[PATCH] D138958: [clang] Better UX for Clang’s unwind-affecting attributes

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


================
Comment at: clang/include/clang/Basic/Attr.td:4133
+def NoUnwind : DeclOrStmtAttr {
+  let Spellings = [Clang<"nounwind", /*allowInC =*/0>];
+  let Accessors = [Accessor<"isClangNoUnwind", [CXX11<"clang", "nounwind">]>];
----------------
aaron.ballman wrote:
> Should this be allowed in C (where structured exception handling is still a thing)?
The only two thing about structured exceptions i know
is that it's abbreviation and that it's a MSVC thing. 
I don't know anything about how "exceptions" work there,
and therefore i do not want to touch it.

https://llvm.org/docs/LangRef.html notes:
```
nounwind
This function attribute indicates that the function never raises an exception. <...>.
However, functions marked nounwind may still trap or generate asynchronous exceptions.
Exception handling schemes that are recognized by LLVM to handle asynchronous exceptions,
such as SEH, will still provide their implementation defined semantics.
```


================
Comment at: clang/include/clang/Basic/Attr.td:4138
+                             "functions and statements">;
+  let SimpleHandler = 1;
+}
----------------
aaron.ballman wrote:
> I would have guessed we'd want to help the user out in a case like: `[[clang::nounwind]] void func() noexcept(false);`, given that this stuff can creep in via macros?
Could you please clarify, what do you mean by "help the user" in this case?
Given
```
[[clang::nounwind]] void func() noexcept(false);

void qqq();

void foo() {
  try {
    func();
  } catch (...) {
    qqq();
  }
}
```
we already end up with
```
; Function Attrs: mustprogress noinline nounwind optnone uwtable
define dso_local void @_Z3foov() #0 {
entry:
  call void @_Z4funcv() #2
  ret void
}

; Function Attrs: nounwind
declare void @_Z4funcv() #1

attributes #0 = { mustprogress noinline nounwind optnone uwtable "frame-pointer"="all" "min-legal-vector-width"="0" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #1 = { nounwind "frame-pointer"="all" "no-trapping-math"="true" "stack-protector-buffer-size"="8" "target-cpu"="x86-64" "target-features"="+cx8,+fxsr,+mmx,+sse,+sse2,+x87" "tune-cpu"="generic" }
attributes #2 = { nounwind }
```
and if i drop `[[clang::nounwind]]`, `landingpad` is back.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D138958/new/

https://reviews.llvm.org/D138958