Why does a fastdebug build compile with -O3 *and* -O0?

[Severin Gehwolf]

Hi,

I've noticed that a fastdebug build of recent hs-rt compiles objects
with -O3 *and* -O0. Is this a bug or a feature? If it's a feature,
what's the rationale?

I'm not sure what GCC does with that, but it appears it just uses -O0
(last seen option?).

Example:
/usr/bin/g++ -DLINUX -D_GNU_SOURCE -DAMD64 -DASSERT -DCHECK_UNHANDLED_OOPS -I. -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/share/vm/prims -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/share/vm -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/share/vm/precompiled -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/cpu/x86/vm -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/os_cpu/linux_x86/vm -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/os/linux/vm -I/home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/os/posix/vm -I../generated -DHOTSPOT_BUILD_USER="\"sgehwolf\"" -DHOTSPOT_LIB_ARCH=\"amd64\" -DHOTSPOT_VM_DISTRO="\"OpenJDK\""  -DTARGET_OS_FAMILY_linux -DTARGET_ARCH_x86 -DTARGET_ARCH_MODEL_x86_64 -DTARGET_OS_ARCH_linux_x86 -DTARGET_OS_ARCH_MODEL_linux_x86_64 -DTARGET_COMPILER_gcc -DCOMPILER2 -DCOMPILER1 -fPIC -fno-rtti -fno-exceptions -D_REENTRANT -fcheck-new -fvisibility=hidden -m64  -pipe -fno-strict-aliasing -g -fno-omit-frame-pointer -O3  -O0 -fstack-protector-all --param ssp-buffer-size=1 -DVM_LITTLE_ENDIAN -D_LP64=1 -Werror -Wpointer-arith -Wsign-compare -Wundef -Wunused-function -Wunused-value -Wformat=2 -Wreturn-type -Woverloaded-virtual -Wtype-limits   -DDTRACE_ENABLED -c -MMD -MP -MF ../generated/dependencies/virtualMemoryTracker.o.d -fpch-deps -o virtualMemoryTracker.o /home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/share/vm/services/virtualMemoryTracker.cpp 
Compiling /home/sgehwolf/Documents/openjdk/upstream-sources/openjdk9-hs-rt/hotspot/src/share/vm/runtime/virtualspace.cpp

Cheers,
Severin