RFR: 8275405: Linking error for classes with lambda template parameters and virtual functions

[Magnus Ihse Bursie]

On Wed, 20 Oct 2021 08:11:34 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

> We encountered the following linking error when trying to build Generational ZGC on Windows:
> 
> jvm.exp : error LNK2001: unresolved external symbol "const ZBasicOopIterateClosure<class <lambda_9767402e468f9fc654281195f9700e48> >::`vftable'" (??_7?$ZBasicOopIterateClosure at V<lambda_9767402e468f9fc654281195f9700e48>@@@@6B@)
> 
> 
> I narrowed this down to a simple reproducer, which doesn't link when built through the HotSpot build system:
> 
> #include <functional>
> std::function<void()> = [](){};
> 
> 
> I found that we have a line in our make files that filters out symbols that contain the string vftable (though it checks the mangled name, so a bit hard to find):
> 
> else ifeq ($(call isTargetOs, windows), true)
>   DUMP_SYMBOLS_CMD := $(DUMPBIN) -symbols *.obj
>   FILTER_SYMBOLS_AWK_SCRIPT := \
>       '{ \
>         if ($$7 ~ /??_7.*@@6B@/ && $$7 !~ /type_info/) print $$7; \
>       }'
> 
> 
> The following line prints the vftable symbol if it doesn't contain the string 'type_info':
>  if ($$7 ~ /??_7.*@@6B@/ && $$7 !~ /type_info/) print $$7;
> 
> The printed values are added to a list of symbols that get filtered out of the mapfile, which is then passed to the linker.
> 
> I can get the code to link if I add a second exception for vftable symbols containing the string 'lambda':
>  if ($$7 ~ /??_7.*@@6B@/ && $$7 !~ /type_info/ && $$7 !~ /lambda/) print $$7;
> 
> I did an additional experiment where I completely removed this filtering of vftable symbols. When I did that the linker complained that we used more than 64K symbols.

Looks good to me.

make/hotspot/lib/JvmMapfile.gmk line 109:

> 107:   #
> 108:   # Some usages of C++ lambdas require the vftable symbol of classes that use
> 109:   # the lambda type as a template parameter. The usage of those classes wont

Suggestion:

  # the lambda type as a template parameter. The usage of those classes won't

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Marked as reviewed by ihse (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/6030