Build fix: Turn off _GLIBCXX_DEBUG on gcc 9

[nhaehnle]

Summary

Enabling _GLIBCXX_DEBUG (implied by LLVM_ENABLE_EXPENSIVE_CHECKS) causes
std::min (and presumably others) to no longer be constexpr, which in turn
causes the build to fail.

This seems like a bug in the GCC STL. This change works around it.

Change-Id: I5fc471caa9c4de3ef4e87aeeac8df1b960e8e72c

[hans]

What happens if one uses libstdc++ 9 but with Clang?

[serge-sans-paille]

Thanks for working on this! I ran into this once and manually undefined the macro, so the workaround looks decent. However, I share @hans concerns about the detection part. You most likely want to compile minimal C++ program using std::min (and maybe other functions) to detect if the actual standard library used matches our requirements.

[jwakely]

I think this affects the versions of std::min(std::initializer_list) and std::max(std::initializer_list), and std::{min,max,minmax}_element. It's already fixed for GCC 10.

Testing for GCC is definitely wrong, it needs to test for the library. If __GLIBCXX__ is defined then you're using libstdc++, and for versions since 7.1.0 _GLIBCXX_RELEASE tells you the release version, so you can use that to detect libstdc++ from GCC 10.

[jwakely]

P.S. you could use _GLIBCXX_ASSERTIONS instead for affected versions of libstdc++. That is a subset of _GLIBCXX_DEBUG that has far fewer checks, but is still much better than not checking anything at all.

[serge-sans-paille]

@nhaehnle : something like

CHECK_CXX_SOURCE_COMPILES("
#define _GLIBCXX_DEBUG
#include <algorithm>

int main(int argc, char ** argv) {
    static constexpr int data []= {0, 1};
    constexpr const int* min_elt  = std::min_element(&data[0], &data[2]) ;
    return 0;
}" CXX_SUPPORTS_GLIBCXX8DEBUG)

[nhaehnle]

Thank you all for the advice.

Changing the patch to use a compile check and fall back to
_GLIBCXX_ASSERTIONS.

[serge-sans-paille]

LGTM

llvm/cmake/modules/HandleLLVMOptions.cmake

foreach (flags_var_to_scrub
       string (REGEX REPLACE "(^| )[/-]D *NDEBUG($| )" " "
         "${flags_var_to_scrub}" "${${flags_var_to_scrub}}")
     endforeach()
   endif()
 endif()
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)
   add_definitions(-DEXPENSIVE_CHECKS)
+
+  # In some libstdc++ versions, std::min_element is not constexpr when
+  # _GLIBCXX_DEBUG is enabled.
+  CHECK_CXX_SOURCE_COMPILES("
+    #define _GLIBCXX_DEBUG
+    #include <algorithm>
+    int main(int argc, char** argv) {
+      static constexpr int data[] = {0, 1};
+      constexpr const int* min_elt = std::min_element(&data[0], &data[2]);
+      return 0;
+    }" CXX_SUPPORTS_GLIBCXX_DEBUG)
+  if(CXX_SUPPORTS_GLIBCXX_DEBUG)
     add_definitions(-D_GLIBCXX_DEBUG)
+  else()
+    add_definitions(-D_GLIBCXX_ASSERTIONS)
+  endif()
 endif()
 
 string(TOUPPER "${LLVM_ABI_BREAKING_CHECKS}" uppercase_LLVM_ABI_BREAKING_CHECKS)
 
 if( uppercase_LLVM_ABI_BREAKING_CHECKS STREQUAL "WITH_ASSERTS" )
   if( LLVM_ENABLE_ASSERTIONS )
     set( LLVM_ENABLE_ABI_BREAKING_CHECKS 1 )
   endif()