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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
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture