RFR (2xS): 8181318: Allow C++ library headers on Solaris Studio

Daniel D. Daugherty daniel.daugherty at oracle.com
Mon Jun 5 17:22:32 UTC 2017

On 6/5/17 10:59 AM, Erik Osterlund wrote:
> Hi Dan,
>>> On 5 Jun 2017, at 18:31, Daniel D. Daugherty <daniel.daugherty at oracle.com> wrote:
>>> On 6/5/17 10:19 AM, Erik Österlund wrote:
>>> Hi David,
>>>> On 2017-06-05 14:45, David Holmes wrote:
>>>> Hi Erik,
>>>>> On 5/06/2017 8:38 PM, Erik Österlund wrote:
>>>>> Hi David,
>>>>>> On 2017-06-02 03:30, David Holmes wrote:
>>>>>> Hi Erik,
>>>>>>> On 2/06/2017 12:50 AM, Erik Österlund wrote:
>>>>>>> Hi David,
>>>>>>>> On 2017-06-01 14:33, David Holmes wrote:
>>>>>>>> Hi Erik,
>>>>>>>> Just to be clear it is not the use of <limits> that I am concerned about, it is the -library=stlport4. It is the use of that flag that I would want to check in terms of having no affect on any existing code generation.
>>>>>>> Thank you for the clarification. The use of -library=stlport4 should not have anything to do with code generation. It only says where to look for the standard library headers such as <limits> that are used in the compilation units.
>>>>>> The potential problem is that the stlport4 include path eg:
>>>>>> ./SS12u4/lib/compilers/include/CC/stlport4/
>>>>>> doesn't only contain the C++ headers (new, limits, string etc) but also a whole bunch of regular 'standard' .h headers that are _different_ to those found outside the stlport4 directory ie the ones we would currently include. I don't know if the differences are significant, nor whether those others may be found ahead of the stlport4 version. But that is my concern about the effects on the code.
>>>>> While I do not think exchanging these headers will have any behavioral impact, I agree that we can not prove so as they are indeed different header files. That is a good point.
>>>>> However, I think that makes the stlport4 case stronger rather than weaker. We already use stlport4 for our gtest testing (because it is required and does not build without it). And if those headers would indeed have slightly different behaviour as you imply, it further motivates using the same standard library when compiling the product as the testing code. If they were to behave slightly differently, it might be that our gtest tests does not catch hidden bugs that only manifest when building with a different set of headers used for the product build. I therefore find it exceedingly dangerous to stay on two standard libraries (depending on if test code or product code is compiled) compared to consistently using the same standard library across all compilations. So for me, the larger the risk is of them behaving differently is, the bigger the motivation is to use stlport4 consistently.
>>>> Regardless of what gtest does if you want to switch the standard libraries used by the product then IMHO that should go through a vetting process no weaker than that for changing the toolchain, as you effectively are doing that.
>>> I talked to Erik Joelsson about how to compare two builds. He introduced me to our compare.sh script that is used to compare two builds.
>>> I built a baseline without these changes and a new build with these changes applied, both on a Solaris SPARC T7 machine. Then I compared them with ./compare.sh -2dirs {$BUILD1}/hotspot/variant-server/libjvm/objs {$BUILD2}/hotspot/variant-server/libjvm/objs -libs --strip
>>> This compares the object files produced when compiling hotspot in build 1 and build 2 after stripping symbols.
>>> First it reported:
>>> Libraries...
>>>    Size    : Symbols :  Deps   : Disass   :
>>>            :* diff  *:         :          : ./dtrace.o
>>>            :* diff  *:         :*   38918*: ./jni.o
>>>            :* diff  *:         :*   23226*: ./unsafe.o
>>> It seems like all symbols were not stripped here on these mentioned files and constituted all differences in the disassembly. So I made a simple sed filter to filter out symbol names in the disassembly with the regexp <.*>.
>>> The result was:
>>> Libraries...
>>>    Size    : Symbols :  Deps   : Disass   :
>>>            :* diff  *:         :          : ./dtrace.o
>>>            :* diff  *:         :          : ./jni.o
>>>            :* diff  *:         :          : ./unsafe.o
>>> This shows that not a single instruction was emitted differently between the two builds.
>>> I also did the filtering manually on jni.o and unsafe.o in emacs to make sure I did not mess up.
>>> Are we happy with this, or do you still have doubts that this might result in different code or behavior?
>> Just to be clear: The current experiment changes both the header and
>> the standard library right? If so, then the compare.sh run works for
>> validating that using the new header file will not result in a change
>> in behavior. However, that comparison doesn't do anything for testing
>> a switch in the standard libraries right?
> The -xnolib guards are still there in the LDFLAGS. That is, the linker will not allow anything to link against either standard library. I have manually confirmed this by doing the sanity check of comparing the NEEDED entries in the dynamic section of the libjvm.so elf file using elfdump. It has no references to neither libstlport4 nor libCstd with or without my changes.
> Summary: the changes do not add any linktime dependencies to either standard library, and we are still guarded in the sense that if such dependencies were to accidentally be introduced, it would not build. The only difference then would be slightly different code generation of object files. And their disassemblies have been confirmed not to differ by even a single instruction generated differently.

