Note: this post is also cross-posted to the Bytecode Alliance blog here.

This is a blog post outlining the odyssey I recently took to implement the Wasm exception-handling proposal in Wasmtime, the open-source WebAssembly engine for which I’m a core team member/maintainer, and its Cranelift compiler backend.

When first discussing this work, I made an off-the-cuff estimate in the Wasmtime biweekly project meeting that it would be “maybe two weeks on the compiler side and a week in Wasmtime”. Reader, I need to make a confession now: I was wrong and it was not a three-week task. This work spanned from late March to August of this year (roughly half-time, to be fair; I wear many hats). Let that be a lesson!1

In this post we’ll first cover what exceptions are and why some languages want them (and what other languages do instead) – in particular what the big deal is about (so-called) “zero-cost” exception handling. Then we’ll see how Wasm has specified a bytecode-level foundation that serves as a least-common denominator but also has some unique properties. We’ll then take a roundtrip through what it means for a compiler to support exceptions – the control-flow implications, how one reifies the communication with the unwinder, how all this intersects with the ABI, etc. – before finally looking at how Wasmtime puts it all together (and is careful to avoid performance pitfalls and stay true to the intended performance of the spec).

Why Exceptions?

Many readers will already be familiar with exceptions as they are present in languages as widely varied as Python, Java, JavaScript, C++, Lisp, OCaml, and many more. But let’s briefly review so we can (i) be precise what we mean by an exception, and (ii) discuss why exceptions are so popular.

Exception handling is a mechanism for nonlocal flow control. In particular, most flow-control constructs are intraprocedural (send control to other code in the current function) and lexical (target a location that can be known statically). For example, if statements and loops both work this way: they stay within the local function, and we know exactly where they will transfer control. In contrast, exceptions are (or can be) interprocedural (can transfer control to some point in some other function) and dynamic (target a location that depends on runtime state).2

To unpack that a bit: an exception is thrown when we want to signal an error or some other condition that requires “unwinding” the current computation, i.e., backing out of the current context; and it is caught by a “handler” that is interested in the particular kind of exception and is currently “active” (waiting to catch that exception). That handler can be in the current function, or in any function that has called it. Thus, an exception throw and catch can result in an abnormal, early return from a function.

One can understand the need for this mechanism by considering how programs can handle errors. In some languages, such as Rust, it is common to see function signatures of the form fn foo(...) -> Result<T, E>. The Result type indicates that foo normally returns a value of type T, but may produce an error of type E instead. The key to making this ergonomic is providing some way to “short-circuit” execution if an error is returned, propagating that error upward: that is, Rust’s ? operator, for example, which turns into essentially “if there was an error, return that error from this function”.3 This is quite conceptually nice in many ways: why should error handling be different than any other data flow in the program? Let’s describe the type of results to include the possibility of errors; and let’s use normal control flow to handle them. So we can write code like

fn f() -> Result<u32, Error> {
if bad {
return Err(Error::new(...));
}
Ok(0)
}

fn g() -> Result<u32, Error> {
let result = f()?;
Ok(result + 1)
}

and we don’t have to do anything special in g to propagate errors from f further, other than use the ? operator.

But there is a cost to this: it means that every error-producing function has a larger return type, which might have ABI implications (another return register at least, if not a stack-allocated representation of the Result and the corresponding loads/stores to memory), and also, there is at least one conditional branch after every call to such a function that checks if we need to handle the error. The dynamic efficiency of the “happy path” (with no thrown exceptions) is thus impacted. Ideally, we skip any cost unless an error actually occurs (and then perhaps we accept slightly more cost in that case, as tradeoffs often go).

It turns out that this is possible with the help of the language runtime. Consider what happens if we omit the Result return types and error checks at each return. We will need to reach the code that handles the error in some other way. Perhaps we can jump directly to this code somehow?

The key idea of “zero-cost exception handling” is to get the compiler to build side-tables to tell us where this code – known as a “handler” – is. We can walk the callstack, visiting our caller and its caller and onward, until we find a function that would be interested in the error condition we are raising. This logic is implemented with the help of these side-tables and some code in the language runtime called the “unwinder” (because it “unwinds” the stack). If no errors are raised, then none of this logic is executed at runtime. And we no longer have our explicit checks for error returns in the “happy path” where no errors occur. This is why the common term for this style of error-handling is called “zero-cost”: more precisely, it is zero-cost when no errors occur, but the unwinding in case of error can still be expensive.

