unordered_map<int,v8::internal::compiler::Node *,v8::base::hash<int>,std::__1::equal_to<int>,v8::internal::ZoneAllocator<std::__1::pair<const int,v8::internal::compiler::Node *> > >::operator[]

04 mksnapshot!v8::internal::compiler::NodeCache<int,v8::base::hash<int>,std::__1::equal_to<int> >::Find

05 mksnapshot!v8::internal::compiler::CommonNodeCache::FindInt32Constant

06 mksnapshot!v8::internal::compiler::MachineGraph::Int32Constant

07 mksnapshot!v8::internal::compiler::CodeAssembler::Int32Constant

08 mksnapshot!v8::internal::FromConstexpr_IterationKind_constexpr_IterationKind_0

09 mksnapshot!v8::internal::DownCastForTorqueClass_JSFunction_0

0a mksnapshot!v8::internal::Cast_JSFunction_0

0b mksnapshot!v8::internal::CollectCallFeedback_0

0c mksnapshot!v8::internal::CollectCallFeedback_1

0d mksnapshot!v8::internal::CallWithSpread_WithFeedbackAssembler::GenerateCallWithSpread_WithFeedbackImpl

0e mksnapshot!v8::internal::Builtins::Generate_CallWithSpread_WithFeedback

0f mksnapshot!v8::internal::`anonymous namespace'::BuildWithCodeStubAssemblerCS

10 mksnapshot!v8::internal::SetupIsolateDelegate::SetupBuiltinsInternal

11 mksnapshot!v8::internal::Isolate::Init

12 mksnapshot!v8::internal::Isolate::InitWithoutSnapshot

13 mksnapshot!v8::SnapshotCreator::SnapshotCreator

14 mksnapshot!v8::internal::CreateSnapshotDataBlobInternal

15 mksnapshot!`anonymous namespace'::CreateSnapshotDataBlob

16 mksnapshot!main

17 mksnapshot!invoke_main

18 mksnapshot!__scrt_common_main_seh

19 KERNEL32!BaseThreadInitThunk

1a ntdll!RtlUserThreadStart

By the way, you can just set "enable_iterator_debugging = true" in out/Release/args.gn rather than editing BUILD.gn if you like 😊.

-Seth

[Jakob Gruber]

Sounds related: https://bugs.chromium.org/p/chromium/issues/detail?id=1126116.

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