[asan] Unconditionally enable GC of globals on COFF

[eugenis]

Summary

This change restores pre-r301225 behavior, where linker GC compatible global
instrumentation was used on COFF targets disregarding -f(no-)data-sections and/or
/Gw flags.

This instrumentation puts each global in a COMDAT with an ASan descriptor for that global.
It effectively enables -fdata-sections, but limits it to ASan-instrumented globals.

[pcc]

Why?

[eugenis]

Apparently the ODR detector in Asan on Windows has issues with full data-sections. This way we can have limited GC for user globals.

[eugenis]

r301374

lib/CodeGen/BackendUtil.cpp

 
 // Check if ASan should use GC-friendly instrumentation for globals.
 // First of all, there is no point if -fdata-sections is off (expect for MachO,
 // where this is not a factor). Also, on ELF this feature requires an assembler
 // extension that only works with -integrated-as at the moment.
 static bool asanUseGlobalsGC(const Triple &T, const CodeGenOptions &CGOpts) {
   switch (T.getObjectFormat()) {
   case Triple::MachO:
-    return true;
   case Triple::COFF:
-    return CGOpts.DataSections;
+    return true;
   case Triple::ELF:
     return CGOpts.DataSections && !CGOpts.DisableIntegratedAS;
   default:
     return false;
   }
 }
 
 static void addAddressSanitizerPasses(const PassManagerBuilder &Builder,

test/CodeGen/asan-globals-gc.cpp

-// RUN: %clang_cc1 -fsanitize=address -emit-llvm -o - -triple x86_64-windows-msvc %s | FileCheck %s --check-prefix=WITHOUT-GC
+// RUN: %clang_cc1 -fsanitize=address -emit-llvm -o - -triple x86_64-windows-msvc %s | FileCheck %s --check-prefix=WITH-GC
 // RUN: %clang_cc1 -fsanitize=address -emit-llvm -o - -triple x86_64-windows-msvc -fdata-sections %s | FileCheck %s --check-prefix=WITH-GC
 
 int global;
 
 // WITH-GC-NOT: call void @__asan_register_globals
 // WITHOUT-GC: call void @__asan_register_globals