RFR: 8187676: Disable harmless uninitialized warnings for two files

[Rahul Raghavan]

Hi Erik,

Please note that this 8187676 seems to be related to 8160404.
    https://bugs.openjdk.java.net/browse/JDK-8160404
    (RelocationHolder constructors have bugs)

As per the latest notes comments added for 8160404-jbs, I will submit 
webrev/RFR soon and will request help confirm similar issues with latest 
gcc7 gets solved.

Thanks,
Rahul

On Tuesday 19 September 2017 07:07 PM, Erik Helin wrote:
> Hi all,
> 
> with gcc 7.1.1 from Fedora 26 on x86-64 there are warnings about the 
> potential usage of maybe uninitialized memory in 
> src/hotspot/cpu/x86/assembler_x86.cpp and in 
> src/hotspot/cpu/x86/interp_masm_x86.cpp.
> 
> The problems arises from the class RelocationHolder in 
> src/hotspot/share/code/relocInfo.hpp which has the private fields:
>    enum { _relocbuf_size = 5 };
>    void* _relocbuf[ _relocbuf_size ];
> 
> and the default constructor for RelocationHolder does not initialize the 
> elements of _relocbuf. I _think_ this is an optimization, 
> RelocationHolder is used *a lot* and setting the elements of 
> RelocationHolder::_relocbuf to NULL (or some other value) in the default 
> constructor might result in a performance penalty. Have a look in 
> build/linux-x86_64-normal-server-fastdebug/hotspot/variant-server/gensrc/adfiles 
> and you will see that RelocationHolder is used all over the place :)
> 
> AFAICS all users of RelocationHolder::_relocbuf take care to not use 
> uninitialized memory, which means that this warning is wrong, so I 
> suggest we disable the warning -Wmaybe-uninitialized for 
> src/hotspot/cpu/x86/assembler_x86.cpp.
> 
> The problem continues because the class Address in 
> src/hotspot/cpu/x86/assembler_x86.hpp has a private field, 
> `RelocationHolder _rspec;` and the default constructor for Address does 
> not initialize _rspec._relocbuf (most likely for performance reasons). 
> The class Address also has a default copy constructor, which will copy 
> all the elements of _rspec._relocbuf, which will result in a read of 
> uninitialized memory. However, this is a benign usage of uninitialized 
> memory, since we take no action based on the content of the 
> uninitialized memory (it is just copied byte for byte).
> 
> So, in this case too, I suggest we disable the warning -Wuninitialized 
> for src/hotspot/cpu/x86/assembler_x86.hpp.
> 
> What do you think?
> 
> Patch:
> http://cr.openjdk.java.net/~ehelin/8187676/00/
> 
> --- old/make/hotspot/lib/JvmOverrideFiles.gmk    2017-09-19 
> 15:11:45.036108983 +0200
> +++ new/make/hotspot/lib/JvmOverrideFiles.gmk    2017-09-19 
> 15:11:44.692107277 +0200
> @@ -32,6 +32,8 @@
>   ifeq ($(TOOLCHAIN_TYPE), gcc)
>     BUILD_LIBJVM_vmStructs.cpp_CXXFLAGS := -fno-var-tracking-assignments 
> -O0
>     BUILD_LIBJVM_jvmciCompilerToVM.cpp_CXXFLAGS := 
> -fno-var-tracking-assignments
> +  BUILD_LIBJVM_assembler_x86.cpp_CXXFLAGS := -Wno-maybe-uninitialized
> +  BUILD_LIBJVM_interp_masm_x86.cpp_CXXFLAGS := -Wno-uninitialized
>   endif
> 
>   ifeq ($(OPENJDK_TARGET_OS), linux)
> 
> Issue:
> https://bugs.openjdk.java.net/browse/JDK-8187676
> 
> Testing:
> - Compiles with:
>    - gcc 7.1.1 and glibc 2.25 on Fedora 26
>    - gcc 4.9.2 and glibc 2.12 on OEL 6.4
> - JPRT
> 
> Thanks,
> Erik