[compiler-rt][crt] make test case nontrivial in check_cxx_section_exists

[jcai19]

Summary

.init_array gets optimized away when building with -O2 and as a result,
check_cxx_section_exists failed to pass -DCOMPILER_RT_HAS_INITFINI_ARRAY
when building crtbegin.o and crtend.o, which causes binaries linked with
them encounter segmentation fault. See https://crbug.com/855759 for
details. This change prevents .init_array section to be optimized away
even with -O2 or higher optimization level.

[phosek]

Could we make the function non-trivial e.g. __attribute__((constructor)) void f() { asm volatile ("nop"::); } to ensure that it's not optimized out even at -O2?

[joerg]

Can you declare a volatile variable and set that from f instead?

[jcai19]

Thanks for the comments. I have update my change accordinlgy.

[jcai19]

Remove uncessary new line character.

[joerg]

LGTM

compiler-rt/trunk/lib/crt/CMakeLists.txt

function(check_cxx_section_exists section output)
   else()
     set(${output} FALSE PARENT_SCOPE)
   endif()
 
   file(REMOVE_RECURSE ${TARGET_NAME})
 endfunction()
 
 check_cxx_section_exists(".init_array" COMPILER_RT_HAS_INITFINI_ARRAY
-  SOURCE "__attribute__((constructor)) void f() {}\nint main() { return 0; }\n")
+  SOURCE "volatile int x;\n__attribute__((constructor)) void f() {x = 0;}\nint main() { return 0; }\n")
 
 append_list_if(COMPILER_RT_HAS_STD_C11_FLAG -std=c11 CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_INITFINI_ARRAY -DCRT_HAS_INITFINI_ARRAY CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FPIC_FLAG -fPIC CRT_CFLAGS)
 append_list_if(COMPILER_RT_HAS_WNO_PEDANTIC -Wno-pedantic CRT_CFLAGS)
 
 foreach(arch ${CRT_SUPPORTED_ARCH})
   add_compiler_rt_runtime(clang_rt.crtbegin