<AWT Dev> RFR: JDK-8200178 Remove mapfiles for JDK native libraries

Volker Simonis volker.simonis at gmail.com
Fri Mar 23 17:24:24 UTC 2018

Hi Magnus,

thanks for addressing this long standing issue! I haven't looked at
the changes, but just want to share some general and historical notes:

- Compiling with "-fvisibility=hidden" which hides all symbols expect
the ones explicitly exported with
"__attribute__((visibility("default")))" has been requested by SAP
back in 2007 even before we had OpenJDK (see "Use -fvisibility=hidden
for gcc compiles" https://bugs.openjdk.java.net/browse/JDK-6588413)
and finally pushed into the OpenJKD around 2010.
- "-fvisibility=hidden" gave us performance improvements of about 5%
(JBB2005) and 2% (JVM98) on Linux/IA64 and 1,5% (JBB2005) and 0,5%
(JVM98) on Linux/PPC64 because the compiler could use faster calls for
non exported symbols. This improvement was only very small on x86
- "-fvisibility=hidden"/"__attribute__((visibility("default")))"
applies BEFORE using the map files in the linking step (i.e. hidden
symbols can't be exported any more even if mentioned in the map file)
- because of the performance improvements we got by using
"-fvisibility=hidden" it was worth while using it even though we had
the mapfiles at the end of the process.

Then we had several mail threads (which you probably remember because
you were involved :) where we discussed to either remove the map files
completely or instead generate them automatically during the build:


The main arguments against removing the map files at that time were:

1. the danger to re-export all symbols of statically linked libraries
(notably libstdc++ at that time)
2. loosing exports of compiler generated symbols like vtables which
are required by the Serviceability Agent

Point 1 is not a problem today, because I don't think we do any static
linking any more. If we still do it under some circumstances, this
problem should be re-evaluated.

Point 2 is only relevant for HotSpot. But because of "8034065: GCC 4.3
and later doesn't export vtable symbols any more which seem to be
needed by SA" (https://bugs.openjdk.java.net/browse/JDK-8034065),
exporting such symbols trough a map files doesn't work any more
anyway. So this isn't a problem either.

So to cut a long story short - I think the time is ripe to get rid of
the map files. Thumbs up from me (meant as moral support, not as a
concrete review :)


