Wasm: how to set __heap_base ?

[vshymanskyy]

I run into this problem while running a wasm file on ESP32 and other devices (yeagh..). I'm using Wasm3 interpreter, and created this example:

https://github.com/wasm3/wasm3-arduino/tree/master/wasm_apps/tinygo

Now I'm trying to use the Linear Memory, but many embedded devices cannot afford to allocate even a single Wasm page (64KiB), so we allocate as much as we can... And it works with AssemblyScript, Rust for example. In TinyGo, I get __heap_base = 65536 and can't find a way to lower it.

With this picture in mind I tried:

tinygo  build -target wasm                \
        -panic trap -wasm-abi generic     \
        -heap-size 4096                   \
        -ldflags="-z,stack-size=2048"     \
        -o app.wasm app.go

Still getting __heap_base = 65536.

Would appreciate any help. Thanks!

[aykevl]

I believe this is a linker limitation. Ideally __heap_base would be put right after the end of the static data (stack and .data) but IIRC I couldn't find a way to do that. If you have ideas how to fix that, that would be great (such as a special symbol for the end of .data).

However, there is one way you can influence the heap size and that is with the -heap-size flag. Unfortunately, IIRC it is not possible to set it to something other than a multiple of 64K otherwise wasm-ld will complain.

I would like to see TinyGo supported by this wasm runtime so I hope this can be fixed somehow.

[vshymanskyy]

@aykevl I think I achieved (at least patially) what I wanted with

-ldflags="-z stack-size=2048 --max-memory=65536"

Yup.. the problem was hidden in the , character ;)