[Libre-soc-isa] [Bug 996] New: add shaddsw or replace shadduw with shaddsw since i32 indexes are waay more common than u32

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=996

            Bug ID: 996
           Summary: add shaddsw or replace shadduw with shaddsw since i32
                    indexes are waay more common than u32
           Product: Libre-SOC's first SoC
           Version: unspecified
          Hardware: Other
                OS: Linux
            Status: CONFIRMED
          Severity: enhancement
          Priority: ---
         Component: Specification
          Assignee: lkcl at lkcl.net
          Reporter: programmerjake at gmail.com
                CC: libre-soc-isa at lists.libre-soc.org
   NLnet milestone: ---

as mentioned in:
https://bugs.libre-soc.org/show_bug.cgi?id=966#c17

> 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).

> those easy cases are:
> 1. the compiler can prove the index is a constant, in which case just using
> ld/st immediate is sufficient.
> 2. the compiler can promote all uses of the index variable to a 64-bit value
> throughout the code thereby avoiding needing any sign extensions (a common
> optimization compilers already do), in which case it can just use shadd.

Note that easy case #2 also covers u32 index variables that the compiler can
prove don't wrap around.

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