Simple case analyses generate too many branches

Take the following example,

f :: Int -> Bool
f a = case a of
        1  -> True
        5  -> True
        8  -> True
        9  -> True
        11 -> True
        19 -> True
        _  -> False
{-# NOINLINE f #-}

main = print $ f 8

This gets lowered to the following Core (by GHC 7.8),

f
f =
  \ a_s3EH ->
    case a_s3EH of _ { I# ds_s3EJ ->
    case ds_s3EJ of _ {
      __DEFAULT -> False;
      1 -> True;
      5 -> True;
      8 -> True;
      9 -> True;
      11 -> True;
      19 -> True
    }
    }

I have expected GHC to lower this into a nice string of logical operations, with perhaps a couple of branches at the end to determine the result.

Unfortunately, this is not what happens. Instead the C-- is a sea of branches,

       c3F7:
           _s3EK::I64 = I64[R1 + 7];
           if (%MO_S_Lt_W64(_s3EK::I64, 9)) goto c3Fz; else goto c3FA;
       c3Fz:
           if (%MO_S_Lt_W64(_s3EK::I64, 5)) goto c3Fq; else goto c3Fr;
       c3Fq:
           if (_s3EK::I64 != 1) goto c3Ff; else goto c3Fg;
       c3Fr:
           if (%MO_S_Lt_W64(_s3EK::I64, 8)) goto c3Fn; else goto c3Fo;
       c3Fn:
           if (_s3EK::I64 != 5) goto c3Ff; else goto c3Fg;
       c3Fo:
           if (_s3EK::I64 != 8) goto c3Ff; else goto c3Fg;
       c3FA:
           if (%MO_S_Lt_W64(_s3EK::I64, 11)) goto c3Fw; else goto c3Fx;
       c3Fw:
           if (_s3EK::I64 != 9) goto c3Ff; else goto c3Fg;
       c3Fx:
           if (%MO_S_Lt_W64(_s3EK::I64, 19)) goto c3Ft; else goto c3Fu;
       c3Ft:
           if (_s3EK::I64 != 11) goto c3Ff; else goto c3Fg;
       c3Fu:
           if (_s3EK::I64 != 19) goto c3Ff; else goto c3Fg;
       c3Ff:
           R1 = False_closure+1;
           Sp = Sp + 8;
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
       c3Fg:
           R1 = True_closure+2;
           Sp = Sp + 8;
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
...

Which gets turned into the branchy assembler that you would expect. To my surprise even the LLVM backend isn't able to bring this back into a branchless form.

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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