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by zarzavat 269 days ago
> AND-masking does not really allow for producing the necessary traps for invalid accesses.

Why does it need to trap? Can't they just make it UB?

Specifying that invalid accesses always trap is going to degrade performance, that's not a 64-bit problem, that's a spec problem. Even if you define it in WASM, it's still UB in the compiler so you aren't saving anyone from UB they didn't already have. Just make the trapping guarantee a debug option only.
1 comments
_nalply 269 days ago
It's WASM. WASM runs in a sandbox and you can't have UB on the hardware level. Imagine someone exploiting the behavior of some browser when UB is triggered. Except that the programmer is not having nasal demons [1] but some poor user, like a mom of four children in Abraska running a website on her cell phone.

[1]: http://catb.org/jargon/html/N/nasal-demons.html

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zarzavat 269 days ago
The UB in this case is "you may get another value in the sandboxed memory region if you dereference an invalid pointer, rather than a guaranteed trap". You can still have UB even in a sandbox.

Seems like they got overly attached to the guaranteed trapping they got on 32-bit and wanted to keep it even though it's totally not worth the cost of bounds checking every pointer access. Save the trapping for debug mode only.

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_nalply 269 days ago
Ah, so you meant UB = unspecified behavior, not UB = undefined behavior.

Maybe. Bugs that come from spooky behavior at a distance are notoriously hard to debug, especially in production, and it's worthwile to pay for it to avoid that.

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