This is the status quo for exception-handling implementations in most production languages: for example, in the C++ world, exception handling is commonly implemented via the Itanium C++ ABI4, which defines a comprehensive set of tables emitted by the compiler and a complex dance between the system unwinding library and compiler-generated code to find and transfer control to handlers. Handler tables and stack unwinders are common in interpreted and just-in-time (JIT)-compiled language implementations, too: for example, SpiderMonkey has try notes on its bytecode (so named for “try blocks”) and a HandleException function that walks stack frames to find a handler.

The Wasm Exception-Handling Spec

The WebAssembly specification now (since version 3.0) has exception handling. This proposal was a long time in the making by various folks in the standards, toolchain and browser worlds, and the CG (standards group) has now merged it into the spec and included it in the recently-released “Wasm 3.0” milestone. If you’re already familiar with the proposal, you can skip over this section to the Cranelift- and Wasmtime-specific bits below.

First: let’s discuss why Wasm needs an extension to the bytecode definition to support exceptions. As we described above, the key idea of zero-cost exception handling is that an unwinder visits stack frames and looks for handlers, transferring control directly to the first handler it finds, outside the normal function return path. Because the call stack is protected, or not directly readable or writable from Wasm code (part of Wasm’s control-flow integrity aspect), an unwinder that works this way necessarily must be a privileged part of the Wasm runtime itself. We can’t implement it in “userspace” because there is no way for Wasm bytecode to transfer control directly back to a distant caller, aside from a chain of returns. This missing functionality is what the extension to the specification adds.

The implementation comes down to only three opcodes (!), and some new types in the bytecode-level type system. (In other words – given the length of this post – it’s deceptively simple.) These opcodes are:

try_table, which wraps an inner body, and specifies handlers to be active during that body. For example:

(block $b1    ;; defines a label for a forward edge to the end of this block
(block $b2  ;; likewise, another label
(try_table
(catch $tag1 $b1) ;; exceptions with tag `$tag1` will be caught by code at $b1
(catch_all $b2)   ;; all other exceptions will be caught by code at $b2

body...)))

In this example, if an exception is thrown from within the code in body, and it matches one of the specified tags (more below!), control will transfer to the location defined by the end of the given block. (This is the same as other control-flow transfers in Wasm: for example, a branch br $b1 also jumps to the end of $b1.)

This construct is the single all-purpose “catch” mechanism, and is powerful enough to directly translate typical try/catch blocks in most programming languages with exceptions.

throw: an instruction to directly throw a new exception. It carries the tag for the exception, like: throw $tag1.

throw_ref, used to rethrow an exception that has already been caught and is held by reference (more below!).

And that’s it! We implement those three opcodes and we are “done”.

Payloads

That’s not the whole story, of course. Ordinarily a source language will offer the ability to carry some data as part of an exception: that is, the error condition is not just one of a static set of kinds of errors, but contains some fields as well. (E.g.: not just “file not found”, but “file not found: $PATH”.)

One could build this on top of an bytecode-level exception-throw mechanism that only had throw/catch with static tags, with the help of some global state, but that would be cumbersome; instead, the Wasm specification offers payloads on each exception. For full generality, this payload can actually take the form of a list of values; i.e., it is a full product type (struct type).

We alluded to “tags” above but didn’t describe them in detail. These tags are key to the payload definition: each tag is effectively a type definition that specifies its list of payload value types as well. (Technically, in the Wasm AST, a tag definition names a function type with only parameters, no returns, which is a nice way of reusing an existing entity/concept.) Now we show how they are defined with a sample module:

(module
;; Define a "tag", which serves to define the specific kind of exception
;; and specify its payload values.
(tag $t (param i32 i64))

(func $f (param i32 i64)
;; Throw an exception, to be caught by whatever handler is "closest"
;; dynamically.
(throw $t (local.get 0) (local.get 1)))

(func $g (result i32 i64)
(block $b (result i32 i64)
;; Run a body below, with the given handlers (catch-clauses)
;; in-scope to catch any matching exceptions.
;;
;; Here, if an exception with tag `$t` is thrown within the body,
;; control is transferred to the end of block `$b` (as if we had
;; branched to it), with the payload values for that exception
;; pushed to the operand stack.
(try_table (catch $t $b)
(call $f (i32.const 1) (i64.const 2)))
(i32.const 3)
(i64.const 4))))

Here we’ve defined one tag (the Wasm text format lets us attach a name $t, but in the binary format it is only identified by its index, 0), with two payload values. We can throw an exception with this tag given values of these types (as in function $f) and we can catch it if we specify a catch destination as the end of a block meant to return exactly those types as well. Here, if function $g is invoked, the exception payload values 1 and 2 will be thrown with the exception, which will be caught by the try_table; the results of $g will be 1 and 2. (The values 3 and 4 are present to allow the Wasm module to validate, i.e. have correct types, but they are dynamically unreachable because of the throw in $f and will not be returned.)

