921586 - 'may be used uninitialized' warnings in StructuredClone.cpp

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jim Blandy :jimb]

Attachment: Avoid 'may be used uninitialized' warnings in StructuredClone.cpp.

Specifically, it seems that G++ 4.8.1 is unable to tell that the call to
AutoCompartment::~AutoCompartment is protected by the Maybe::constructed
flag.

The "solution" is not to use Maybe here; although Debugger.cpp uses
Maybe<AutoCompartment> everywhere, it gets away with it because it always
actually constructs the Maybe(?!?).

It's a shame to let the compiler's stupidity shape the code, but that
particular warning is valuable elsewhere, so I feel that we shouldn't just
disable the warning.

In file included from /home/jimb/moz/dbg/js/src/jsobjinlines.h:20:0,
                 from /home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:44:
/home/jimb/moz/dbg/js/src/jscompartmentinlines.h: In function ‘bool JS_StructuredClone(JSContext*, JS::Value, JS::Value*, const JSStructuredCloneCallbacks*, void*)’:
/home/jimb/moz/dbg/js/src/jscompartmentinlines.h:45:35: warning: ‘ac.js::AutoCompartment::origin_’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     cx_->leaveCompartment(origin_);
                                   ^
/home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:1521:41: note: ‘ac.js::AutoCompartment::origin_’ was declared here
         mozilla::Maybe<AutoCompartment> ac;
                                         ^
In file included from /home/jimb/moz/dbg/js/src/jsobjinlines.h:20:0,
                 from /home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:44:
/home/jimb/moz/dbg/js/src/jscompartmentinlines.h:45:35: warning: ‘ac.js::AutoCompartment::cx_’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     cx_->leaveCompartment(origin_);
                                   ^
/home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp:1521:41: note: ‘ac.js::AutoCompartment::cx_’ was declared here
         mozilla::Maybe<AutoCompartment> ac;
                                         ^

Comment 1

[Mike Hordecki [:mhordecki] (UTC+2)]

Interesting, I cannot reproduce the warning. I'm using gcc 4.8.1 20130725 (the default one on Arch) with a default build configuration. Which version are you using?

Comment 2

[Jim Blandy :jimb]

I'm using g++ (GCC) 4.8.1 20130603 (Red Hat 4.8.1-1). I don't get the warning on shell-only builds, but I do when I build as part of the whole Firefox tree, with the following .mozconfig:

ac_add_options --enable-debug='-g3 -O1'
ac_add_options --disable-optimize
ac_add_options --enable-trace-malloc

It seems like this is #defining MOZ_GLUE_IN_PROGRAM on the command line and not MFBT_IMPL. Here's the whole command:

c++ -o StructuredClone.o -c  -I./../../dist/system_wrappers_js -include /home/jimb/moz/dbg/js/src/config/gcc_hidden.h -DENABLE_PARALLEL_JS -DENABLE_BINARYDATA -DMOZ_GLUE_IN_PROGRAM -DNO_NSPR_10_SUPPORT -DEXPORT_JS_API -DJS_HAS_CTYPES -DDLL_PREFIX=\"lib\" -DDLL_SUFFIX=\".so\" -DUSE_ZLIB -Ictypes/libffi/include -I/home/jimb/moz/dbg/js/src/../../mfbt/double-conversion -I/home/jimb/moz/dbg/js/src/../../intl/icu/source/common -I/home/jimb/moz/dbg/js/src/../../intl/icu/source/i18n  -I/home/jimb/moz/dbg/js/src -I. -I./../../dist/include  -I/home/jimb/moz/dbg/obj-bug/dist/include/nspr      -I/home/jimb/moz/dbg/js/src  -fPIC   -DMOZILLA_CLIENT -include ./js-confdefs.h -MD -MP -MF .deps/StructuredClone.o.pp  -Wall -Wpointer-arith -Woverloaded-virtual -Werror=return-type -Wtype-limits -Wempty-body -Werror=conversion-null -Wsign-compare -Wno-invalid-offsetof -Wcast-align -fno-rtti -ffunction-sections -fdata-sections -fno-exceptions -fno-math-errno -std=gnu++0x -pthread -pipe  -DDEBUG -D_DEBUG -DTRACING -g3 -O1 -fno-omit-frame-pointer  -DUSE_SYSTEM_MALLOC=1 -DENABLE_ASSEMBLER=1 -DENABLE_JIT=1  /home/jimb/moz/dbg/js/src/vm/StructuredClone.cpp

Comment 3

[Jim Blandy :jimb]

Attachment: StructuredClone.ii

This attachment is the output from preprocessing StructuredClone.cpp, so it doesn't depend on any header files or preprocessor macro definitions. You can compile it and see the warning with the command:

c++ -o StructuredClone.o -c -fPIC -Wall -Wpointer-arith -Woverloaded-virtual -Werror=return-type -Wtype-limits -Wempty-body -Werror=conversion-null -Wsign-compare -Wno-invalid-offsetof -Wcast-align -fno-rtti -ffunction-sections -fdata-sections -fno-exceptions -fno-math-errno -std=gnu++0x -pthread -pipe -g3 -O1 -fno-omit-frame-pointer StructuredClone.ii

Comment 4

[Jim Blandy :jimb]

Comment on attachment 811311 [details] [diff] [review]
Avoid 'may be used uninitialized' warnings in StructuredClone.cpp.

Mike isn't available to look at this; Waldo, would you be willing to check it out?

Comment 5

[Nicholas Nethercote [inactive]]

Comment on attachment 811311 [details] [diff] [review]
Avoid 'may be used uninitialized' warnings in StructuredClone.cpp.

Review of attachment 811311 [details] [diff] [review]:
-----------------------------------------------------------------

Stealing -- looks fine to me.

Comment 6

[Jim Blandy :jimb]

https://hg.mozilla.org/integration/mozilla-inbound/rev/ab67bdb018d8

Comment 7

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/ab67bdb018d8