mirror of https://github.com/WebAssembly/wasi-sdk
Sy Brand
6 months ago
commit
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# Support for C++ Exceptions
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The released artifacts for wasi-sdk at this time do not support C++ exceptions.
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LLVM and Clang, however, have support for C++ exceptions in WebAssembly and this
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is intended to serve as documentation of the current state of affairs of using
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C++ exceptions. It should be noted though that the current status of C++
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exceptions support is not intended to be the final state of support, and this is
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all continuing to be iterated on over time.
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## Building wasi-sdk with exceptions
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When building the sysroot with wasi-sdk you can pass `-DWASI_SDK_EXCEPTIONS=ON`
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to enable support for C++ exceptions. For example:
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```shell script
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$ cmake -G Ninja -B build/sysroot -S . \
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-DCMAKE_TOOLCHAIN_FILE=$path/to/wasi-sdk-p1.cmake \
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-DWASI_SDK_EXCEPTIONS=ON
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```
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The C++ standard library will be compiled with support for exceptions for the
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desired targets and the resulting sysroot supports using exceptions.
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## Compiling code with C++ exceptions
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Currently extra compilation flags are required to fully support C++ exceptions.
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Without these flags programs using C++ exceptions will not work correctly:
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* `-fwasm-exceptions` - needed to enable the WebAssembly exception-handling
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proposal.
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* `-mllvm -wasm-use-legacy-eh=false` - indicates that the standard WebAssembly
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exception-handling instructions should be used.
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* `-lunwind` - links in support for unwinding which C++ exceptions requires.
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This can be specified for example with:
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```shell script
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$ export CFLAGS="-fwasm-exceptions -mllvm -wasm-use-legacy-eh=false"
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$ export LDFLAGS="-lunwind"
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```
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## Limitations
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Currently C++ exceptions support in wasi-sdk does not support shared libraries.
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Fixing this will require resolving some miscellaneous build issues in this
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repository itself.
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## Future Plans
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There are a few tracking issues with historical discussion about C++ exceptions
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support in wasi-sdk such as [#334](https://github.com/WebAssembly/wasi-sdk/issues/334)
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and [#565](https://github.com/WebAssembly/wasi-sdk/issues/565). The major
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remaining items are:
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* Figure out support for shared libraries.
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* Determine how to ship a sysroot that supports both with-and-without
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exceptions.
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* Figure out how to avoid the need for extra compiler flags when using
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exceptions.
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* Figure out if a new wasm target is warranted.
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@ -0,0 +1,28 @@
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diff --git a/libunwind/src/assembly.h b/libunwind/src/assembly.h
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index f8e83e138eff..c5097d25b0c6 100644
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--- a/libunwind/src/assembly.h
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+++ b/libunwind/src/assembly.h
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@@ -249,6 +249,9 @@ aliasname: \
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#define WEAK_ALIAS(name, aliasname)
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#define NO_EXEC_STACK_DIRECTIVE
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+#elif defined(__wasm__)
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+#define NO_EXEC_STACK_DIRECTIVE
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+
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// clang-format on
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#else
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diff --git a/libunwind/src/config.h b/libunwind/src/config.h
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index deb5a4d4d73d..23c9f012cbcf 100644
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--- a/libunwind/src/config.h
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+++ b/libunwind/src/config.h
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@@ -66,7 +66,8 @@
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#define _LIBUNWIND_EXPORT
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#define _LIBUNWIND_HIDDEN
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#else
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- #if !defined(__ELF__) && !defined(__MACH__) && !defined(_AIX)
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+ #if !defined(__ELF__) && !defined(__MACH__) && !defined(_AIX) && \
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+ !defined(__wasm__)
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#define _LIBUNWIND_EXPORT __declspec(dllexport)
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#define _LIBUNWIND_HIDDEN
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#else
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@ -0,0 +1,17 @@
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#include <iostream>
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#include <stdlib.h>
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int main() {
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#ifdef __wasm_exception_handling__
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try {
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throw std::runtime_error("An error occurred");
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abort();
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} catch (const std::runtime_error& e) {
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// ..
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return 0;
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}
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abort();
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#else
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return 0;
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#endif
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}
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