This is an instance where Wasm, being a bytecode, can afford to generalize a bit relative to real-metal ISAs and offer conveniences to the Wasm producer (i.e., toolchain generating Wasm modules). In this sense, it is a little more like a compiler IR. In contrast, most other exception-throw ABIs have a fixed definition of payload, e.g., one or two machine register-sized values. In practice some producers might choose a small fixed signature for all exception tags anyway, but there is no reason to impose such an artificial limit if there is a compiler and runtime behind the Wasm in any case.

Unwind, Cleanup, and Destructors

So far, we’ve seen how Wasm’s primitives can allow for basic exception throws and catches, but what about languages with scoped resources, e.g. C++ with its destructors? If one writes something like

struct Scoped {
Scoped() {}
~Scoped() { cleanup(); }
};

void f() {
Scoped s();
throw my_exception();
}

then the throw should transfer control out of f and upward to whatever handler matches, but the destructor of s still needs to run and call cleanup. This is not quite a “catch” because we don’t want to terminate the search: we aren’t actually handling the error condition.

The usual approach to compile such a program is to “catch and rethrow”. That is, the program is lowered to something like

try {
throw ...
} catch_any(e) {
cleanup();
rethrow e;
}

where catch_any catches any exception propagating past this point on the stack, and rethrow re-throws the same exception.

Wasm’s exception primitives provide exactly the pieces we need for this: a catch_all_ref clause, which catches all exceptions and boxes the caught exception as a reference; and a throw_ref instruction, which re-throws a previously-caught exception.5

In actuality there is a two-by-two matrix of “catch” options: we can catch a specific tag or catch_all; and we can catch and immediately unpack the exception into its payload values (as we saw above), or we can catch it as a reference. So we have catch, catch_ref, catch_all, and catch_all_ref.6

Dynamic Identity and Compositionality

There is one final detail to the Wasm proposal, and in fact it’s the part that I find the most interesting and unique. Given the above introduction, and any familiarity with exception systems in other language semantics and/or runtime systems, one might expect that the “tags” identifying kinds of exceptions and matching throws with particular catch handlers would be static labels. In other words, if I throw an exception with tag $tA, then the first handler for $tA anywhere up the stack, from any module, should catch it.

However, one of Wasm’s most significant properties as a bytecode is its emphasis on isolation. It has a distinction between static modules and dynamic instances of those modules, and modules have no “static members”: every entity (e.g., memory, table, or global variable) defined by a module is replicated per instance of that module. This creates a clean separation between instances and means that, for example, one can freely reuse a common module (say, some kind of low-level glue or helper module) with separate instances in many places without them somehow communicating or interfering with each other.

Consider what happens if we have an instance A that invokes some other (dynamically provided) function reference which ultimately invokes a callback in A. Say that the instance throws an exception from within its callback in order to unwind all the way to its outer stack frames, across the intermediate functions in some other Wasm instance(s):

A.f   ---------call--------->   B.g   --------call--------->    A.callback
^                                                                  v
catch $t                                                           throw $t
|                                                                  |
`----------------------------<-------------------------------------'

The instance A expects that the exception that it throws from its callback function to f is a local concern to that instance only, and that B cannot interfere. After all, if the exception tag is defined inside A, and Wasm preserves modularity, then B should not be able to name that tag to catch exceptions by that tag, even if it also uses exception handling internally. The two modules should not interact: that is the meaning of modularity, and it permits us to reason about each instance’s behavior locally, with the effects of “the rest of the world” confined to imports and exports.

Unfortunately, if one designed a straightforward “static” tag-matching scheme, this might not be the case if B were an instance of the same module as A: in that case, if B also used a tag $t internally and registered handlers for that tag, it could interfere with the desired throw/catch behavior, and violate modularity.

So the Wasm exception handling standard specifies that tags have dynamic instances as well, just as memories, tables and globals do. (Put in programming-language theory terms, tags are generative.) Each instance of a module creates its own dynamic identities for the statically-defined tags in those modules, and uses those dynamic identities to tag exceptions and find handlers. This means that no matter what instance B is, above, if instance A does not export its tag $t for B to import, there is no way for B to catch the thrown exception explicitly (it can still catch all exceptions, and it may do so and rethrow to perform some cleanup). Local modular reasoning is restored.

