RFR: 8187676: Disable harmless uninitialized warnings for two files

[Erik Helin]

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