s390x: Basic support for IaddIfcout

[uweigand]

This adds enough support for the IaddIfcout opcode to make the
code emitted by dynamic_addr work on s390x.

Note: On s390x, the condition code mask that has to be used to
implement unsigned_add_overflow_condition does not match any of
the masks for the "normal" condition codes, so this design is
not really a good match for s390x ...

[cfallin]

Thanks!

Just a request for a little more detail/description below to help future readers, but otherwise LGTM.

As a meta-level comment, I do think that a more self-contained instruction here -- maybe "add and trap on overflow" or something like that? -- would probably be better, as it wouldn't require special handling of flags. (The original design was pretty x86-centric, for sure.) But that's a future refactor, and this should work for now!

[cfallin]

Could we add a comment here noting what these constants are (which condition flags) and how the combination of the add instruction generated by IaddIfcout and the condcode here will properly trap?

Perhaps also a note in unsigned_add_overflow_condition() would be helpful in order to describe how the condition code is actually specially handled in the one place this is known to be used.

[uweigand]

Added comments.

As a meta-level comment, I do think that a more self-contained instruction here -- maybe "add and trap on overflow" or something like that? -- would probably be better, as it wouldn't require special handling of flags.

Either that, or at least some new value like IntCC::Carry (analogous to the existing IntCC::Overflow).