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8f5c09b37c
Emit function declarations for functions with `#[linkage="extern_weak"]`
Currently, when declaring an extern weak function in Rust, we use the following syntax:
```rust
unsafe extern "C" {
#[linkage = "extern_weak"]
static FOO: Option<unsafe extern "C" fn() -> ()>;
}
```
This allows runtime-checking the extern weak symbol through the Option.
When emitting LLVM-IR, the Rust compiler currently emits this static as an i8, and a pointer that is initialized with the value of the global i8 and represents the nullabilty e.g.
```
`@FOO` = extern_weak global i8
`@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO`
```
This approach does not work well with CFI, where we need to attach CFI metadata to a concrete function declaration, which was pointed out in https://github.com/rust-lang/rust/issues/115199.
This change switches to emitting a proper function declaration instead of a global i8. This allows CFI to work for extern_weak functions. Example:
```
`@_rust_extern_with_linkage_FOO` = internal global ptr `@FOO`
...
declare !type !61 !type !62 !type !63 !type !64 extern_weak void `@FOO(double)` unnamed_addr #6
```
We keep initializing the Rust internal symbol with the function declaration, which preserves the correct behavior for runtime checking the Option.
r? `@rcvalle`
cc `@jakos-sec`
try-job: test-various
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|---|---|---|
| .. | ||
| add-canonical-jump-tables-flag.rs | ||
| add-cfi-normalize-integers-flag.rs | ||
| add-enable-split-lto-unit-flag.rs | ||
| dbg-location-on-cfi-blocks.rs | ||
| emit-type-checks-attr-no-sanitize.rs | ||
| emit-type-checks.rs | ||
| emit-type-metadata-attr-cfi-encoding.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-const-generics.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-drop-in-place.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-function-types.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-lifetimes.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-method-secondary-typeid.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-paths.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-pointer-types.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-primitive-types.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-repr-transparent-types.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-sequence-types.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-trait-types.rs | ||
| emit-type-metadata-id-itanium-cxx-abi-user-defined-types.rs | ||
| emit-type-metadata-itanium-cxx-abi-generalized.rs | ||
| emit-type-metadata-itanium-cxx-abi-normalized-generalized.rs | ||
| emit-type-metadata-itanium-cxx-abi-normalized.rs | ||
| emit-type-metadata-itanium-cxx-abi.rs | ||
| emit-type-metadata-trait-objects.rs | ||
| external_weak_symbols.rs | ||
| generalize-pointers.rs | ||
| normalize-integers.rs | ||