Uncaught RuntimeError: unaligned memory access (wasm only build)

Александр Гурьянов

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Dec 10, 2024, 3:39:46 PM12/10/24

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Hi, periodically my WebAssembly (Wasm) crashes with this error: "Uncaught RuntimeError: unaligned memory access." As far as I understand, Wasm can perform unaligned memory access. So why am I encountering this error?

Linker flags:

"-sUSE_SDL=2" "-sUSE_SDL_IMAGE=2" "-sUSE_SDL_MIXER=2" "-sUSE_SDL_TTF=2" "-sUSE_ZLIB=1"
"-sSDL2_IMAGE_FORMATS=['bmp']"
"-sSTACK_SIZE=1Mb"
"-sTOTAL_MEMORY=512Mb"
"-sALLOW_MEMORY_GROWTH=0"
"-sPTHREAD_POOL_SIZE=6"
"-sPTHREAD_POOL_SIZE_STRICT=0"
"-sFORCE_FILESYSTEM=1"
"-sEXPORT_NAME=VCMI"
"-sMODULARIZE=1"
"-sINVOKE_RUN=0"
"-sEXPORTED_RUNTIME_METHODS=['addRunDependency', 'removeRunDependency', 'FS', \
'lengthBytesUTF8', 'stringToUTF8', 'UTF8ToString', 'callMain', 'run']"
"-sEXPORTED_FUNCTIONS=['_main', '_malloc', '_free']"
"-pthread"
"-fwasm-exceptions"
"-sMALLOC=mimalloc"

"-Oz"

# for debugging
# "-O1"
"--profiling-funcs"
# "-sASSERTIONS=2"

Full error stack (not very useful):

Alon Zakai

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Dec 10, 2024, 5:17:17 PM12/10/24

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AFAIK the one place wasm may trap on unaligned data is atomic operations. See

https://github.com/WebAssembly/threads/blob/main/proposals/threads/Overview.md#alignment ("Unlike normal memory accesses, misaligned atomic accesses trap")

You may need to avoid packing data if it will be accessed using atomics. Also atomic SIMD data may need special handling so it is fully aligned.

- Alon

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Sam Clegg

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Dec 10, 2024, 5:22:48 PM12/10/24

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Also, I'm a little confused about why there are no wasm frames on the stack. If you are using DevTools you should be able to see the exact wasm instruction that caused that trap, or at the very least it should give you the byte offset of the bad instruction and the function name/number.

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Александр Гурьянов

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Dec 11, 2024, 4:07:39 AM12/11/24

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1. I use only the simplest atomics, like atomic_int, atomic_bool, etc. I will try to figure it out.

2. Regarding the Wasm stack, I am also a bit frustrated. In Firefox, when you save a log, it never contains the Wasm stack. To get it, you need to manually click on the triangle near the error message to view the detailed stack. Unfortunately, these reports came from users who didn’t do this. Also, this problem never happens in Chrome (no reports yet).

ср, 11 дек. 2024 г. в 01:22, 'Sam Clegg' via emscripten-discuss <emscripte...@googlegroups.com>:

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