Comment by thewavelength
7 hours ago
Why is a relatively new technology like WASM being limited to 32-bit pointers? Why repeat the same mistake again?
> Web is 32-bit. Your 64-bit structs will break. This was the root cause of most of my bugs. WASM is 32-bit address space, pointers are 4 bytes not 8.
1: Letting your code break on pointer size changes is a quite bad sign imho (it's a sign that many other things are probably done with aliasing,etc and has a high risk of breaking due to undefined behaviour once gcc/clang gets around to utilizing it for an optimization).
2: iirc WASM was initially designed to be shimmable via Asm.JS to force laggards(Apple, Google) to implement it, Asm.JS in turn relied on specific rules in JS to get reliable 32bit arithmetic (but impossible for 64bit).
Wasm64 is implemented and works in Chrome and Firefox.. Apple is lagging again with Safari.
Thanks!
1: True, although it also limits the addressable memory and the typical 4GB limit seems less these days. I’m thinking of large apps like Figma running in the browser.
2: Will existing 32-bit WASM binaries break on WASM64 engines or does the binary have a flag for compatibility?
1: Something like Figma could probably offload some of the memory pressure to GPU textures. (But they'd probably run into safety browser limits before that).
2: Most runtimes are 64bit already, A runtime detecting a wasm32 binary will just continue to generate code with the current JIT compiler whilst WASM64 will require another JIT (and perhaps memory system since WASM32 runtimes are often based on "hacks" where 4gb of address space is reserved but not given real memory so that the JIT compiler gets an easier job without security implications).
what would make it break? i think the program just calls a 64 bit wasm memory function if it uses the capability
Because 64-bit WASM can be quite a bit slower than 32-bit WASM:
https://spidermonkey.dev/blog/2025/01/15/is-memory64-actuall...
TL;DR: wasm64 requires explicit heap bounds checks, while in wasm32 the memory mapping hardware does it for free.
E.g. quote:
"The only reason to use Memory64 is if you actually need more than 4GB of memory.
Memory64 won’t make your code faster or more “modern”. 64-bit pointers in WebAssembly simply allow you to address more memory, at the cost of slower loads and stores."
I believe 32-bit was chosen partially due to implementation efficiency reasons. It makes sense because you can allocate a 4GB mapping, so there is no need for a second software virtual memory layer. Also perhaps they internally require tagged pointers, which are much cheaper, especially if aligned, if the pointer is only 32 bits
WASM has a (pointer + i32) address mode, and the effective address is 33 bits. So WASM implementations use 8GB mappings ...
64 bit was added in WebAssembly 2.0 (finished in 2022 according to Wikipedia). I know what doesn't answer any it wasn't there in the first place.
32 is better for a lot of things like simd. the strength of it is wasm can do both types now and js can't unfortunately. a number in js is strictly 64.
Not sure about SIMD. If you mean WASM, the main advantage of WASM32 over WASM64 is execution speed if it runs on a 64-bit runtime. This is because pointer accesses are simply 64-bit base pointer + 32/33-bit offset (the 32-bit pointer value in WASM program + some 32-bit optional offset). Since the offset in the memory access is already trimmed to 32/33-bit (in a 32-bit half of the register) at machine instruction level there is no possibility to escape the 8GB virtual memory cage that Chrome allocates, thus no need for additional runtime checks. WASM64, on the other hand, can escape without such checks.
Because a web page shouldn’t use 4 GB of ram, and the win is that each pointer can be half the size (better for memory and cache).
The real mistake is requiring pointer to be 64 bit when most programs don’t use it.
You sounds like the misattributed Bill Gates of 2026.
No? Most consumer desktops have 8 or 16 GB and phones less. You want to use more than half for a web page?
For reference 4 GB is 8x more than a ps3.
Even before RAM got very expensive recently, it had already plateaued. Like 32 GB was still considered a lot for a PC and was about the same price as a decade prior.