Once we have tags as dynamic entities, just like Wasm memories, we can take the same approach that we do for the other entities to allow them to be imported and exported. Thus, visibility of exception payloads and ability for modules to catch certain exceptions is completely controlled by the instantiation graph and the import/export linking, just as for all other Wasm storage.

This is surprising (or at least was to me)! It creates some pretty unique implementation challenges in the unwinder – in essence, it means that we need to know about instance identity for each stack frame, not just static code location and handler list.

Compiling Exceptions in Cranelift

Before we implement the primitives for exception handling in Wasmtime, we need to support exceptions in our underlying compiler backend, Cranelift.

Why should this be a compiler concern? What is special about exceptions that makes them different from, say, new Wasm instructions that implement additional mathematical operators (when we already have many arithmetic operators in the IR), or Wasm memories (when we already have loads/stores in the IR)?

In brief, the complexities come in three flavors: new kinds of control flow, fundamentally different than ordinary branches or calls in that they are “externally actuated” (by the unwinder); a new facet of the ABI (that we get to define!) that governs how the unwinder interacts with compiled code; and interactions between the “scoped” nature of handlers and inlining in particular. We’ll talk about each below.

Note that much of this discussion started with an RFC for Wasmtime/Cranelift, which had been posted way back in August of 2024 by Daniel Hillerstrom with help from my colleague Nick Fitzgerald, and was discussed then; many of the choices within were subsequently refined as I discovered interesting nuances during implementation and we talked them through.

Control Flow

There are a few ways to think about exception handlers from the point of view of compiler IR (intermediate representation). First, let’s recognize that exception handling (i) is a form of control flow, and (ii) has all the same implications various compiler stages that other kinds of control flow do. For example, the register allocator has to consider how to get registers into the right state whenever control moves from one basic block to the next (“edge moves”); exception catches are a new kind of edge, and so the regalloc needs to be aware of that, too.

One could see every call or other opcode that could throw as having regular control-flow edges to every possible handler that could match. I’ll call this the “regular edges” approach. The upside is that it’s pretty simple to retrofit: one “only” needs to add new kinds of control-flow opcodes that have out-edges, but that’s already a kind of thing that IRs have. The disadvantage is that, in functions with a lot of possible throwing opcodes and/or handlers, the overhead can get quite high. And control-flow graph overhead is a bad kind of overhead: many analyses’ runtimes are heavily dependent the edge and node (basic block) counts, sometimes superlinearly.

The other major option is to build a kind of implicit new control flow into the IR’s semantics. For example, one could lower the source-language semantics of a “try block” down to regions in the IR, with one set of handlers attached. This is clearly more efficient than adding out-edges from (say) every callsite within the try-block to every handler in scope. On the other hand, it’s hard to understate how invasive this change would be. This means that every traversal over IR, analyzing dataflow or reachability or any other property, has to consider these new implicit edges anyway. In a large established compiler like Cranelift, we can lean on Rust’s type system for a lot of different kinds of refactors, but changing a fundamental invariant goes beyond that: we would likely have a long tail of issues stemming from such a change, and it would permanently increase the cognitive overhead of making new changes to the compiler. In general we want to trend toward a smaller, simpler core and compositional rather than entangled complexity.

Thus, the choice is clear: in Cranelift we opted to introduce one new instruction, try_call, that calls a function and catches (some) exceptions. In other words, there are now two possible kinds of return paths: a normal return or (possibly one of many) exceptional return(s). The handled exceptions and block targets are enumerated in an exception table. Because there are control-flow edges stemming from this opcode, it is a block terminator, like a conditional branch. It looks something like (in Cranelift’s IR, CLIF):

function %f0(i32) -> i32, f32, f64 {
sig0 = (i32) -> f32 tail
fn0 = %g(i32) -> f32 tail

block0(v1: i32):
v2 = f64const 0x1.0
;; exception-catching callsite
try_call fn0(v1), sig0, block1(ret0, v2), [ tag0: block2(exn0), default: block3(exn0) ]

;; normal return path
block1(v3: f32, v4: f64):
v5 = iconst.i32 1
return v5, v3, v4

;; exception handler for tag0
block2(v6: i64):
v7 = ireduce.i32 v6
v8 = iadd_imm.i32 v7, 1
v9 = f32const 0x0.0
return v8, v9, v2

;; exception handler for all other exceptions
block2(v10: i64):
v11 = ireduce.i32 v10
v12 = f32.const 0x0.0
v13 = f64.const 0x0.0
return v11, v12, v13
}

There are a few aspects to note here. First, why are we only concerned with calls? What about other sources of exceptions? This is an important invariant in the IR: exception throws are only externally sourced. In other words, if an exception has been thrown, if we go deep enough into the callstack, we will find that that throw was implemented by calling out into the runtime. The IR itself has no other opcodes that throw! This turns out to be sufficient: (i) we only need to build what Wasmtime needs, here, and (ii) we can implement Wasm’s throw opcodes as “libcalls”, or calls into the Wasmtime runtime. So, within Cranelift-compiled code, exception throws always happen at callsites. We can thus get away with adding only one opcode, try_call, and attach handler information directly to that opcode.

