--disable-warnings-as-errors does not work for HotSpot build

Passing --disable-warnings-as-errors to configure not works in jdk9/hs-rt repo on Linux-x64 OS. Running following configure&build command give compilation error:
 sh ./configure --disable-warnings-as-errors --enable-native-coverage --enable-debug
make all
...
/home/dmitry/work/bugs/disable_warnings_as_errors/jdk9-hs-rt/hotspot/src/share/vm/gc/shared/genCollectedHeap.cpp: In function ‘ScratchBlock* removeSmallestScratch(ScratchBlock**)’:
/home/dmitry/work/bugs/disable_warnings_as_errors/jdk9-hs-rt/hotspot/src/share/vm/gc/shared/genCollectedHeap.cpp:1032:33: error: ‘smallest_ptr’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   *smallest_ptr = smallest->next;
                                 ^
cc1plus: all warnings being treated as errors
make[8]: *** [genCollectedHeap.o] Error 1
...

The issue reproducible with --enable-native-coverage flag to configure.