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

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.

Subscribers: dberris, mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D67628

llvm-svn: 372038

Author: jcai19

compiler-rt/lib/crt/CMakeLists.txt

@@ -70,7 +70,7 @@ function(check_cxx_section_exists section output)
 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)