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fix nounwind attribute logic #63909

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fix nounwind attribute logic #63909

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2201adc
fix nounwind attribute logic
RalfJung Aug 26, 2019
edcfc26
remove a useless test
RalfJung Aug 26, 2019
dcf3a47
fix nounwind on extern "C" declarations
RalfJung Aug 27, 2019
47e4193
move a FIXME to a better place
RalfJung Aug 27, 2019
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12 changes: 4 additions & 8 deletions src/librustc_codegen_llvm/attributes.rs
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Expand Up @@ -268,22 +268,18 @@ pub fn from_fn_attrs(
// optimize based on this!
false
} else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::UNWIND) {
// If a specific #[unwind] attribute is present, use that
// If a specific #[unwind] attribute is present, use that.
// FIXME: We currently assume it can unwind even with `#[unwind(aborts)]`.
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true
} else if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::RUSTC_ALLOCATOR_NOUNWIND) {
// Special attribute for allocator functions, which can't unwind
false
} else if let Some(id) = id {
let sig = cx.tcx.normalize_erasing_late_bound_regions(ty::ParamEnv::reveal_all(), &sig);
if cx.tcx.is_foreign_item(id) {
// Foreign items like `extern "C" { fn foo(); }` are assumed not to
if cx.tcx.is_foreign_item(id) && sig.abi != Abi::Rust && sig.abi != Abi::RustCall {
// Foreign non-Rust items like `extern "C" { fn foo(); }` are assumed not to
// unwind
false
} else if sig.abi != Abi::Rust && sig.abi != Abi::RustCall {
// Any items defined in Rust that *don't* have the `extern` ABI are
// defined to not unwind. We insert shims to abort if an unwind
// happens to enforce this.
false
} else {
// Anything else defined in Rust is assumed that it can possibly
// unwind
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26 changes: 24 additions & 2 deletions src/test/codegen/extern-functions.rs
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Expand Up @@ -6,14 +6,36 @@
extern {
// CHECK: Function Attrs: nounwind
// CHECK-NEXT: declare void @extern_fn
fn extern_fn();
// CHECK-NOT: Function Attrs: nounwind
fn extern_fn(); // assumed not to unwind
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This part of the test seems dubious to me. Wouldn't code that bubbles Rust panics through C (the kind of code for which #62603 got merged) also have an extern "C" declaration like this where the panic enters back into Rust?

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We don't have a stable vs nightly difference in semantics. That would be a bad mess.

We document panicking from extern "C" as UB, but (similar to #62825) are willing to patch things such that code exploiting this UB is not broken until we provide adequate alternatives to said code.

So, I think this function should not be unwind. I am waiting for @alexcrichton to confirm.

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So, I think this function should not be unwind

This is right. The check should say CHECK-NOT: nounwind.

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I believe this is correct, all imported FFI functions, by default, should be tagged with nounwind

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But doesn't this mean that when e.g. mozjpeg calls C that calls Rust (the situation due to which #62603 was accepted), and a panic passes from C to Rust, we still violate nounwind?

This PR removes the nounwind on the first edge the panic crosses (Rust -> C), but there's another edge (C -> Rust) and we should not have nounwind there either.

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This PR removes the nounwind on the first edge the panic crosses (Rust -> C), but there's another edge (C -> Rust) and we should not have nounwind there either.

Yes, that's correct, that is, the check should be CHECK-NOT: nounwind. Otherwise, the second example in #63943, which is equivalent to mozjpeg use case, would still be miscompiled.

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Okay, I have implemented that.

// CHECK-NOT: nounwind
// CHECK: declare void @unwinding_extern_fn
#[unwind(allowed)]
fn unwinding_extern_fn();
// CHECK-NOT: nounwind
// CHECK: declare void @aborting_extern_fn
#[unwind(aborts)]
fn aborting_extern_fn(); // FIXME: we don't have the attribute here
}

extern "Rust" {
// CHECK-NOT: nounwind
// CHECK: declare void @rust_extern_fn
fn rust_extern_fn();
// CHECK-NOT: nounwind
// CHECK: declare void @rust_unwinding_extern_fn
#[unwind(allowed)]
fn rust_unwinding_extern_fn();
// CHECK-NOT: nounwind
// CHECK: declare void @rust_aborting_extern_fn
#[unwind(aborts)]
fn rust_aborting_extern_fn(); // FIXME: we don't have the attribute here
}

pub unsafe fn force_declare() {
extern_fn();
unwinding_extern_fn();
aborting_extern_fn();
rust_extern_fn();
rust_unwinding_extern_fn();
rust_aborting_extern_fn();
}
3 changes: 3 additions & 0 deletions src/test/codegen/nounwind-extern.rs
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Expand Up @@ -2,5 +2,8 @@

#![crate_type = "lib"]

// The `nounwind` attribute does not get added by rustc; it is present here because LLVM
// analyses determine that this function does not unwind.
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// CHECK: Function Attrs: norecurse nounwind
pub extern fn foo() {}
15 changes: 15 additions & 0 deletions src/test/codegen/unwind-extern.rs
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@@ -0,0 +1,15 @@
// compile-flags: -C opt-level=0

#![crate_type = "lib"]
#![feature(unwind_attributes)]

// make sure these all do *not* get the attribute
// CHECK-NOT: nounwind

pub extern fn foo() {} // right now we don't abort-on-panic, so we also shouldn't have `nounwind`
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#[unwind(allowed)]
pub extern fn foo_allowed() {}
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pub extern "Rust" fn bar() {}
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#[unwind(allowed)]
pub extern "Rust" fn bar_allowed() {}
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