CheerpX JIT compilation error when running ipython

[NiltonVolpato]

Description

When running ipython in WebVM, CheerpX's JIT compiler produces invalid WebAssembly, causing compilation failures and triggering fail.wasm downloads.

Steps to Reproduce

1. Build an Alpine-based WebVM image with ipython installed (pip install ipython)
2. Boot the VM and run ipython
3. Observe console errors

Error Output

CompileError: WebAssembly.compile(): Compiling function #8 failed: expected 0 elements on the stack for fallthru, found 1 @+35000

Uncaught CompileError: WebAssembly.Module(): Compiling function #0 failed: expected 0 elements on the stack for fallthru, found 1 @+7503
    at aog (3ed10a5f-2986-4e86-83f5-79758cd9d49b:1:15612)
    at 0073be8a:0xf168a
    at 0073be8a:0x3dba0
    at MessagePort.Iv (3ed10a5f-2986-4e86-83f5-79758cd9d49b:1:60559)

Multiple similar errors follow with different offsets (+6262, +6771, +5806, +5887).

Environment

• Base image: i386/alpine:edge
• Python: 3.12
• ipython: 9.10.0 (latest)
• Browser: Chrome 144.0.7559.97

Analysis

The error indicates the JIT-compiled WASM has invalid stack behavior ("expected 0 elements on the stack for fallthru, found 1"). This appears to be triggered by specific x86 code paths in ipython/Python that produce invalid WASM during JIT compilation.

The fail.wasm download is CheerpX's fallback mechanism when compilation fails.

[NiltonVolpato]

fail-wasm-files.zip
Attaching the fail.wasm file that was automatically downloaded when the JIT compilation failed. This should help with debugging the issue.

[NiltonVolpato]

Additional Analysis

Using wasm-objdump from wabt on the fail.wasm files reveals the issue:

0000077: error: table elem type must be a reference type
0000098: error: table elem type must be a reference type

All generated fail.wasm files have the same structural issue - invalid table element types. Here's the structure of the largest one (692KB):

Sections:
     Type start=0x0000000d end=0x00000014 (size=0x00000007) count: 1
   Import start=0x00000019 end=0x00000024 (size=0x0000000b) count: 1
 Function start=0x00000029 end=0x00000070 (size=0x00000047) count: 70
    Table start=0x00000075 end=0x00000080 (size=0x0000000b) count: 1
   Export start=0x00000085 end=0x0000008c (size=0x00000007) count: 1
     Elem start=0x00000091 end=0x00000169 (size=0x000000d8) count: 1
   Custom start=0x0000016e end=0x000001c6 (size=0x00000058) "compilationHints"
   Custom start=0x000001cb end=0x00006c0a (size=0x00006a3f) "metadata.code.branch_hint"
     Code start=0x00006c0f end=0x000acf55 (size=0x000a6346) count: 70

Each run of ipython generates different fail.wasm files (5 different ones ranging from 55KB to 692KB), all with the same table element type issue.

[saarikoski-jules]

Hi @NiltonVolpato. Thanks for reporting this issue. I've been unable to reproduce the error, could you send me the ext2 image you're using, your html file and the exact commands you're working with, so that I can take a closer look.

[NiltonVolpato]

This image: alpine_python_minimal_20260206_21746725002.ext2

And it's live now too: https://niltonvolpato.github.io/webvm/

To reproduce just run: ipython. Some other python programs seem to trigger the same issue.