https://cs.chromium.org/chromium/src/build/config/compiler/BUILD.gn?type=cs&l=1329 contains this block:

  if (!is_asan) {
    common_optimize_on_cflags += [
      # Put data in separate COMDATs. This allows the linker
      # to put bit-identical constants at the same address even if
      # they're unrelated constants, which saves binary size.
      # This optimization can't be used when ASan is enabled because
      # it is not compatible with the ASan ODR checker.
      "/Gw",
    ]
  }

This was added in https://codereview.chromium.org/832583007 . Due to this, llvm r301225 had to be rowed back in LLVM r301374 (http://llvm.org/viewvc/llvm-project?view=revision&revision=301374)


On comment 14 in  issue 715315 , pcc said: """
I looked closely at the implementation of the ODR checker with eugenis and found a potential bug: after we rebuild a global [0] we do not copy the comdat from the original global. That may have caused duplicate symbol link errors with the ODR checker enabled. 

[0] http://llvm-cs.pcc.me.uk/lib/Transforms/Instrumentation/AddressSanitizer.cpp#1830
"""



We should investigate what exactly the ODR issue was (Slava, do you remember?), fix that, always pass /Gw even in is_asan builds, and then reinstantiate llvm r301225.