The next characteristic of note is that handlers are ordinary basic blocks. Thus may not seem remarkable unless one has seen other compiler IRs, such as LLVM’s, where exception handlers are definitely special: they start with “landing pad” instructions, and cannot be branched to as ordinary basic blocks. That might look something like:

function %f() {
block0:
;; Callsite defining a return value `v0`, with normal
;; return path to `block1` and exception handler `block2`.
v0 = try_call ..., block1, [ tag0: block2 ]

block1:
;; Normal return; use returned value.
return v0

block2 exn_handler: ;; Specially-marked block!
;; Exception handler payload value.
v1 = exception_landing_pad
...
}

This bifurcation of kinds of blocks (normal and exception handler) is undesirable from our point of view: just as exceptional edges add a new cross-cutting concern that every analysis and transform needs to consider, so would new kinds of blocks with restrictions. It was an explicit design goal (and we have tests that show!) that the same block can be both an ordinary block and a handler block – not because that would be common, necessarily (handlers usually do very different things than normal code paths), but because it’s one less weird quirk of the IR.

But then if handlers are normal blocks, the data flow question becomes very interesting. An exception-catching call, unlike every other opcode in our IR, has conditionally-defined values: that is, its normal function return value(s) are available only if the callee returns normally, and the exception payload value(s), which are passed in from the unwinder and carry information about the caught exception, are available only if the callee throws an exception that we catch. How can we ensure that these values are represented such that they can only be used in valid ways? We can’t make them all regular SSA definitions of the opcode: that would mean that all successors (regular return and exceptional) get to use them, as in:

function %f() {
block0:
;; Callsite defining a return value `v0`, with normal return path
;; to `block1` and exception handler `block2`.
v0 = try_call ..., block1, [ tag0: block2 ]

block1:
;; Use `v0` legally: it is defined on normal return.
return v0

block2:
;; Oops! We use `v0` here, but the normal return value is undefined
;; when an exception is caught and control reaches this handler block.
return v0
}

This is the reason that a compiler may choose to make handler blocks special: by bifurcating the universe of blocks, one ensures that normal-return and exceptional-return values are used only where appropriate. Some compiler IRs reify exceptional return payloads via “landing pad” instructions that must start handler blocks, just as phis start regular blocks (in phi- rather than blockparam-based SSA). But, again, this bifurcation is undesirable.

Our insight here, after a lot of discussion, was to put the definitions where they belong: on the edges. That is, regular returns are only defined once we know we’re following the regular-return edge, and likewise for exception payloads. But we don’t want to have special instructions that must be in the successor blocks: that’s a weird distributed invariant and, again, likely to lead to bugs when transforming IR. Instead, we leverage the fact that we use blockparam-based SSA and we widen the domain of allowable block-call arguments.

Whereas previously one might end a block like brif v1, block2(v2, v3), block3(v4, v5), i.e. with blockparams assigned values in the chosen successor via a list of value-uses in the branch, we now allow (i) SSA values, (ii) a special “normal return value” sentinel, or (iii) a special “exceptional return value” sentinel. The latter two are indexed because there can be more than one of each. So one can write a block-call in a try_call as block2(ret0, v1, ret1), which passes the two return values of the call and a normal SSA value; or block3(exn0, exn1), which passes just the two exception payload values. We do have a new well-formedness check on the IR that ensures that (i) normal returns are used only in the normal-return blockcall, and exception payloads are used only in the handler-table blockcalls; (ii) normal returns’ indices are bounded by the signature; and (iii) exception payloads’ indices are bounded by the ABI’s number of exception payload values; but all of these checks are local to the instruction, not distributed across blocks. That’s nice, and conforms with the way that all of our other instructions work, too. (Block-call argument types are then checked against block-parameter types in the successor block, but that happens the same as for any branch.) So we have, repeating from above, a callsite like

block1:
try_call fn0(v1), block2(ret0), [ tag0: block3(exn0, exn1) ]

with all of the desired properties: only one kind of block, explicit control flow, and SSA values defined only where they are legal to use.

