How lower 1024 bytes of Linear Memory is used?

[pepyakin]

How lower 1024 bytes is used?

Emscripten generated code like this:

(set_local $0
  (call $_ext_memcpy
    (i32.const 1)
    (get_local $5)
    (get_local $2)
  )
)

I'm wondering is this could be correct?

[kripken]

What is ext_memcpy? Perhaps the first argument is not what it seems.

[pepyakin]

@kripken Oops. Sorry, it is my version of memcpy. I managed to reproduce this with an ordinal $_memcpy.

[kripken]

That does look wrong then, unless the source code has a literal pointer to location 1 in memory hardcoded? What test case did you see this with?

In general, it is wrong to emit something like this, the lower 1024 bytes are used for a few special things like a temp copy area for alignment issues etc., so no global data or stack or heap could be there.

[pepyakin]

I guess, it is not hardcoded. The line of code for which this code is generated looks like this.

[kripken]

Yeah, something's getting miscompiled then. I'd guess on the rust side just because the other parts are better tested, but can't be sure. Anyhow, if you can reduce this to a non-rust testcase I could help debug that.

[pepyakin]

Anyhow, if you can reduce this to a non-rust testcase I could help debug that.

Ok, will try!

[pepyakin]

Turns out that this code could be perfectly valid! It seems that it is legal to execute memcpy with any non-zero pointers given that len is 0.