On Fri, Mar 23, 2018 at 5:05 PM, mandy chung <mandy.chung at oracle.com> wrote:
> This is a very good change and no more mapfile to maintain!!
> Please do file JBS issues for the component teams to clean up their exports.
> Mandy
> On 3/23/18 7:30 AM, Erik Joelsson wrote:
>> I have looked at the build changes and they look good.
>> Will you file followups for each component team to look over their
>> exported symbols, at least for the libraries with $(EXPORT_ALL_SYMBOLS)? It
>> sure looks like there is some technical debt laying around here.
>> /Erik
>> On 2018-03-23 06:56, Magnus Ihse Bursie wrote:
>>> With modern compilers, we can use compiler directives (such as
>>> _attribute__((visibility("default"))), or __declspec(dllexport)) to control
>>> symbol visibility, directly in the source code. This has historically not
>>> been present on all compilers, so we had to resort to using mapfiles (also
>>> known as linker scripts).
>>> This is no longer the case. Now all compilers we use support symbol
>>> visibility directives, in one form or another. We should start using this.
>>> Since this has been the only way to control symbol visibility on Windows,
>>> for most of the shared code, we already have proper JNIEXPORT decorations in
>>> place.
>>> If we fix the remaining platform-specific files to have proper JNIEXPORT
>>> tagging, then we can finally get rid of mapfiles.
>>> This fix removed mapfiles for all JDK libraries. It does not touch
>>> hotspot libraries nor JDK executables; they will have to wait for a future
>>> fix -- this was complex enough. This change will not have any impact on
>>> macosx, since we do not use mapfiles there, but instead export all symbols.
>>> (This is not a good idea, but I'll address that separately.) This change
>>> will also have a minimal impact on Windows. The only reason Windows is
>>> impacted at all, is that some changes needed by Solaris and Linux were
>>> simpler to fix for all platforms.
>>> I have strived for this change to have no impact on the actual generated
>>> code. Unfortunately, this was not possible to fully achieve. I do not
>>> believe that these changes will have any actual impact on the product,
>>> though. I will present the differences more in detail further down. Those
>>> who are not interested can probably skip that.
>>> The patch has passed tier1 testing and is currently running tier2 and
>>> tier3. Since the running code is more or less (see caveat below) unmodified,
>>> I don't expect any testing issues.
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8200178
>>> WebRev:
>>> http://cr.openjdk.java.net/~ihse/JDK-8200178-remove-mapfiles/webrev.01
>>> Details on changes:
>>> Most of the source code changes are (unsurprisingly) in java.base and
>>> java.desktop. Remaining changes are in jdk.crypto.ucrypto,
>>> jdk.hotspot.agent, jdk.jdi and jdk.jdwp.agent.
>>> Source code changes does almost to 100% consists in decorating an
>>> exported function with JNIEXPORT. I have also followed the long-standing
>>> convention of adding JNICALL. This is a no-op on non-Windows platforms, so
>>> for most of the changes this is purely cosmetic (and possibly adding in
>>> robustness, should the function ever be used on Windows in the future). I
>>> have also followed the stylistic convention of putting "JNIEXPORT <return
>>> type> JNICALL" on a separate line. For some functions, however, this might
>>> cause a change in calling convention on Windows. Since this can not apply to
>>> exported functions on Windows (otherwise they would already have had
>>> JNIEXPORT), I do not think this matters anything.
>>> A few libraries did not have a mapfile, on Linux and/or Solaris. This
>>> actually meant that all symbols were exported. It is highly unclear if this
>>> was known and intended by the original make rule writer. I have emulated
>>> this by adding the flag $(EXPORT_ALL_SYMBOLS) to these libraries. Hopefully,
>>> we can remove this flag and fix proper exported symbols in the future.
>>> I have run the complete build using COMPARE_BUILD, and made a thourough
>>> analysis of the differences for Linux and Solaris. All native libraries have
>>> symbol differences, but most of them are trivial and/or harmless. As a
>>> result, most libraries have disasm differences as well, but these too seem
>>> trivial and harmless. The differences in symbols that are common to all
>>> libraries include:
>>>  * Internal symbols such as __bss_start, _edata, _end and _fini are now
>>> global. (They are imported as such from the compiler libraries/archives, and
>>> we have no linker script to override this behavior).
>>>  * The versioning tag SUNWprivate_1.1 is not included, and thus neither
>>> the .gnu.version_d symbol.
>>>  * There are a few differences in the symbol and/or mangling of some
>>> local functions. I'm not sure what's causing this,
>>> but it's unlikely to have any effect on the product.
>>> Another common source for change in symbols is due to previous platform
>>> differences. For instance, if we had "JNIEXPORT int JNICALL do_foo() { ...
>>> }", but do_foo was not in the mapfile, the symbol was exported on Windows
>>> but not on Linux and Solaris. (Presumable since it was not needed there,
>>> even though it was compiled for those platforms as well.) Now, with the
>>> mapfiles gone, do_foo() will be exported on all platforms. And contrary,
>>> functions that are compiled on all platforms, and were exported in mapfiles,
>>> but now have gotten an JNIEXPORT decoration, will now be visible even on
>>> Windows. (This accounts for half of the noticed symbol differences on