All of this may seem somewhat obvious in hindsight, but as attested by the above GitHub discussions and Cranelift weekly meeting minutes, it was far from clear when we started how to design all of this to maximize simplicity and generality and minimize quirks and footguns. I’m pretty happy with our final design: it feels like a natural extension of our core blockparam-SSA control flow graph, and I managed to put it into the compiler without too much trouble at all (well, a few PRs and associated fixes to Cranelift and regalloc2 functionality and testing; and I’m sure I’ve missed a few).

Data Flow and ABI

So we have defined an IR that can express exception handlers – what about the interaction between this function body and the unwinder? We will need to define a different kind of semantics to nail down that interface: in essence, it is a property of the ABI (Application Binary Interface).

As mentioned above, existing exception-handling ABIs exist for native code, such as compiled C++. While we are certainly willing to draw inspiration from native ABIs and align with them as much as makes sense, in Wasmtime we already define our own ABI7, and so we are not necessarily constrained by existing standards.

In particular, there is a very good reason we would prefer not to: to unwind to a particular exception handler, register state must be restored as specified in the ABI, and the standard Itanium ABI requires the usual callee-saved (“non-volatile”) registers on the target ISA to be restored. But this requires (i) having the register state at time of throw, and (ii) processing unwind metadata at each stack frame as we walk up the stack, reading out values of saved registers from stack frames. The latter is already supported with a generic “unwind pseudoinstruction” framework I built four years ago, but would still add complexity to our unwinder, and this complexity would be load-bearing for correctness; and the former is extremely difficult with Wasmtime’s normal runtime-entry trampolines. So we instead choose to have a simpler exception ABI: all try_calls, that is, callsites with handlers, clobber all registers. This means that the compiler’s ordinary register-allocation behavior will save all live values to the stack and restore them on either a normal or exceptional return. We only have to restore the stack (stack pointer and frame pointer registers) and redirect the program counter (PC) to a handler.

The other aspect of the ABI that matters to the exception-throw unwinder is exceptional payload. The native Itanium ABI specifies two registers on most platforms (e.g.: rax and rdx on x86-64, or x0 and x1 on aarch64) to carry runtime-defined playload; so for simplicity, we adopt the same convention.

That’s all well and good; now how do we implement try_call with the appropriate register-allocator behavior to conform to this? We already have fairly complex ABI handling (machine-independent and five different architecture implementations) in Cranelift, but it follows a general pattern: we generate a single instruction at the register-allocator level, and emit uses and defs with fixed-register constraints. That is, we tell regalloc that parameters must be in certain registers (e.g., rdi, rsi, rcx, rdx, r8, r9 on x86-64 System-V calling-convention platforms, or x0 up to x7 on aarch64 platforms) and let it handle any necessary moves. So in the simplest case, a call might look like (on aarch64), with register-allocator uses/defs and constraints annotated:

bl (call) v0 [def, fixed(x0)], v1 [use, fixed(x0)], v2 [use, fixed(x1)]

It is not always this simple, however: calls are not actually always a single instruction, and this turned out to be quite problematic for exception-handling support. In particular, when values are returned in memory, as the ABI specifies they must be when there are more return values than registers, we add (or added, prior to this work!) load instructions after the call to load the extra results from their locations on the stack. So a callsite might generate instructions like

bl v0 [def, fixed(x0)], ..., v7 [def, fixed(x7)] # first eight return values
ldr v8, [sp]     # ninth return value
ldr v9, [sp, #8] # tenth return value

and so on. This is problematic simply because we said that the try_call was a terminator; and it is at the IR level, but no longer at the regalloc level, and regalloc expects correctly-formed control-flow graphs as well. So I had to do a refactor to merge these return-value loads into a single regalloc-level pseudoinstruction, and in turn this cascaded into a few regalloc fixes (allowing more than 256 operands and more aggressively splitting live-ranges to allow worst-case allocation, plus a fix to the live range-splitting fix and a fuzzing improvement).

There is one final question that might arise when considering the interaction of exception handling and register allocation in Cranelift-compiled code. In Cranelift, we have an invariant that the register allocator is allowed to insert moves between any two instructions – register-to-register, or loads or stores to/from spill-slots in the stack frame, or moves between different spill-slots – and indeed it does this whenever there is more state than fits in registers. It also needs to insert edge moves “between” blocks, because when jumping to another spot in the code, we might need the register values in a differently-assigned configuration. When we have an unwinder that jumps to a different spot in the code to invoke a handler, we need to ensure that all the proper moves have executed so the state is as expected.