So your current changes use the stlport4 include path for both product
build and 'gtest' build. You've verified the following:

- The product binaries do not change even one instruction with the
   new include path.
- The options to keep us from linking to anything in stlport4 are
   still in place.
- You've manually verified that there are no linkage dependencies
   in the resulting binaries.

If I have all that right, then I think you've covered your bases.


> Thanks,
> /Erik
>> Dan
>>> Thanks,
>>> /Erik
>>>> Cheers,
>>>> David
>>>>> Thanks,
>>>>> /Erik
>>>>>> Thanks,
>>>>>> David
>>>>>> -----
>>>>>>> Specifically, the man pages for CC say:
>>>>>>> <man>
>>>>>>>         -library=lib[,lib...]
>>>>>>>             Incorporates  specified  CC-provided libraries into compilation and
>>>>>>>             linking.
>>>>>>>             When the -library option is used to specify a CC-provided library,
>>>>>>>             the  proper  -I paths are set during compilation and the proper -L,
>>>>>>>             -Y, -P, and -R paths and -l options are set during linking.
>>>>>>> </man>
>>>>>>> As we are setting this during compilation and not during linking, this corresponds to setting the right -I paths to find our C++ standard library headers.
>>>>>>> My studio friends mentioned I could double-check that we did indeed not add a dependency to any C++ standard library by running elfdump on the generated libjvm.so file and check if the NEEDED entries in the dynamic section look right. I did and here are the results:
>>>>>>>        [0]  NEEDED          0x2918ee   libsocket.so.1
>>>>>>>        [1]  NEEDED          0x2918fd   libsched.so.1
>>>>>>>        [2]  NEEDED          0x29190b   libdl.so.1
>>>>>>>        [3]  NEEDED          0x291916   libm.so.1
>>>>>>>        [4]  NEEDED          0x291920   libCrun.so.1
>>>>>>>        [5]  NEEDED          0x29192d   libthread.so.1
>>>>>>>        [6]  NEEDED          0x29193c   libdoor.so.1
>>>>>>>        [7]  NEEDED          0x291949   libc.so.1
>>>>>>>        [8]  NEEDED          0x291953   libdemangle.so.1
>>>>>>>        [9]  NEEDED          0x291964   libnsl.so.1
>>>>>>>       [10]  NEEDED          0x291970   libkstat.so.1
>>>>>>>       [11]  NEEDED          0x29197e   librt.so.1
>>>>>>> This list does not include any C++ standard libraries, as expected (libCrun is always in there even with -library=%none, and as expected no libstlport4.so or libCstd.so files are in there). The NEEDED entries in the dynamic section look identical with and without my patch.
>>>>>>>> I'm finding the actual build situation very confusing. It seems to me in looking at the hotspot build files and the top-level build files that -xnolib is used for C++ compilation & linking whereas -library=%none is used for C compilation & linking. But the change is being applied to $2JVM_CFLAGS which one would think is for C compilation but we don't have $2JVM_CXXFLAGS, so it seems to be used for both!
>>>>>>> I have also been confused by this when I tried adding CXX flags through configure that seemed to not be used. But that's a different can of worms I suppose.
>>>>>>> Thanks,
>>>>>>> /Erik
>>>>>>>> David
>>>>>>>>> On 1/06/2017 7:36 PM, Erik Österlund wrote:
>>>>>>>>> Hi David,
>>>>>>>>>> On 2017-06-01 08:09, David Holmes wrote:
>>>>>>>>>> Hi Kim,
>>>>>>>>>> On 1/06/2017 3:51 PM, Kim Barrett wrote:
>>>>>>>>>>>> On May 31, 2017, at 9:22 PM, David Holmes <david.holmes at oracle.com> wrote:
>>>>>>>>>>>> Hi Erik,
>>>>>>>>>>>> A small change with big questions :)
>>>>>>>>>>>> On 31/05/2017 11:45 PM, Erik Österlund wrote:
>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>> It would be desirable to be able to use harmless C++ standard library headers like <limits> in the code as long as it does not add any link-time dependencies to the standard library.
>>>>>>>>>>>> What does a 'harmless' C++ standard library header look like?
