wasm-ld assert hit with -flto in latest Emscripten TOT

[floooh]

cc @sbc100 (this is related to our Discord exchange here: https://discord.com/channels/453584038356058112/590215642444202044/1381731153240657930)

Please include the following in your bug report:

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 4.0.10-git (fc8aa00e2e7d3c166cc7330df60957f557d22fe4)
clang version 21.0.0git (https:/github.com/llvm/llvm-project 8f7e57485ee73205e108d74abb5565d5c63beaca)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: /Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin
Build config: +assertions

NOTE: see comment #24544 (comment) for the actual good/bad versions

The first bad Emscripten SDK hash is (literally the first version after 4.0.10):

https://chromium.googlesource.com/emscripten-releases/+/65c4e7ad07766fdc9dc514159bd5ac4449393e17

The last good Emscripten SDK hash is:

https://chromium.googlesource.com/emscripten-releases/+/8103ffedfb0c42d231c6af6859a5a1a832260b43

Symptoms

wasm-ld hits an assert when working on a Dear ImGui function with -flto:

Assertion failed: (llvm::isUIntN(BitWidth, val) && "Value is not an N-bit unsigned value"), function APInt, file APInt.h, line 128.

Full details are in the readme of the reproducer repository.

Reproduction

Follow the steps outlined in the readme here:

https://github.com/floooh/cimgui-sokol-starterkit/tree/wasm-ld-crash

...the compilation breaks with an wasm-ld assertion, the full console output of the error is:

/opt/homebrew/bin/cmake -E cmake_link_script CMakeFiles/demo.dir/link.txt --verbose=1
/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/emscripten/emcc -O3 -DNDEBUG -flto --shell-file ../sokol/shell.html -sUSE_WEBGL2=1 -sNO_FILESYSTEM=1 -sASSERTIONS=0 -sMALLOC=emmalloc --closure=1 @CMakeFiles/demo.dir/objects1.rsp -o demo.html @CMakeFiles/demo.dir/linkLibs.rsp
Assertion failed: (llvm::isUIntN(BitWidth, val) && "Value is not an N-bit unsigned value"), function APInt, file APInt.h, line 128.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace.
Stack dump:
0.      Program arguments: /Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/wasm-ld -o demo.wasm CMakeFiles/demo.dir/demo.c.o libsokol.a libcimgui.a -L/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto -L/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/emscripten/src/lib -lGL-webgl2-getprocaddr -lal -lhtml5 -lstubs -lnoexit -lc -lemmalloc -lcompiler_rt -lc++-noexcept -lc++abi-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /var/folders/dz/g9ydwg8973z9nn5bvffcwf3h0000gn/T/tmpju1azu1elibemscripten_js_symbols.so --strip-debug -u__cxa_atexit --export=_emscripten_stack_alloc --export=__wasm_call_ctors --export=emscripten_stack_get_current --export=_emscripten_stack_restore --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --table-base=1 --global-base=1024
1.      Running pass 'Function Pass Manager' on module 'ld-temp.o'.
2.      Running pass 'WebAssembly Instruction Selection' on function '@_ZN5ImGui12ColorPicker4EPKcPfiPKf'
 #0 0x000000010972f3c0 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x17f3c0)
 #1 0x000000010972d178 llvm::sys::RunSignalHandlers() (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x17d178)
 #2 0x000000010972fa7c SignalHandler(int, __siginfo*, void*) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x17fa7c)
 #3 0x000000018b810624 (/usr/lib/system/libsystem_platform.dylib+0x1804ac624)
 #4 0x000000018b7d688c (/usr/lib/system/libsystem_pthread.dylib+0x18047288c)
 #5 0x000000018b6dfc60 (/usr/lib/system/libsystem_c.dylib+0x18037bc60)
 #6 0x000000018b6deeec (/usr/lib/system/libsystem_c.dylib+0x18037aeec)
 #7 0x000000010a26ef74 llvm::SelectionDAG::getNOT(llvm::SDLoc const&, llvm::SDValue, llvm::EVT) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xcbef74)
 #8 0x000000010a075670 (anonymous namespace)::DAGCombiner::visitSUB(llvm::SDNode*) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xac5670)
 #9 0x000000010a072cf0 (anonymous namespace)::DAGCombiner::combine(llvm::SDNode*) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xac2cf0)
#10 0x000000010a06fea4 llvm::SelectionDAG::Combine(llvm::CombineLevel, llvm::BatchAAResults*, llvm::CodeGenOptLevel) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xabfea4)
#11 0x000000010a2d0774 llvm::SelectionDAGISel::CodeGenAndEmitDAG() (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xd20774)
#12 0x000000010a2cfcac llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xd1fcac)
#13 0x000000010a2cce78 llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xd1ce78)
#14 0x000000010a2cb300 llvm::SelectionDAGISelLegacy::runOnMachineFunction(llvm::MachineFunction&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0xd1b300)
#15 0x0000000109cbd4c4 llvm::MachineFunctionPass::runOnFunction(llvm::Function&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x70d4c4)
#16 0x00000001099204e0 llvm::FPPassManager::runOnFunction(llvm::Function&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x3704e0)
#17 0x0000000109928338 llvm::FPPassManager::runOnModule(llvm::Module&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x378338)
#18 0x0000000109920eb8 llvm::legacy::PassManagerImpl::run(llvm::Module&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x370eb8)
#19 0x000000010b61ffdc codegen(llvm::lto::Config const&, llvm::TargetMachine*, std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>, unsigned int, llvm::Module&, llvm::ModuleSummaryIndex const&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x206ffdc)
#20 0x000000010b61f274 llvm::lto::backend(llvm::lto::Config const&, std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>, unsigned int, llvm::Module&, llvm::ModuleSummaryIndex&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x206f274)
#21 0x000000010b6085fc llvm::lto::LTO::runRegularLTO(std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x20585fc)
#22 0x000000010b607b3c llvm::lto::LTO::run(std::__2::function<llvm::Expected<std::__2::unique_ptr<llvm::CachedFileStream, std::__2::default_delete<llvm::CachedFileStream>>> (unsigned int, llvm::Twine const&)>, llvm::FileCache) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/lib/libLLVM.dylib+0x2057b3c)
#23 0x0000000104489644 lld::wasm::BitcodeCompiler::compile() (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x1003cd644)
#24 0x000000010448bed8 lld::wasm::SymbolTable::compileBitcodeFiles() (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x1003cfed8)
#25 0x0000000104472000 lld::wasm::(anonymous namespace)::LinkerDriver::linkerMain(llvm::ArrayRef<char const*>) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x1003b6000)
#26 0x000000010446e10c lld::wasm::link(llvm::ArrayRef<char const*>, llvm::raw_ostream&, llvm::raw_ostream&, bool, bool) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x1003b210c)
#27 0x00000001040bee7c lld::unsafeLldMain(llvm::ArrayRef<char const*>, llvm::raw_ostream&, llvm::raw_ostream&, llvm::ArrayRef<lld::DriverDef>, bool) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x100002e7c)
#28 0x00000001040bda40 lld_main(int, char**, llvm::ToolContext const&) (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x100001a40)
#29 0x00000001040be0f4 main (/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/lld+0x1000020f4)
#30 0x000000018b436b98
emcc: error: '/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/bin/wasm-ld -o demo.wasm CMakeFiles/demo.dir/demo.c.o libsokol.a libcimgui.a -L/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/lto -L/Users/floh/projects/cimgui-sokol-starterkit/emsdk/upstream/emscripten/src/lib -lGL-webgl2-getprocaddr -lal -lhtml5 -lstubs -lnoexit -lc -lemmalloc -lcompiler_rt -lc++-noexcept -lc++abi-noexcept -lsockets -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr /var/folders/dz/g9ydwg8973z9nn5bvffcwf3h0000gn/T/tmpju1azu1elibemscripten_js_symbols.so --strip-debug -u__cxa_atexit --export=_emscripten_stack_alloc --export=__wasm_call_ctors --export=emscripten_stack_get_current --export=_emscripten_stack_restore --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-table -z stack-size=65536 --no-growable-memory --initial-heap=16777216 --no-entry --table-base=1 --global-base=1024' failed (received SIGABRT (-6))

