[compiler-rt][crt] Pass -fno-lto in check_cxx_section_exists

[hans]

Summary

Otherwise it doesn't work when building with -DLLVM_ENABLE_LTO=thin

(We hit this in Chromium in https://bugs.chromium.org/p/chromium/issues/detail?id=966403)

[thakis]

This lgtm, but it suggests there's no upstream bot building llvm with lto, and running tests with it. I'm not sure I want to be in the business of owning test coverage for that config -- should we have an LTO bot on the llvm buildbot master?

[phosek]

compiler-rt/lib/crt/CMakeLists.txt

22

I believe this should be done on line 79 as append_list_if(COMPILER_RT_HAS_FNO_LTO_FLAG -fno-lto CRT_CFLAGS), try_compile_flags is only used to test whether the compiler supports .init/.fini section but doesn't change flags used to compile crtbegin.o/crtend.o.

[hans]

In D62315#1513898, @thakis wrote:

This lgtm, but it suggests there's no upstream bot building llvm with lto, and running tests with it. I'm not sure I want to be in the business of owning test coverage for that config -- should we have an LTO bot on the llvm buildbot master?

As Bob pointed out on the Chromium bug, there is http://lab.llvm.org:8011/builders/clang-with-lto-ubuntu but that doesn't include compiler-rt. It's Sony's bot. I'll ask if they want to include compiler-rt.

compiler-rt/lib/crt/CMakeLists.txt

22

We're already building crtbegin.o/crtend.o with -fno-lto -- I think it gets appended to all runtime build targets at some point.

It's really the "check whether .init/.fini is support" part I'm going after here, because that doesn't build with the normal compiler-rt flags, and so it doesn't work in thin-lto builds.

[hans]

compiler-rt/lib/crt/CMakeLists.txt

22

Ping?

[hans]

Ping?

[thakis]

The ping is for phosek, right?

[hans]

In D62315#1610846, @thakis wrote:

The ping is for phosek, right?

Yes.

[phosek]

LGTM

compiler-rt/lib/crt/CMakeLists.txt

function(check_cxx_section_exists section output)
 
   string(REGEX MATCHALL "<[A-Za-z0-9_]*>" substitutions
          ${CMAKE_C_COMPILE_OBJECT})
 
   set(try_compile_flags "${ARG_FLAGS}")
   if(CMAKE_C_COMPILER_ID MATCHES Clang AND CMAKE_C_COMPILER_TARGET)
     list(APPEND try_compile_flags "-target ${CMAKE_C_COMPILER_TARGET}")
   endif()
+  append_list_if(COMPILER_RT_HAS_FNO_LTO_FLAG -fno-lto try_compile_flags)
 
   string(REPLACE ";" " " extra_flags "${try_compile_flags}")
 
   set(test_compile_command "${CMAKE_C_COMPILE_OBJECT}")
   foreach(substitution ${substitutions})
     if(substitution STREQUAL "<CMAKE_C_COMPILER>")
       string(REPLACE "<CMAKE_C_COMPILER>" "${CMAKE_C_COMPILER} ${CMAKE_C_COMPILER_ARG1}"
              test_compile_command ${test_compile_command})