>>>>>>>>>>> Header-only (doesn't require linking), doesn't run afoul of our
>>>>>>>>>>> [vm]assert macro, and provides functionality we presently lack (or
>>>>>>>>>>> only handle poorly) and would not be easy to reproduce.
>>>>>>>>>> And how does one establish those properties exist for a given header file? Just use it and if no link errors then all is good?
>>>>>>>>> Objects from headers that are not ODR-used such as constant folded expressions are not imposing link-time dependencies to C++ libraries. The -xnolib that we already have in the LDFLAGS will catch any accidental ODR-uses of C++ objects, and the JVM will not build if that happens.
>>>>>>>>> As for external headers being included and not playing nicely with macros, this has to be evaluated on a case by case basis. Note that this is a problem that occurs when using system headers (that we are already using), as it is for using C++ standard library headers. We even run into that in our own JVM when e.g. the min/max macros occasionally slaps us gently in the face from time to time.
>>>>>>>>>>> The instigator for this is Erik and I are working on a project that
>>>>>>>>>>> needs information that is present in std::numeric_limits<> (provided
>>>>>>>>>>> by the <limits> header).  Reproducing that functionality ourselves
>>>>>>>>>>> would require platform-specific code (with all the complexity that can
>>>>>>>>>>> imply).  We'd really rather not re-discover and maintain information
>>>>>>>>>>> that is trivially accessible in every standard library.
>>>>>>>>>> Understood. I have no issue with using <limits> but am concerned by the state of stlport4. Can you use <limits> without changing -library=%none?
>>>>>>>>> No, that is precisely why we are here.
>>>>>>>>>>>>> This is possible on all supported platforms except the ones using the solaris studio compiler where we enforce -library=%none in both CFLAGS and LDFLAGS.
>>>>>>>>>>>>> I propose to remove the restriction from CFLAGS but keep it on LDFLAGS.
>>>>>>>>>>>>> I have consulted with the studio folks, and they think this is absolutely fine and thought that the choice of -library=stlport4 should be fine for our CFLAGS and is indeed what is already used in the gtest launcher.
>>>>>>>>>>>> So what exactly does this mean? IIUC this allows you to use headers for, and compile against "STLport’s Standard Library implementation version 4.5.3 instead of the default libCstd". But how do you then not need to link against libstlport.so ??
>>>>>>>>>>>> https://docs.oracle.com/cd/E19205-01/819-5267/bkakg/index.html
>>>>>>>>>>>> "STLport is binary incompatible with the default libCstd. If you use the STLport implementation of the standard library, then you must compile and link all files, including third-party libraries, with the option -library=stlport4”
>>>>>>>>>>> It means we can only use header-only parts of the standard library.
>>>>>>>>>>> This was confirmed / suggested by the Studio folks Erik consulted,
>>>>>>>>>>> providing such limited access while continuing to constrain our
>>>>>>>>>>> dependency on the library.  Figuring out what can be used will need to
>>>>>>>>>>> be determined on a case-by-case basis.  Maybe we could just link with
>>>>>>>>>>> a standard library on Solaris too.  So far as I can tell, Solaris is
>>>>>>>>>>> the only platform where we don't do that.  But Erik is trying to be
>>>>>>>>>>> conservative.
>>>>>>>>>> Okay, but the docs don't seem to acknowledge the ability to use, but not link to, stlport4.
>>>>>>>>> Not ODR-used objects do not require linkage. (http://en.cppreference.com/w/cpp/language/definition)
>>>>>>>>> I have confirmed directly with the studio folks to be certain that accidental linkage would fail by keeping our existing guards in the LDFLAGS rather than the CFLAGS.
