| Issue |
139630
|
| Summary |
[WebAssembly][Clang] Add intrinsics to make and test for an externref sentinel
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
hoodmane
|
#139580 adds the ability to test whether an `externref` is null or not. This can
be used for in-band error signalling, but it can lead to trouble because some
_javascript_ functions care about the difference between null and undefined, and
if we use null to signal errors we can't handle a real null value. There are
several possible solutions:
1. Change my functions that return an `__externref_t` to `__externref_t f(int
*status, <other args>);` (This doesn't require any change to clang.)
2. Somehow let clang deal with functions with multiple return values and give it
a pair of return values an `externref` and a status `i32`
3. Similarly to 2, make the return value a single wasm-gc struct that's a pair
4. Use a struct with no fields as a sentinel
This fourth case could look like:
```C
if (error_condition) {
return __builtin_wasm_ref_sentinel_extern();
}
```
and then check like:
```C
__externref_t res = some_func();
if (__builtin_wasm_ref_is_sentinel_extern(res)) {
// handle the error
}
// Otherwise it represents an actual JS value.
```
We would have a type like
```wat
(type $sentinel (struct)) ;; 0x5f, 0x00
```
Then `__builtin_wasm_ref_sentinel_extern()` would translate to:
```wat
struct.new $sentinel ;; 0xfb, 0x00, <type index of $sentinel>
extern.convert_any ;; 0xfb, 27
```
and `__builtin_wasm_ref_is_sentinel_extern()` translates to:
```wat
any.convert_extern ;; 0xfb, 26
ref.test $sentinel ;; 0xfb, 20, <type index of $sentinel>
```
It's also worth noting that if we needed to, we could make a second one-value type with:
```
(type $sentinel1 (struct (ref null noextern)))
```
and similarly by adding more `(ref null noextern)` fields we could make as many distinct
struct-based singleton "sentinel" types as we want.
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