| efa236e5 | 05-Jun-2025 |
Chris Fallin <[email protected]> |
Cranelift: implement an "unwinder" crate and exception throws in filetests. (#10919)
This commit introduces the next major piece of machinery (after the
previously-landed `try_call` support) that we
Cranelift: implement an "unwinder" crate and exception throws in filetests. (#10919)
This commit introduces the next major piece of machinery (after the
previously-landed `try_call` support) that we will eventually use to
implement Wasm exceptions in Wasmtime. In particular, it implements a
generic unwinder as a new crate that supports (i) walking a stack
produced by Cranelift code, (ii) serializing Cranelift exception
metadata to compact tables (in a way very similar to address maps in
Wasmtime, so they will be mappable directly from disk), (iii) using
these serialized tables to find handlers during a stack-walk, and (iv)
jumping to handlers (i.e., actually unwinding). This crate is currently
used in the filetests runner, and will next be used in Wasmtime.
The commit first performs code-motion: it moves stack-walking code from
Wasmtime to `cranelift-unwinder`. This itself has no functional effect,
but isolates the code that understands contiguous sequences of Cranelift
frames ("activations") from that which is specific to Wasmtime's
activation delimiters and metadata.
It then implements a compact exception-table format. This format uses
the `object` crate's mechanisms for directly referencing in-memory
arrays of little-endian `u32`s in a way that will allow us to find
handlers when mapping exception metadata directly from an ELF section in
a `.cwasm` (for example). The format consists of four sorted `u32`
arrays in a way that allows us to look up a callsite first, then search
its sorted array of handler offsets by tags.
It next implements the actual unwind control flow: it contains an
assembly stub for each supported architecture that transfers control to
a PC, SP, and FP value "up the stack", with payload values placed in the
payload registers we have defined per our exception ABI in Cranelift.
Finally, it puts these pieces together in the filetest runner. Note that
the runtest does a lot "by hand": we don't have entry and exit
trampolines as we do in Wasmtime, so the filetest contains three
functions, with the middle one invoking the "throw hostcall" and entry
and exit trampolines around it grabbing the appropriate entry/exit FPs
and exit PC. The dance to call back to host code is also somewhat
delicate, as we haven't done this before. The `JITModule`'s linking +
relocation support does not seem sufficient to properly define a symbol,
so instead we scan for `func_addr` instructions referencing a well-known
name (`__cranelift_throw`) and replace them with `iconst`s with the
function address at runtime, baking it in. This is somewhat ugly, but it
works. All of these filetest-specific details will be handled much more
nicely in the Wasmtime version of this functionality, as we have proper
abstractions for entry/exit trampolines and hostcalls.
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