[libc++] Require a C++20 capable compiler.

[ldionne]

Summary

This enforces libcxx and its benchmarks are compiled by a C++20 capable
compiler. Based on review comments in D103413.

[ldionne]

Commandeering to rebase on top of my D110285, which conflicts with this.

[ldionne]

Rebase onto main

[saghir]

This seems to be breaking one of PPC buildbots: https://lab.llvm.org/buildbot/#/builders/18/builds/2553/steps/7/logs/stdio
Can you please take a look when you get a chance?

Thanks!

[Mordante]

In D110338#3022576, @saghir wrote:

This seems to be breaking one of PPC buildbots: https://lab.llvm.org/buildbot/#/builders/18/builds/2553/steps/7/logs/stdio
Can you please take a look when you get a chance?

Thanks!

Looking at the log I see a no longer supported combination -DLLVM_ENABLE_PROJECTS='\''clang;compiler-rt;libcxx;libcxxabi'\''' and GCC 7.3.1. Libc++ requires a C++20 capable compiler. Instead of using GCC directly it's possible to use a bootstrap build. Using
DLLVM_ENABLE_PROJECTS='clang;compiler-rt' -DLLVM_ENABLE_RUNTIMES='libcxx;libcxxabi' should fix your issue. See https://libcxx.llvm.org/BuildingLibcxx.html#bootstrapping-build for more information. Please let us know when you need more information.

[carlosgalvezp]

Hi @Idionne @Mordante !

I believe this commit broke the whole delivery pipeline of .deb bionic-snapshot packages:

https://llvm-jenkins.debian.net/job/llvm-toolchain-bionic-binaries/architecture=amd64,distribution=bionic,label=amd64/2228/console

CMake Error in /build/llvm-toolchain-snapshot-14~++20210926063815+c4ae4a745dbd/libcxx/benchmarks/CMakeLists.txt:
  The compiler feature "cxx_std_20" is not known to CXX compiler

  "GNU"

  version 7.3.0.

Would it be possible to revert/fix? Currently we cannot get new fixes to trunk as deb packages since the pipeline is broken.

[Mordante]

In D110338#3027111, @carlosgalvezp wrote:

Hi @Idionne @Mordante !

I believe this commit broke the whole delivery pipeline of .deb bionic-snapshot packages:

Can you try whether you can use the -DLLVM_ENABLE_RUNTIMES solution works for you?
(See the post before yours)

[carlosgalvezp]

In D110338#3027771, @Mordante wrote:

In D110338#3027111, @carlosgalvezp wrote:

Hi @Idionne @Mordante !

I believe this commit broke the whole delivery pipeline of .deb bionic-snapshot packages:

Can you try whether you can use the -DLLVM_ENABLE_RUNTIMES solution works for you?
(See the post before yours)

Unfortunately I'm not in control of that build pipeline, I thought this was the correct place to report these issues. I'll see if I find the right place and forward your tips there, thanks!

[Mordante]

In D110338#3028032, @carlosgalvezp wrote:

In D110338#3027771, @Mordante wrote:

In D110338#3027111, @carlosgalvezp wrote:

Hi @Idionne @Mordante !

I believe this commit broke the whole delivery pipeline of .deb bionic-snapshot packages:

Can you try whether you can use the -DLLVM_ENABLE_RUNTIMES solution works for you?
(See the post before yours)

Unfortunately I'm not in control of that build pipeline, I thought this was the correct place to report these issues. I'll see if I find the right place and forward your tips there, thanks

No problem, since you reported it I thought you were also in control of the pipeline.
@sylvestre.ledru can you look at using -DLLVM_ENABLE_RUNTIMES for the Debian/Ubuntu nightly builds.
If you have questions feel free to leave a message in the libcxx Discord channel.

[sylvestre.ledru]

Yeah, it is ongoing work but it will take a few more days/weeks
here is the work:
https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/merge_requests/73

[ldionne]

GCC 7.3 has been unsupported since the LLVM 12 release, see the discussion started at https://lists.llvm.org/pipermail/llvm-dev/2021-March/148881.html. Please use https://libcxx.llvm.org/BuildingLibcxx.html#bootstrapping-build as @Mordante suggested -- it will also result in a working libc++ as opposed to a maybe-working-libc++-but-nobody-has-tested-that-for-years.

If you need some more time to fix this on your end, please apply a revert locally, but reverting upstream is not an option at this point.

[carlosgalvezp]

In D110338#3028614, @sylvestre.ledru wrote:

Yeah, it is ongoing work but it will take a few more days/weeks
here is the work:
https://salsa.debian.org/pkg-llvm-team/llvm-toolchain/-/merge_requests/73

Just for my understanding, is this work only for branch "12" or will it also apply to "snapshot"?

[sylvestre.ledru]

12 will be merged into 13 and then snapshot :)

[sylvestre.ledru]

For the release notes:
https://reviews.llvm.org/D111043

Which version of gcc are supported btw?

[Mordante]

We support the latest release of GCC, at the moment GCC 11.
This is a link to libc++'s platform support https://libcxx.llvm.org/#platform-and-compiler-support

libcxx/CMakeLists.txt

 
 # Required flags ==============================================================
 function(cxx_add_basic_build_flags target)
 
   # Require C++20 for all targets. C++17 is needed to use aligned allocation
   # in the dylib. C++20 is needed to use char8_t.
   set_target_properties(${target} PROPERTIES
     CXX_STANDARD 20
-    CXX_STANDARD_REQUIRED NO
+    CXX_STANDARD_REQUIRED YES
     CXX_EXTENSIONS NO)
 
   # When building the dylib, don't warn for unavailable aligned allocation
   # functions based on the deployment target -- they are always available
   # because they are provided by the dylib itself with the excepton of z/OS.
   if (ZOS)
     target_add_compile_flags_if_supported(${target} PRIVATE -fno-aligned-allocation)
   else()

libcxx/benchmarks/CMakeLists.txt

 # Benchmark tests configuration
 #==============================================================================
 add_custom_target(cxx-benchmarks)
 set(BENCHMARK_OUTPUT_DIR ${CMAKE_CURRENT_BINARY_DIR})
 set(BENCHMARK_LIBCXX_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-libcxx)
 set(BENCHMARK_NATIVE_INSTALL ${CMAKE_CURRENT_BINARY_DIR}/benchmark-native)
 
 add_library(               cxx-benchmarks-flags INTERFACE)
-target_compile_features(   cxx-benchmarks-flags INTERFACE cxx_std_17)
+target_compile_features(   cxx-benchmarks-flags INTERFACE cxx_std_20)
 target_compile_options(    cxx-benchmarks-flags INTERFACE -O2 -fsized-deallocation)
 target_include_directories(cxx-benchmarks-flags INTERFACE "${BENCHMARK_LIBCXX_INSTALL}/include"
                                                 INTERFACE "${LIBCXX_SOURCE_DIR}/test/support")
 
 add_library(           cxx-benchmarks-flags-native INTERFACE)
 target_link_libraries( cxx-benchmarks-flags-native INTERFACE cxx-benchmarks-flags)
 target_compile_options(cxx-benchmarks-flags-native INTERFACE ${BENCHMARK_NATIVE_TARGET_FLAGS})
 target_link_options(   cxx-benchmarks-flags-native INTERFACE ${BENCHMARK_NATIVE_TARGET_FLAGS} "-L${BENCHMARK_NATIVE_INSTALL}/lib")