flang: Fix build with latest libc++

[tstellar]

I think this first stopped working with
9548366. This patch fixes the following error:

/home/runner/work/llvm-project/llvm-project/flang/runtime/io-api-minimal.cpp:153:11: error: '__libcpp_verbose_abort' is missing exception specification 'noexcept'
153 | void std::__libcpp_verbose_abort(char const format, ...) {
| ^
| noexcept
/mnt/build/bin/../include/c++/v1/__verbose_abort:30:28: note: previous declaration is here
30 | printf, 1, 2) void __libcpp_verbose_abort(const char __format, ...) _LIBCPP_VERBOSE_ABORT_NOEXCEPT;
| ^
1 error generated.

[llvmbot]

@llvm/pr-subscribers-flang-runtime

Author: Tom Stellard (tstellar)

Changes

I think this first stopped working with
9548366. This patch fixes the following error:

/home/runner/work/llvm-project/llvm-project/flang/runtime/io-api-minimal.cpp:153:11: error: '__libcpp_verbose_abort' is missing exception specification 'noexcept'
153 | void std::__libcpp_verbose_abort(char const format, ...) {
| ^
| noexcept
/mnt/build/bin/../include/c++/v1/__verbose_abort:30:28: note: previous declaration is here
30 | printf, 1, 2) void __libcpp_verbose_abort(const char __format, ...) _LIBCPP_VERBOSE_ABORT_NOEXCEPT;
| ^
1 error generated.

---

Full diff: https://github.com/llvm/llvm-project/pull/127362.diff

1 Files Affected:

• (modified) flang/runtime/io-api-minimal.cpp (+6-1)

diff --git a/flang/runtime/io-api-minimal.cpp b/flang/runtime/io-api-minimal.cpp
index 68768427be0c2..e53089e02d93d 100644
--- a/flang/runtime/io-api-minimal.cpp
+++ b/flang/runtime/io-api-minimal.cpp
@@ -150,7 +150,12 @@ bool IODEF(OutputLogical)(Cookie cookie, bool truth) {
 // Provide own definition for `std::__libcpp_verbose_abort` to avoid dependency
 // on the version provided by libc++.
 
-void std::__libcpp_verbose_abort(char const *format, ...) {
+#if !defined(_LIBCPP_VERBOSE_ABORT_NOEXCEPT)
+  #define _LIBCPP_VERBOSE_ABORT_NOEXCEPT
+#endif
+
+
+void std::__libcpp_verbose_abort(char const *format, ...) _LIBCPP_VERBOSE_ABORT_NOEXCEPT {
   va_list list;
   va_start(list, format);
   std::vfprintf(stderr, format, list);

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[mmuetzel]

I can't comment on the general question whether it is supported by libc++ to use headers from the STL without linking to the library.
But the proposed change looks like a valid fix for the existing work-around to me. 👍

[mmuetzel]

Fwiw, a similar issue was previously reported in #113059 and fixed(?) with #113310.

[ldionne]

The right way of fixing this would be:

void std::__libcpp_verbose_abort(char const *format, ...)
    noexcept(noexcept(std::__libcpp_verbose_abort("")))
{
  // ...
}

This allows unconditionally using the right noexcept-ness based on how the base function has been declared.

About the more general question of whether libc++ supports being used header-only (without linking against the dylib), the answer is no, but some stuff will still happen to work if you do that. That's a brittle setup though, for example I expect that you would encounter a linker error if you tried using std::shared_ptr or std::sort(int*, int*) anywhere since those are in the dylib. Is there a reason why you don't link against the libc++ built library?

[mmuetzel]

Is there a reason why you don't link against the libc++ built library?

That is a very good question that I was pondering myself repeatedly.
I hope it is ok to ask @sscalpone for clarification.

[tstellar]

@ldionne I've implemented your suggested fix.

[ldionne]

LGTM.

I think Flang should figure out why they're not linking against libc++ since that will break other stuff in the future. But in the interest of fixing the current state of trunk, this patch works and doesn't make anything worse than it already is.

[tstellar]

/cherry-pick 2b340c1

[llvmbot]

/pull-request #127805

[Meinersbur]

I had already tried to fix this, but had to revert in 81c85ea due to a buildbot failure. Thanks for taking care.