The answer here turns out to be a careful argument that we don’t need to do anything at all. (That’s the best kind of solution to a problem, but only if one is correct!) The crux of the argument has to do with critical edges. A critical edge is one from a block with multiple successors to one with multiple predecessors: for example, in the graph

A    D
/ \  /
B   C

where A can jump to B or C, and D can also jump to C, then A-to-C is a critical edge. The problem with critical edges is that there is nowhere to put code that has to run on the transition from A to C (it can’t go in A, because we may go to B or C; and it can’t go in C, because we may have come from A or D). So the register allocator prohibits them, and we “split” them when generating code by inserting empty blocks (e below) on them:

A    D
/ \   |
|   e  |
|   \ /
B    C

The key insight is that a try_call always has more than one successor as long as it has a handler (because it must always have a normal return-path successor too)8; and in this case, because we split critical edges, the immediate successor block on the exception-catch path has only one predecessor. So the register allocator can always put its moves that have to run on catching an exception in the successor (handler) block rather than the predecessor block. Our rule for where to put edge moves prefers the successor (block “after” the edge) unless it has multiple in-edges, so this was already the case. The only thing we have to be careful about is to record the address of the inserted edge block, if any (e above), rather than the IR-level handler block (C above), in the handler table.

And that’s pretty much it, as far as register allocation is concerned!

We’ve now covered the basics of Cranelift’s exception support. At this point, having landed the compiler half but not the Wasmtime half, I context-switched away for a bit, and in the meantime, bjorn3 picked this support up right away as a means to add panic-unwinding support to rustc_codegen_cranelift, the Cranelift-based Rust compiler backend. With a few small changes they contributed, and a followup edge-case fix and a refactor, panic-unwinding support in rustc_codegen_cranelift was working. That was very good intermediate validation that what I had built was usable and relatively solid.

Exceptions in Wasmtime

We have a compiler that supports exceptions; we understand Wasm exception semantics; let’s build support into Wasmtime! How hard could it be?

Challenge 1: Garbage Collection Interactions

I started by sketching out the codegen for each of the three opcodes (try_table, throw, and throw_ref). My mental model at the very beginning of this work, having read but not fully internalized the Wasm exception-handling proposal, was that I would be able to implement a “basic” throw/catch first, and then somehow build the exnref objects later. And I had figured I could build exnrefs in a (in hindsight) somewhat hacky way, by aggregating values together in a kind of tuple and creating a table of such tuples indexed by exnrefs, just as Wasmtime does for externrefs.

This understanding quickly gave way to a deeper one when I realized a few things:

Exception objects (exnrefs) can carry references to other GC objects (that is, GC types can be part of the payload signature of an exception), and GC objects can store exnrefs in fields. Hence, exnrefs need to be traced, and can participate in GC cycles; this either implies an additional collector on top of our GC collector (ugh) or means that exception objects needs to be on the GC heap when GC is enabled.

We’ll need a host API to introspect and build exception objects, and we already have nice host APIs for GC objects.

There was a question in an extensively-discussed PR whether we could build a cheap “subset” implementation that doesn’t mandate the existence of a GC heap for storing exception objects. This would be great in theory for guests that use exceptions for C-level setjmp/longjmp but no other GC features. However, it’s a little tricky for a few reasons. First, this would require the subset to exclude throw_ref (so we don’t have to invent another kind of exception object storage). But it’s not great to subset the spec – and throw_ref is not just for GC guest languages, but also for rethrows. Second, more generally, this is additional maintenance and testing surface that we’d rather not have for now. Instead we expect that we can make GC cheap enough, and its growth heuristic smart enough that a “frequent setjmp/longjmp” stress-test of exceptions (for example) should live within a very small (e.g., few-kilobyte) GC heap, essentially approximating the purpose-built storage. My colleague Nick Fitzgerald (who built and is driving improvements to Wasmtime’s GC support) wrote up a nice issue describing the tradeoffs and ideas we have.

All of that said, we’ll only build one exception object implementation – great! – but it will have to be a new kind of GC object. This spawned a large PR to build out exception objects first, prior to actual support for throwing and catching them, with host APIs to allocate them and inspect their fields. In essence, they are structs with immutable fields and with a less-exposed type lattice and no subtyping.

Challenge 2: Generative Tags and Dynamic Identity

So there I was, implementing the throw instruction’s libcall (runtime implementation), and finally getting to the heart of the matter: the unwinder itself, which walks stack frames to find a matching exception handler. This is the final bit of functionality that ties it all together. We’re almost there!

