Stream: git-wasmtime

Topic: wasmtime / issue #10281 Cranelift: RISCV frame pointer po...

Wasmtime GitHub notifications bot (Feb 24 2025 at 10:45):

JonasKruckenberg added the bug label to Issue #10281.

Wasmtime GitHub notifications bot (Feb 24 2025 at 10:45):

JonasKruckenberg added the cranelift label to Issue #10281.

Wasmtime GitHub notifications bot (Feb 24 2025 at 10:45):

JonasKruckenberg opened issue #10281:

Cranelift on RISCV (when frame pointers are enabled) generated code where the fp register always points to the next frame pointer like so:

----  frame ----
ra,
fp,   <- fp points here
...
---- frame ----

while LLVM generates code where fp points one word above the frame like so:

----  frame ---- <- fp points here
ra,
fp,
...
---- frame ----

While this is not a problem in isolation it means that stack walkers (for backtraces, perf stack traces, unwinding) cannot walk across a boundary where LLVM generated code calls cranelift generated code and vice-versa.

Since we cannot really change LLVMs behavior cranelift should match LLVM even though that means a really nasty breaking change for cranelift consumers that rely on the frame pointer position (nasty since code would continue to compile just be broken at runtime in weird ways).

AFAIK supporting this change in wasmtime would be quite trivial, it would just mean changing this from 0 to -2 * size_of::<usize>()
https://github.com/bytecodealliance/wasmtime/blob/de469e2790f0c33f99df483f2dae4ceb47e11c8e/crates/wasmtime/src/runtime/vm/arch/riscv64.rs#L22
which would mean this assertion no longer holds https://github.com/bytecodealliance/wasmtime/blob/07eeac40525246425073011bd707edd298443a96/crates/wasmtime/src/runtime/vm/traphandlers/backtrace.rs#L252
and the corresponding code in save_last_wasm_exit_fp_pc needs to be adjusted
https://github.com/bytecodealliance/wasmtime/blob/70a37939d367e83ab62002bad64fb11e763f3d2f/crates/cranelift/src/compiler.rs#L1173-L1176

Wasmtime GitHub notifications bot (Feb 25 2025 at 17:33):

alexcrichton added the cranelift:area:riscv64 label to Issue #10281.

Wasmtime GitHub notifications bot (Feb 27 2025 at 17:58):

fitzgen added the wasmtime:debugging label to Issue #10281.

Wasmtime GitHub notifications bot (Feb 27 2025 at 17:58):

fitzgen added the cranelift:area:debug label to Issue #10281.

Last updated: Feb 28 2025 at 03:10 UTC