Description
tl;dr there should be a panic build option to short-circuit runtime.runtimePanicAt to just unreachable vs. printing a string.
I was trying to produce a WASM progra, that could accept & return a string from/to the host, similar to this wazero example.
I compiled with tinygo v0.30.0, tinygo build -o main.wasm -no-debug -panic=trap -scheduler=none -gc=leaking -target=wasm main.go.
Inspecting the output with wasmer, I found wasi_snapshot_preview1 fd_write being imported. My program does not print anything, and only imports unsafe.
Converting the WASM to WAT, I found that the runtime will unconditionally try to print during a runtimePanic. This feels pretty unavoidable, as funcs in the unsafe package like Slice or String can panic, but are also useful for passing memory between the guest and host.
Relevant WAT snippet:
(func $runtime.runtimePanicAt (type 4) (param i32 i32)
i32.const 65536
i32.const 22
call $runtime.printstring
local.get 0
local.get 1
call $runtime.printstring
i32.const 10
call $runtime.putchar
unreachable)
(func $runtime.printstring (type 4) (param i32 i32)
local.get 1
i32.const 0
local.get 1
i32.const 0
i32.gt_s
select
local.set 1
loop ;; label = @1
local.get 1
if ;; label = @2
local.get 0
i32.load8_u
call $runtime.putchar
local.get 1
i32.const 1
i32.sub
local.set 1
local.get 0
i32.const 1
i32.add
local.set 0
br 1 (;@1;)
end
end)
(func $runtime.putchar (type 1) (param i32)
(local i32 i32)
i32.const 65716
i32.load
local.tee 1
i32.const 119
i32.le_u
if ;; label = @1
i32.const 65716
local.get 1
i32.const 1
i32.add
local.tee 2
i32.store
local.get 1
i32.const 65720
i32.add
local.get 0
i32.store8
local.get 0
i32.const 255
i32.and
i32.const 10
i32.ne
i32.const 0
local.get 1
i32.const 119
i32.ne
select
i32.eqz
if ;; label = @2
i32.const 65668
local.get 2
i32.store
i32.const 1
i32.const 65664
i32.const 1
i32.const 65856
call $runtime.fd_write
drop
i32.const 65716
i32.const 0
i32.store
end
return
end
i32.const 65581
i32.const 18
call $runtime.runtimePanicAt
unreachable)Call graph:
flowchart TD
A[runtime.slicePanic] & B[runtime.nilPanic] --> C[runtime.runtimePanicAt]
C --> D[runtime.printstring] & E[runtime.putchar]
D --> E
E --> F[runtime.fd_write]
Rust has the build option panic_immediate_abort to omit any string formatting on panic, for similar reasons of small code size & minimal dependencies. Adding something similar to TinyGo would allow for WASM programs that include the runtime without also requiring a WASI-compliant host.