[sbc100]

Hmm.. its hard to see how the offend roll could be the blame, it only contains:

2025-06-06 sbc@chromium.org [ci] Fix `emsdk: command not found` error. NFC (#24513)
2025-06-06 me@rreverser.com Remove libformatString.js (#24508)
2025-06-06 sbc@chromium.org Mark test_dylink_pthread_static_data and flaky. NFC (#24501)
2025-06-06 me@rreverser.com Allow using Closure Compiler with debug flags (#24510)

I don't see how any of those 4 commits could cause an llvm crash..

Can you try installing 4.0.10-asserts (since 4.0.10 itself is a release build of the sdk which doesn't include llvm assertions).

[floooh]

Argh you're right, the assert triggers in 4.0.10-asserts.

I wasn't aware that tot builds have asserts enabled but regular releases not. Ok, that means I might need to go back much further in my bisects... I'll try to do more bisecting tomorrow.

At the same time that means though that the problem might not be all that critical, at least I haven't noticed any problems in my WASM apps so far that use Dear ImGui.

I'd still like to get to the bottom of the assert though, so I'll try to do another bisect tomorrow (I guess I'll first check *-assert releases builds, and then narrow it down to a specific non-release version.

[floooh]

To already narrow it down a bit:

• 3.1.72-asserts: good
• 3.1.73-asserts: bad

I'll try to find the specific commit between those two versions tomorrow :)

[floooh]

Ok, last good version: https://chromium.googlesource.com/emscripten-releases/+/848e67bef087f70999f57976248ec4358d459bbd

First bad version: https://chromium.googlesource.com/emscripten-releases/+/2f2fc539017bc8e062699b5f05043a0dc2381ea4

Also makes much more sense since this pulls in LLVM updates.

[sbc100]

Any chance you could share the bull repro case? Also I guess we should probably move this bug the llvm-project since its really an upstream bug.

[floooh]

bull repro case

Typo for 'full repro case'?

The repro case is here, with instruction steps in the readme (I only tested on macOS, but the install_emsk.sh helper script should work on any zsh/bash system):

https://github.com/floooh/cimgui-sokol-starterkit/tree/wasm-ld-crash?tab=readme-ov-file#wasm-ld-crash-reproducer

...would be good if you could quickly confirm the repro steps in the readme, it's just 2 minutes or so :)

[sbc100]

bull repro case

Typo for 'full repro case'?

Sorry yes, did not mean to insult your repro :)