emitCmmLitSwitch could be better

This is a spin off #10124. While looking at the code generated for

f :: Int -> Bool
f a = case a of
        1  -> True
        2  -> True
        3  -> True
        4  -> True
        8  -> True
        9  -> True
        11 -> True
        19 -> True
        _  -> False

I noticed this Cmm:

       c2tI:
           if (%MO_S_Lt_W64(_s2sJ::I64, 3)) goto c2tw; else goto c2tx;
       c2tw:
           if (%MO_S_Lt_W64(_s2sJ::I64, 2)) goto c2tq; else goto c2tr;
       c2tq:
           if (_s2sJ::I64 != 1) goto c2tg; else goto c2th;
       c2tr:
           if (_s2sJ::I64 != 2) goto c2tg; else goto c2th;

Note that when c2tr is reached, we know 2 ≤ _s2sJ < 3, so _s2sJ already is 2, and this check is redundant.

emitCmmLitSwitch does not take that into account, probably because it also needs to work for floats.

I wonder if it isn’t a bit shady to use an if-then-else tree for floats. Maybe for float types, a sequence of equality tests is more suitable. For all other cases, the code generator could make use of "2 ≤ x < 3 ⇒ x = 2" and skip one check.

Trac metadata

Trac field	Value
Version	7.8.4
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
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CC	simonmar
Operating system
Architecture