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feat(wasm): compile C++ with WebAssembly exception handling

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wasm builds were compiled with -fno-exceptions. A throw did not
compile, libc++'s own throws (vector::at, std::format, filesystem)
aborted the program, and code that relies on exceptions could not be
built for the browser at all.

Every wasm unit and the std module now get -fwasm-exceptions, and the
link adds -lunwind, which code built that way calls into and the driver
does not add. The flag also selects the libc++ under the sysroot's
lib/<triple>/eh, which wasi-sdk 33 and later ship next to the noeh
one. Those libraries use the standardized encoding (try_table, exnref)
while LLVM 23 still emits the legacy one by default, which wasmtime
refuses to load and which would put both in one binary, so the flags
ask for the standard form with -wasm-use-legacy-eh=false. That makes
the setjmp lowering loadable under wasmtime too.

The flags live in one constant, WasmBaselineFlags, which the compile
command and the std module both take through WasmCompileFlags: a BMI
is refused by a unit whose exception setting differs from its own.
VariantId and StdPcmDir hash it, so the first build after the update
starts fresh instead of importing PCMs built without exceptions, and
two releases sharing the Flatpak's cache don't take turns rebuilding
one std PCM. A sysroot without the exception-enabled libraries stops
the build with a line saying so, rather than at the link with "unable
to find library -lunwind".

The result needs an engine with exnref: Chrome 137, Firefox 131,
Safari 18.4, the bundled wasmtime. The runtime's exception-handling
probe tested the legacy encoding; it now tests try_table, every
variants.json entry carries it, and a browser without it is told which
feature it lacks instead of failing with a CompileError. There is no
switch back to -fno-exceptions: engines without exnref are old enough
not to keep a second build for them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Jorijn van der Graaf 2026-10-31 05:54:39 +01:00
commit 63c4040000
Signed by: jorijnvdgraaf
GPG key ID: 2937E59CDCC1BCFB
4 changed files with 124 additions and 37 deletions
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26
implementations/Crafter.Build-Platform.cpp
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@ -804,6 +804,17 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
std::string stdCppm;
if (isWasm) {
stdCppm = std::format("{}/share/libc++/v1/std.cppm", config.sysroot);
// wasm builds link the sysroot's exception-enabled libc++, which
// -fwasm-exceptions in WasmBaselineFlags selects, and the
// libunwind Build adds. A sysroot without them (wasi-sdk before
// 33, or one built without its exceptions option) otherwise stops
// at the first link with "unable to find library -lunwind", which
// says nothing about why it was asked for. The driver looks in eh/
// first and then in the triple's own directory.
const fs::path libDir = fs::path(config.sysroot) / "lib" / NormalizeTarget(config.target);
if (!fs::exists(libDir / "eh" / "libunwind.a") && !fs::exists(libDir / "libunwind.a")) {
return std::format("the WASI sysroot at {} has no C++ exception support (no {}): wasm builds use WebAssembly exception handling and need the exception-enabled libc++ and libunwind that wasi-sdk 33 and later ship. The Flatpak bundles such a sysroot; CRAFTER_BUILD_WASI_SYSROOT selects another", config.sysroot, (libDir / "eh" / "libunwind.a").string());
}
} else if (config.sysroot.empty()) {
stdCppm = GetToolchain().stdCppm.string();
} else {
@ -818,19 +829,12 @@ std::string Crafter::BuildStdPcm(const Configuration& config, fs::path stdPcm) {
std::string sysrootFlag = config.sysroot.empty()
? std::string()
: std::format(" --sysroot={}", config.sysroot);
// wasm32 rejects -march. wasi-libc++ headers require these baselines
// to compile: setjmp.h needs -mllvm -wasm-enable-sjlj (lowered to wasm
// EH); signal.h requires the emulation define; and EH itself isn't
// wired up so -fno-exceptions stays.
// wasm32 rejects -march, and the std module has to be compiled the way
// the units importing it are, a variant pass's flags (e.g.
// -mrelaxed-simd) included, or the BMI is not feature-compatible.
std::string archFlags = isWasm
? std::string(" -fno-exceptions -msimd128 -fno-c++-static-destructors -mllvm -wasm-enable-sjlj -D_WASI_EMULATED_SIGNAL")
? WasmCompileFlags(config)
: std::format(" -march={} -mtune={}", config.march, config.mtune);
// Match the variant codegen flags the consuming TUs are built with
// (e.g. -mrelaxed-simd) so the std BMI is feature-compatible. wasm only;
// wasmVariantFlags is empty on every other path.
if (isWasm) {
for (const std::string& f : config.wasmVariantFlags) archFlags += std::format(" {}", f);
}
// non-wasm cross builds: the driver ranks the host toolchain's own
// libc++ headers (<install>/../include/c++/v1) above --sysroot, so
// the PCM would mix host libc++ with the foreign libc — force the