>>>>>>>>> This is also reasonably well documented already (https://docs.oracle.com/cd/E19205-01/819-5267/bkbeq/index.html).
>>>>>>>>>>>> There are lots of other comments in that document regarding STLport that makes me think that using it may be introducing a fragile dependency into the OpenJDK code!
>>>>>>>>>>>> "STLport is an open source product and does not guarantee compatibility across different releases. In other words, compiling with a future version of STLport may break applications compiled with STLport 4.5.3. It also might not be possible to link binaries compiled using STLport 4.5.3 with binaries compiled using a future version of STLport."
>>>>>>>>>>>> "Future releases of the compiler might not include STLport4. They might include only a later version of STLport. The compiler option -library=stlport4 might not be available in future releases, but could be replaced by an option referring to a later STLport version."
>>>>>>>>>>>> None of that sounds very good to me.
>>>>>>>>>>> I don't see how this is any different from any other part of the
>>>>>>>>>>> process for using a different version of Solaris Studio.
>>>>>>>>>> Well we'd discover the problem when testing the compiler change, but my point was more to the fact that they don't seem very committed to this library - very much a "use at own risk" disclaimer.
>>>>>>>>> If we eventually need to use something more modern for features that have not been around for a decade, like C++11 features, then we can change standard library when that day comes.
>>>>>>>>>>> stlport4 is one of the three standard libraries that are presently
>>>>>>>>>>> included with Solaris Studio (libCstd, stlport4, gcc). Erik asked the
>>>>>>>>>>> Studio folks which to use (for the purposes of our present project, we
>>>>>>>>>>> don't have any particular preference, so long as it works), and
>>>>>>>>>>> stlport4 seemed the right choice (libCstd was, I think, described as
>>>>>>>>>>> "ancient").  Perhaps more importantly, we already use stlport4,
>>>>>>>>>>> including linking against it, for gtest builds. Mixing two different
>>>>>>>>>>> standard libraries seems like a bad idea...
>>>>>>>>>> So we have the choice of "ancient", "unsupported" or gcc :)
>>>>>>>>>> My confidence in this has not increased :)
>>>>>>>>> I trust that e.g. std::numeric_limits<T>::is_signed in the standard libraries has more mileage than whatever simplified rewrite of that we try to replicate in the JVM. So it is not obvious to me that we should have less confidence in the same functionality from a standard library shipped together with the compiler we are using and that has already been used and tested in a variety of C++ applications for over a decade compared to the alternative of reinventing it ourselves.
>>>>>>>>>> What we do in gtest doesn't necessarily make things okay to do in the product.
>>>>>>>>>> If this were part of a compiler upgrade process we'd be comparing binaries with old flag and new to ensure there are no unexpected consequences.
>>>>>>>>> I would not compare including <limits> to a compiler upgrade process as we are not changing the compiler and hence not the way code is generated, but rather compare it to including a new system header that has previously not been included to use a constant folded expression from that header that has been used and tested for a decade. At least that is how I think of it.
>>>>>>>>> Thanks,
>>>>>>>>> /Erik
>>>>>>>>>> Cheers,
>>>>>>>>>> David
>>>>>>>>>>>> Cheers,
>>>>>>>>>>>> David
>>>>>>>>>>>>> Webrev for jdk10-hs top level repository:
>>>>>>>>>>>>> http://cr.openjdk.java.net/~eosterlund/8181318/webrev.00/
>>>>>>>>>>>>> Webrev for jdk10-hs hotspot repository:
>>>>>>>>>>>>> http://cr.openjdk.java.net/~eosterlund/8181318/webrev.01/
>>>>>>>>>>>>> Testing: JPRT.
>>>>>>>>>>>>> Will need a sponsor.
>>>>>>>>>>>>> Thanks,
>>>>>>>>>>>>> /Erik

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