Speed up Data.Char.isDigit

isDigit is currently defined like this:

isDigit                 :: Char -> Bool
isDigit c               =  c >= '0' && c <= '9'

This will short-circuit the right way if you're looking for digits mixed with spaces, but the wrong way if you're looking for digits mixed with letters. It also requires a conditional jump to do that short-circuiting (confirmed by inspecting the assembly). It should be better to use an unsigned comparison instead:

isDigit                 :: Char -> Bool
isDigit c               = (fromIntegral (ord c) :: Word) - 48 <= 9

The interesting section looks like this

        movq 7(%rbx),%rax
        addq $-48,%rax
        cmpq $9,%rax
        setbe %al
        movzbl %al,%eax
        shlq $3,%rax
        movq ghczmprim_GHCziTypes_Bool_closure_tbl(%rax),%rbx
        addq $8,%rbp
        jmp *(%rbp)

or like this with -fllvm:

        movq    7(%rbx), %rax
        addq    $-48, %rax
        cmpq    $10, %rax
        sbbq    %rax, %rax
        andq    $8, %rax
        movq    ghczmprim_GHCziTypes_Bool_closure_tbl(%rax), %rbx
        movq    8(%rbp), %rax
        addq    $8, %rbp
        jmpq    *%rax  # TAILCALL

Trac metadata

Trac field	Value
Version	7.9
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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