[libc] Warn on use of global constructors in `libc`

[jhuber6]

Summary

Clang supports the -Wglobal-constructors flag which will indicate if a
global constructor is being used. The current goal in libc is to make
the constructors constexpr to prevent this from happening with
straight construction. However, there are many other cases where we can
emit a constructor that this won't catch. This should give warning if
someone accidentally introduces a global constructor.

[JonChesterfield]

I'm in favour. Definitely want this for the GPU targets even if the others want to stay warning-free

[michaelrj]

Does this currently give any warnings for an x86_64/linux fullbuild?

[jhuber6]

In D155721#4515398, @michaelrj wrote:

Does this currently give any warnings for an x86_64/linux fullbuild?

Yes,

/home/jhuber/Documents/llvm/llvm-project/libc/src/unistd/getopt.cpp:184:22: warning: declaration requires a global constructor [-Wglobal-constructors]     
  184 | static GetoptContext ctx{                                                                                   
      |                      ^  ~                                                                                 
  185 |     &impl::optarg, &impl::optind,                                                             
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~                                                                     
  186 |     &impl::optopt, &optpos,                                                                        
      |     ~~~~~~~~~~~~~~~~~~~~~~~                                                                          
  187 |     impl::opterr,  reinterpret_cast<FILE *>(__llvm_libc::stderr)};                                          
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~     

  /home/jhuber/Documents/llvm/llvm-project/libc/src/__support/File/linux/file.cpp:160:18: warning: declaration requires a global destructor [-Wglobal-constructors]
  160 | static LinuxFile StdIn(0, stdin_buffer, STDIN_BUFFER_SIZE, _IOFBF, false,                                   
      |                  ^                                                                                        
/home/jhuber/Documents/llvm/llvm-project/libc/src/__support/File/linux/file.cpp:166:18: warning: declaration requires a global destructor [-Wglobal-constructors]
  166 | static LinuxFile StdOut(1, stdout_buffer, STDOUT_BUFFER_SIZE, _IOLBF, false,                          
      |                  ^                                                                                 
/home/jhuber/Documents/llvm/llvm-project/libc/src/__support/File/linux/file.cpp:171:18: warning: declaration requires a global destructor [-Wglobal-constructors]
  171 | static LinuxFile StdErr(2, nullptr, STDERR_BUFFER_SIZE, _IONBF, false,                                        
      |                  ^                                                                                          
3 warnings generated.

[jhuber6]

I landed D155766 and D155762 so this should be okay to enable.

[michaelrj]

LGTM

libc/cmake/modules/LLVMLibCObjectRules.cmake

if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
     list(APPEND compile_options "-Wimplicit-fallthrough")
     list(APPEND compile_options "-Wwrite-strings")
     list(APPEND compile_options "-Wextra-semi")
     if(NOT CMAKE_COMPILER_IS_GNUCXX)
       list(APPEND compile_options "-Wnewline-eof")
       list(APPEND compile_options "-Wnonportable-system-include-path")
       list(APPEND compile_options "-Wstrict-prototypes")
       list(APPEND compile_options "-Wthread-safety")
+      list(APPEND compile_options "-Wglobal-constructors")
     endif()
     if(ADD_FMA_FLAG)
       if(LIBC_TARGET_ARCHITECTURE_IS_X86)
         list(APPEND compile_options "-mavx2")
         list(APPEND compile_options "-mfma")
       elseif(LIBC_TARGET_ARCHITECTURE_IS_RISCV64)
         list(APPEND compile_options "-D__LIBC_RISCV_USE_FMA")
       endif()