[Libre-soc-isa] [Bug 966] create shift-and-add instruction

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=966

--- Comment #16 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #14)
> (In reply to Dmitry Selyutin from comment #13)
> > Is there any agreement and conclusion about this? If necessary I can create
> > a patch which supports a new form to support shaddsw/shadduw simultaneously;
> 
> no. no justification. extsw is sufficient. we have over 100
> instructions to add already

realistically if we're adding only one of shaddsw/shadduw, we should add
shaddsw because a C compiler can trivially use it with array[int_index],
whereas shadduw requires the compiler to first prove the index isn't ever
negative, which is often difficult (all the easy cases are already covered by
ld/st's immediate or by shadd). u32 indexes are very uncommon -- basically
everything uses a constant, size_t/ptrdiff_t, or int.

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