>>> Windows.) I could have made the JNIEXPORT conditional on OS, but I didn't
>>> think the mess in source code were worth the keeping of binary confidence
>>> with the old build.
>>> A third common source for change in symbols is due to exported functions
>>> "leaking" across library borders. For instance, some functions in
>>> java.desktop is compiled in both libawt_xawt and libawt_headless, but they
>>> were previously only included in the mapfile for one of these libraries.
>>> Now, since the visibility is determined by the source code itself, it gets
>>> exported in both libraries. A variant of this is when a library depends on
>>> another JDK library, and includes the header file from that other library,
>>> which in turn declares a function as JNIEXPORT. This will cause the
>>> including library to also export the function. This accounts for the other
>>> half of the changes on Windows. A typical example of this is that multiple
>>> libraries now re-export hotspot symbols from libjvm.so, like jio_fprintf. (I
>>> have not listed the libjvm re-exports below.)
>>> Note that  Java_java_io_FileOutputStream_close0 in
>>> java.base/unix/native/libjava/FileOutputStream_md.c is no longer exported,
>>> and can probably be removed.
>>> Here is a detailed table showing and accounting for all the remaining
>>> differences found on Linux and Solaris:
>>> java.base/unix/native/libjava: Java_java_io_FileOutputStream_close0 is
>>> now also exported on unix platforms due to JNIEXPORT.
>>> java.base/jspawnlauncher: On solaris, we also include
>>> libjava/childproc.o, which
>>> now exports less functions than it used to (it used to export all
>>> functions, now it is compiled with visibility=hidden).
>>> java.base/java(w).exe: Is now also exporting the following symbols due to
>>> added JNIEXPORT in libjli on Windows:
>>> (Yes, executables can export symbols on Windows. Confusing, I know.)
>>>  JLI_AddArgsFromEnvVar
>>>  JLI_CmdToArgs
>>>  JLI_GetAppArgIndex
>>>  JLI_GetStdArgc
>>>  JLI_GetStdArgs
>>>  JLI_InitArgProcessing
>>>  JLI_Launch
>>>  JLI_List_add
>>>  JLI_List_new
>>>  JLI_ManifestIterate
>>>  JLI_MemAlloc
>>>  JLI_MemFree
>>>  JLI_PreprocessArg
>>>  JLI_ReportErrorMessage
>>>  JLI_ReportErrorMessageSys
>>>  JLI_ReportExceptionDescription
>>>  JLI_ReportMessage
>>>  JLI_SetTraceLauncher
>>>  JLI_StringDup
>>> java.desktop:/libawt_xawt: The following symbols are now also exported on
>>> linux and solaris due to JNIEXPORT:
>>>  awt_DrawingSurface_FreeDrawingSurfaceInfo
>>>  awt_DrawingSurface_GetDrawingSurfaceInfo
>>>  awt_DrawingSurface_Lock
>>>  awt_DrawingSurface_Unlock
>>>  awt_GetColor
>>> The following symbols are now also exported on linux and solaris due to
>>> JNIEXPORT (they were previously
>>>  exported only in libawt):
>>>  Java_sun_awt_DebugSettings_setCTracingOn__Z
>>>  Java_sun_awt_DebugSettings_setCTracingOn__ZLjava_lang_String_2
>>>  Java_sun_awt_DebugSettings_setCTracingOn__ZLjava_lang_String_2I
>>>  Java_sun_awt_X11GraphicsConfig_getNumColors
>>> java.desktop:/libawt_headless: The following symbols are now also
>>> exported due to JNIEXPORT (they were previously
>>>  exported only in libawt_xawt and/or libawt):
>>>  Java_sun_java2d_opengl_GLXGraphicsConfig_getGLXConfigInfo
>>>  Java_sun_java2d_opengl_GLXGraphicsConfig_getOGLCapabilities
>>>  Java_sun_java2d_x11_X11PMBlitLoops_updateBitmask
>>>  Java_sun_java2d_x11_X11SurfaceData_isShmPMAvailable
>>>  X11SurfaceData_GetOps
>>> java.desktop/libawt: The following symbols are now also exported on
>>> Windows, due to added
>>>  SurfaceData_InitOps
>>>  mul8table
>>>  div8table
>>>  doDrawPath
>>>  doFillPath
>>>  g_CMpDataID
>>>  initInverseGrayLut
>>>  make_dither_arrays
>>>  make_uns_ordered_dither_array
>>>  path2DFloatCoordsID
>>>  path2DNumTypesID
>>>  path2DTypesID
>>>  path2DWindingRuleID
>>>  sg2dStrokeHintID
>>>  std_img_oda_blue
>>>  std_img_oda_green
>>>  std_img_oda_red
>>>  std_odas_computed
>>> java.desktop/libawt on solaris:
>>> A number of "#pragma weak" directives was previously overridden by the
>>> mapfile.
>>> Now these directives are respected, so these symbols are now weak instead
>>> of local:
>>>  ByteGrayToIntArgbPreConvert_F
>>>  ByteGrayToIntArgbPreScaleConvert_F
>>>  IntArgbBmToFourByteAbgrPreScaleXparOver_F
>>>  IntArgbToIntRgbXorBlit_F
>>>  IntBgrToIntBgrAlphaMaskBlit_F
>>> java.desktop/libawt on solaris: These are now also exported due to
>>> JNIEXPORT in libmlib_image.
>>>  j2d_mlib_ImageCreate
>>>  j2d_mlib_ImageCreateStruct
>>>  j2d_mlib_ImageDelete
>>> java.desktop/libawt on solaris: This is now also exported due to
>>>  GrPrim_CompGetXorColor
>>>  SurfaceData_GetOpsNoSetup
>>>  SurfaceData_IntersectBoundsXYWH
>>>  SurfaceData_SetOps
>>>  Transform_GetInfo
>>>  Transform_transform
>>> java.desktop/libsplashscreen: JNI_OnLoad is now exported on linux and
>>> solaris due to JNIEXPORT.
>>> libspashscreen also had JNIEXPORT (actually a pure _declspec(dllexport))
>>> but no JNICALL, which I added as
>>> a part of converting to JNIEXPORT. The same goes for libmlib_image .
>>> jdk.sctp/libsctp: handleSocketError is now exported on linux and solaris
>>> due to JNIEXPORT in libnio.
>>> java.instrument:/libinstrument: Agent_OnUnload is now also exported on
>>> linux and solaris platforms due to JNIEXPORT.
>>> JLI_ManifestIterate is now also exported on Windows, due to added
>>> JNIEXPORT in libjli.
>>> jdk.management/libmanagement_ext:
>>> Java_com_sun_management_internal_Flag_setDoubleValue is now also exported on
>>> linux and solaris platforms due to JNIEXPORT.
>>> /Magnus

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