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question about optab_libfunc() returning bogus libfuncs for sdivmod_optab

While implementing divmod transform:

I ran into an  issue with optab_libfunc().
It appears optab_libfunc (sdivmod_optab, DImode) returns
a libfunc with name "__divmoddi4", even though such a libfunc
does not exist in libgcc. This happens because in optabs.def
the entry for sdivmod_optab has gen_int_libfunc,
and call to optab_libfunc (sdivmo_optab, DImode) lazily
creates a bogus libfunc "__divmoddi4" by calling gen_int_libfunc().
Jim has given a better explanation for it here:

To work around this issue I set libfunc entry for sdivmod_optab to NULL
and verified that optab_libfunc (sdivmod_optab, DImode) returns NULL_RTX
instead of a bogus libfunc if it's not overriden by the target.
Patch posted for that here:

It passes bootstrap+test on x86_64-unknown-linux-gnu, ppc64le-linux-gnu,
and cross-tested on arm*-*-* and aarch64*-*-*.
Is that change OK ?

This issue is the main blocker for divmod patch, any suggestions for resolving
it would be greatly appreciated.


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