[llvm-bugs] [Bug 50449] New: allow indicating that calls to asm expressions are [[noreturn]]

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50449

            Bug ID: 50449
           Summary: allow indicating that calls to asm expressions are
                    [[noreturn]]
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: gonzalo.gadeschi at gmail.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Example (minimized): https://gcc.godbolt.org/z/nqKq49q7a

 [[noreturn]]  void trap()  {
     asm volatile ("ret");
 }

generates this LLVM-IR:


    define dso_local void @_Z4trapv() #0 {
      call void asm sideeffect "ret", "~{dirflag},~{fpsr},~{flags}"() #1,
!srcloc !4
      ret void
    }

    attributes #0 = { noinline noreturn nounwind optnone uwtable mustprogress }
    attributes #1 = { nounwind }

and there does not appear to be a way to add the LLVM-IR "noreturn" attribute
to the call such that #1 becomes:

    attributes #1 = { nounwind noreturn }


There are a couple of ways in which this could be allowed, e.g.,:

  - supporting: [[noreturn]] asm volatile(....)
  - supporting: asm volatile(... : : : "noreturn")


If this is supported, both the LLVM-IR "call" or "invoke" instructions
generated should have the "noreturn" attribute.


Also, the warning produced:

    warning: function declared 'noreturn' should not return
[-Winvalid-noreturn]

should probably be updated to take inline asm expressions that never return
into account.

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