[libunwind] Only include cet.h if __CET__ defined

[nikic]

Summary

We should not assume that the cet.h header exists just because we're on x86 linux. Only include it if __CET__ is defined. This makes the code more similar to what compiler-rt does in https://github.com/llvm/llvm-project/blob/ee423d93ead39e94c2970b3cc7ef6e6faa75d10b/compiler-rt/lib/builtins/assembly.h#L17 (though that one also has a __has_include() check -- I've not found that to be necessary).

[MaskRay]

Thanks!

though that one also has a __has_include() check -- I've not found that to be necessary).

It's not necessary because libunwind requires to be built with fresh Clang, which definitely includes cet.h in the resource directory when __CET__ is defined.
compiler-rt may be built with LLVM_ENABLE_PROJECTS with slightly older GCC.

[nikic]

In D119697#3321332, @MaskRay wrote:

Thanks!

though that one also has a __has_include() check -- I've not found that to be necessary).

It's not necessary because libunwind requires to be built with fresh Clang, which definitely includes cet.h in the resource directory when __CET__ is defined.
compiler-rt may be built with LLVM_ENABLE_PROJECTS with slightly older GCC.

Uh, really? I couldn't find docs on this, and the llvm-dev discussion on the topic (https://discourse.llvm.org/t/compiler-support-in-libc/57751/1) was only about raising toolchain requirements for libc++. Requiring a recent toolchain for libc++ makes a lot of sense, but not so much for libunwind, which does not have any tight integration with C++ (and is in fact used as a runtime for other languages, which is why I would find it very surprising if building libunwind also requires you to build clang first).

[MaskRay]

In D119697#3321926, @nikic wrote:

In D119697#3321332, @MaskRay wrote:

Thanks!

though that one also has a __has_include() check -- I've not found that to be necessary).

It's not necessary because libunwind requires to be built with fresh Clang, which definitely includes cet.h in the resource directory when __CET__ is defined.
compiler-rt may be built with LLVM_ENABLE_PROJECTS with slightly older GCC.

Uh, really? I couldn't find docs on this, and the llvm-dev discussion on the topic (https://discourse.llvm.org/t/compiler-support-in-libc/57751/1) was only about raising toolchain requirements for libc++. Requiring a recent toolchain for libc++ makes a lot of sense, but not so much for libunwind, which does not have any tight integration with C++ (and is in fact used as a runtime for other languages, which is why I would find it very surprising if building libunwind also requires you to build clang first).

https://reviews.llvm.org/D119351 : "Note that in the near future, libcxx, libcxxabi and libunwind will stop supporting being built with LLVM_ENABLE_PROJECTS altogether."

[mstorsjo]

In D119697#3321926, @nikic wrote:

In D119697#3321332, @MaskRay wrote:

It's not necessary because libunwind requires to be built with fresh Clang, which definitely includes cet.h in the resource directory when __CET__ is defined.
compiler-rt may be built with LLVM_ENABLE_PROJECTS with slightly older GCC.

Uh, really? I couldn't find docs on this, and the llvm-dev discussion on the topic (https://discourse.llvm.org/t/compiler-support-in-libc/57751/1) was only about raising toolchain requirements for libc++. Requiring a recent toolchain for libc++ makes a lot of sense, but not so much for libunwind, which does not have any tight integration with C++ (and is in fact used as a runtime for other languages, which is why I would find it very surprising if building libunwind also requires you to build clang first).

A couple issues are being conflated here. First, libc++ wants to require a fairly new compiler, either one of the last two stable releases of Clang, or the latest GCC. This doesn't extend to other runtime libs, as far as I know.

libunwind, libc++abi and libc++ aren't meant to be built via LLVM_ENABLE_PROJECTS any longer, but that's not about compiler support, but instead about reducing the number of build system combinations that have to be supported.

You can still build libunwind, libc++abi and libc++ with any compiler of your choice; point CMake at llvm-project/runtimes and pass LLVM_ENABLE_RUNTIMES=libunwind, this will build only libunwind, with whatever compiler you configure CMake to use. You can also pass LLVM_ENABLE_RUNTIMES when pointing CMake at llvm-project/llvm, which then would build those runtimes with the newly-built Clang .

[nikic]

In D119697#3322156, @MaskRay wrote:

In D119697#3321926, @nikic wrote:

In D119697#3321332, @MaskRay wrote:

Thanks!

though that one also has a __has_include() check -- I've not found that to be necessary).

It's not necessary because libunwind requires to be built with fresh Clang, which definitely includes cet.h in the resource directory when __CET__ is defined.
compiler-rt may be built with LLVM_ENABLE_PROJECTS with slightly older GCC.

Uh, really? I couldn't find docs on this, and the llvm-dev discussion on the topic (https://discourse.llvm.org/t/compiler-support-in-libc/57751/1) was only about raising toolchain requirements for libc++. Requiring a recent toolchain for libc++ makes a lot of sense, but not so much for libunwind, which does not have any tight integration with C++ (and is in fact used as a runtime for other languages, which is why I would find it very surprising if building libunwind also requires you to build clang first).

https://reviews.llvm.org/D119351 : "Note that in the near future, libcxx, libcxxabi and libunwind will stop supporting being built with LLVM_ENABLE_PROJECTS altogether."

How is that related? Even without LLVM_ENABLE_PROJECTS, runtimes still support two build modes, one rooted at llvm/ (the bootstrap build) and one rooted at runtimes/ (the "default" build). The latter does not use a bootstrapped clang.

[tstellar]

Do we need this in the release/14.x branch?

[nikic]

In D119697#3340838, @tstellar wrote:

Do we need this in the release/14.x branch?

Probably, so other people don't need to patch this as well. I opened https://github.com/llvm/llvm-project/issues/54028.

libunwind/src/assembly.h

  * This file is not part of the interface of this library.
  *
  * ===----------------------------------------------------------------------===
  */
 
 #ifndef UNWIND_ASSEMBLY_H
 #define UNWIND_ASSEMBLY_H
 
-#if (defined(__i386__) || defined(__x86_64__)) && defined(__linux__)
+#if defined(__linux__) && defined(__CET__)
 #include <cet.h>
 #define _LIBUNWIND_CET_ENDBR _CET_ENDBR
 #else
 #define _LIBUNWIND_CET_ENDBR
 #endif
 
 #if defined(__powerpc64__)
 #define SEPARATOR ;