But wait: check out that spec language. We load the “tag address” from the store in step 9: we allocate the exception instance {tag z.tags[x], fields val^n}. What is this tags array on the store (z) in the runtime semantics? Tags have dynamic identity, not static identity! (This is the part where I learned about the thing I described above.)

This was a problem, because I had defined exception tables to associate handlers with tags that were identified by integer (u32) – like most other entities in Cranelift IR, I had figured this would be sufficient to let Wasmtime define indices (say: index of the tag in the module), and then we could compare static tag IDs.

Perhaps this is no problem: the static index defines the entity ID in the module (defined or imported tag), and we can compare that and the instance ID to see if a handler is a match. But how do we get the instance ID from the stack frame?

It turns out that Wasmtime didn’t have a way, because nothing had needed that yet. (This deficiency had been noticed before when implementing Wasm coredumps, but there hadn’t been enough reason or motivation to fix it then.) So I filed an issue with a few ideas. We could add a new field in every frame storing the instance pointer – and in fact this is a simple version of what at least one other production Wasm implementation, in the SpiderMonkey web engine, does (though as described in that [SMDOC] comment, it only stores instance pointers on transitions between frames of different instances; this is enough for the unwinder when walking linearly up the stack). But that would add overhead to every Wasm function (or with SpiderMonkey’s approach, require adding trampolines between instances, which would be a large change for Wasmtime), and exception handling is still used somewhat rarely in practice. Ideally we’d have a “pay-as-you-go” scheme with as little extra complexity as posible.

Instead, I came up with an idea to add “dynamic context” items to exception handler lists. The idea is that we inject an SSA value into the list and it is stored in a stack location that is given in the handler table metadata, so the stack-walker can find it. To Cranelift, this is some arbitrary opaque value; Wasmtime will use it to store the raw instance pointer (vmctx) for use by the unwinder.

This filled out the design to a more general state nicely: it is symmetric with exception payload, in the sense that the compiled code can communicate context or state to the unwinder as it reads the frames, and the unwinder in turn can communicate data to the compiled code when it unwinds.

It turns out – though I didn’t intend this at all at the time – that this also nicely solves the inlining problem. In brief, we want all of our IR to be “local”, not treating the function boundary specially; this way, IR can be composed by the inliner without anything breaking. Storing some “current instance” state for the whole function will, of course, break when we inline a function from one module (hence instance) into another!

Instead, we can give a nice operational semantics to handler tables with dynamic-context items: the unwinder should read left-to-right, updating its “current dynamic context” at each dynamic-context item, and checking for a tag match at tag-handler items. Then the inliner can compose exception tables: when a try_call callsite inlines a function body as its callee, and that body itself has any other callsites, we attach a handler table that simply concatenates the exception table items.

It’s important, here, to point out another surprising fact about Wasm semantics: we cannot do certain optimizations to resolve handlers statically or optimize the handler list, or at least not naively, without global program analysis to understand where tags come from. For example, if we see a handler for tag 0 then one for tag 1, and we see a throw for tag 1 directly inside the try_tables body, we cannot necessarily resolve it: tag 0 and tag 1 could be the same tag!

Wait, how can that be? Well, consider tag imports:

(module
(import "test" "e0" (tag $e0))
(import "test" "e1" (tag $e1))

(func ...
(try_table
(catch $e0 $b0)
(catch $e1 $b1)
(throw $e1)
(unreachable))))

We could instantiate this module giving the same dynamic tag instance twice, for both imports; then the first handler (to block $b0) matches; or separate tags; then the block $b1 matches. The only way to win the optimization game is not to play – we have to preserve the original handler list. Fortunately, that makes the compiler’s job easier. We transcribe the try_table’s handlers directly to Cranelift exception-handler tables, and those directly to metadata in the compiled module, read in exactly that order by the unwinder’s handler-matching logic.

Challenge 3: Rooting

Since exception objects are GC-managed objects, we have to ensure that they are properly rooted: that is, any handles to these objects outside of references inside other GC objects need to be known to the GC so the objects remain alive (and so the references are updated in the case of a moving GC).

Within a Wasm-to-Wasm exception throw scenario, this is fairly easy: the references are rooted in the compiled code on either side of the control-flow transfer, and the reference only briefly passes through the unwinder. As long as we are careful to handle it with the appropriate types, all will work fine.

Passing exceptions across the host/Wasm boundary is another matter, though. We support the full matrix of {host, Wasm} x {host, Wasm} exception catch/throw